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Advances in the Production and Chemistry of the Heaviest Elements Andreas Türler*,†,‡ and Valeria Pershina§

† Laboratory of Radiochemistry and Environmental Chemistry, Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ‡ Laboratory of Radiochemistry and Environmental Chemistry, Department and Chemistry, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland § GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstrasse 1, D-64291 Darmstadt, Germany 8.1.2. Empirical Correlations and Extrapola- tions 1260 8.2. Complex Formation in Aqueous Solutions 1261 9. (Z = 104) 1262 9.1. Theoretical Predictions 1262 9.2. Experimental Results 1264 9.2.1. Gas-Phase Chemistry of Rutherfordium 1265 9.2.2. Liquid-Phase Chemistry of Rutherfordi- CONTENTS um 1268 10. (Z = 105) 1273 1. Introduction 1238 10.1. Theoretical Predictions 1273 2. History of the Discovery of the Transuranium 10.2. Experimental Results 1275 Elements 1239 10.2.1. Gas-Phase Chemistry 1275 2.1. 1239 10.2.2. Liquid-Phase Chemistry of Dubnium 1277 2.2. Transactinides 1240 11. (Z = 106) 1278 3. Nuclear Properties 1242 11.1. Theoretical Predictions 1278 4. Nuclear Reactions of Colliding Nuclei 1243 11.2. Experimental Results 1279 5. Nuclear Experimental Techniques 1244 11.2.1. Gas-Phase Chemistry 1279 6. Relativistic Effects on Chemical Properties 1246 11.2.2. Liquid-Phase Chemistry 1282 6.1. Relativistic Effects on Atomic Electronic 12. (Z = 107) 1282 Shells 1246 12.1. Theoretical Predictions 1282 6.2. Current Relativistic Quantum-Chemical 12.2. Experimental Results 1283 Methods 1247 12.2.1. Gas-Phase Chemistry of Bohrium 1283 6.3. Relativistic Effects and the Future Periodic 13. (Z = 108) 1284 Table of the Elements 1248 13.1. Theoretical Predictions 1284 7. Chemical Techniques To Investigate Transacti- 13.2. Experimental Results 1285 nides 1249 13.2.1. Gas-Phase Chemistry 1285 7.1. Prerequisites for a Chemical Isolation of 13.2.2. Liquid-Phase Chemistry 1286 Heaviest Elements 1249 14. (Z = 109), (Z = 110), 7.1.1. Synthesis of Heaviest Elements 1249 (Z = 111) 1287 7.1.2. Rapid Transport 1251 14.1. Theoretical Predictions 1287 7.1.3. Chemical Isolation, Sample Preparation, 15. (Z = 112) 1288 and Detection 1252 15.1. Theoretical Predictions 1288 7.2. Gas-Phase Chemistry: Isothermal Chroma- 15.2. Experimental Results 1290 tography and Thermochromatography 1252 15.2.1. Gas-Phase Chemistry 1290 7.2.1. Isothermal Chromatography 1253 16. Element 113 1292 7.2.2. Thermochromatography 1255 16.1. Theoretical Predictions 1292 7.3. Liquid-Phase Chemistry 1256 16.2. Experimental Results 1293 7.3.1. Manual Liquid−Liquid Extractions and 16.2.1. Gas-Phase Chemistry 1293 Column Chromatography 1257 17. (Z = 114) 1293 7.3.2. Automated Column Separations 1257 17.1. Theoretical Predictions 1293 7.3.3. Automated Liquid−Liquid Extractions 1258 17.2. Experimental Results 1294 8. Methods To Predict Experimentally Measurable 17.2.1. Gas-Phase Chemistry 1294 Properties of Transactinides 1259 8.1. Volatility 1259 8.1.1. Physisorption and Chemisorption Mod- Special Issue: 2013 Nuclear Chemistry els 1259 Received: June 15, 2012 Published: February 13, 2013

© 2013 American Chemical Society 1237 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review

Figure 1. Current of the Elements with IUPAC approved numbering of groups and element symbols. The names for elements 114 (Flerovium, Fl) and 116 (, Lv) suggested by the team of discoverers have recently been officially accepted by IUPAC4.

18. Element 115, Livermorium (Z = 116), Element elements will it contain? Will we need to introduce the g- 117, and Element 118 1296 orbitals, and will the current principles governing the groups 18.1. Theoretical Predictions 1296 and periods of the Periodic Table still be valid for the heaviest 19. Element 119 and Element 120 1297 elements? These intricate questions are the topic of current 19.1. Theoretical Predictions 1297 research in the chemistry of the heaviest elements. 20. Elements beyond Z = 120 1298 For increasingly heavy nuclei, the electrostatic repulsions of 20.1. Theoretical Predictions 1298 protons cannot be sufficiently compensated by the attractive 21. Conclusions and Outlook 1298 nuclear force through an increasing number of mediating 21.1. Summary of Volatility Studies of the neutrons. Therefore, the heaviest stable known nucleus is Heaviest Elements and Their Compounds 1300 already reached with 208Pb. All of heavier elements, 21.2. Summary of Aqueous Chemistry Studies of including some elements such as Bi, Th, and U that still can be the Heaviest Elements 1300 found in nature as remnants of the last nucleosynthesis process, 21.3. Future Developments 1300 are radioactive and decay preferentially by successive α-particle Author Information 1300 and β-particle emissions back to the last stable element Pb. The Corresponding Author 1300 heaviest nuclide, which was reported to be detected in traces in 244 Notes 1300 nature, is Pu, with a radioactive half-life (t1/2) of 81.2 million Biographies 1300 years. It is conceivable that 244Pu could still be present on earth Acknowledgments 1301 as primordial element or be of cosmic origin due to explosive Abbreviations and Quantities 1301 stellar nucleosynthesis, for example from a nearby supernova References 1302 that occurred after the formation of our solar system. In an attention attracting article, Hoffman et al.5 reported a concentration of about 2350 atoms of 244Pu per gram of 1. INTRODUCTION Bastnaesite, a mineral highly enriched in rare earth elements. After Dmitri Ivanovich Mendeleev and, independently, Julius However, a recent search in Bastnaesite from the same mine Lothar Meyer, and others discovered the ordering principles of using accelerator mass spectrometry remained negative with a the elements, Mendeleev proposed the first valid Periodic Table detection limit of less than 370 atoms per gram (99% upper 1 fi 6 of the Elements nearly 150 years ago. We are now at a point con dence limit). Well established is the presence of small 237 239 where the discovery of the last missing element in the seventh quantities of Np and Pu in nature due to 235 238 7 , namely synthesis of the element with processes on U and U, respectively. Furthermore, recent 8 117, has recently been announced.2,3 Therefore, the Periodic evidence presented by Marinov et al. about the existence of a Table currently contains 118 elements, the lightest being long-lived with atomic number Z = 122 (or and the heaviest yet unnamed eka- (or Uuo, neighboring element) and mass number A = 292 with an −12 ununoctium) as provisional name in IUPAC (International abundance of about 1 × 10 relative to Th in natural Th 9,10 Union of Pure and Applied Chemistry) terminology. However, could not be authenticated. Also the claimed observation of among heavy element chemists, the IUPAC provisional names extremely long-lived, high spin, super- or hyperdeformed 11 are only rarely used. Instead, for elements that have been isomeric states in neutron deficient heavy nuclei, which was reported, but not fully authenticated, quite often only the used as an argument to explain the observation of long-lived atomic number is being used (i.e., element 113 or E113). A 292122, could not be observed in independent experiments.12,13 modern Periodic Table of the Elements is shown in Figure 1. Therefore, all elements heavier than Pu (Z = 94) are man- But has the far end of the Periodic Table of the Elements made. Elements up to Fm (Z = 100) were synthesized in now been reached? What is the heaviest element in the Periodic successive irradiations of ever heavier elements with α-particles System? Are there still undiscovered ones which might even be (4He2+) and neutrons. Weighable amounts of various actinides found in nature? Is there an eighth period and how many were then produced by successive neutron capture and β−-

1238 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review decay in nuclear reactors. Transactinide elements are only theory that can reveal how relativistic effects influence synthesized in heavy ion fusion reactions at high power chemical properties: only a comparison of the observed accelerators on a “one atom at a time” level with beams ranging behavior with that predicted on the basis of relativistic vs from O to Zn, with 294118 currently being the heaviest nonrelativistic calculations does allow assessing the importance observed nucleus.14 Some theoretical studies place the limits of and magnitude of relativistic effects. nuclei that can no longer exist as bound entities beyond Z ≅ Theoretical chemical research on the heaviest elements is not 300 and A ≅ 960.15 Even though such nuclei could theoretically less challenging than the experimental one and should be based exist as so-called hyperheavy bubble nuclei, the heaviest on the most accurate relativistic electronic structure calculations elements should exist in the form of atoms. To qualify as a in order to reliably predict properties and experimental , the nucleus of the longest lived behavior of the new elements and their compounds. It also should exist >10−14 s,16 which is the time needed for the requires the development of special approaches that bridge formation of an . The number of electrons that calculations with quantities that cannot be so easily predicted can be arranged around a nucleus is limited. Modern relativistic from calculations. Due to the recent spectacular developments electronic structure theory that takes into account quantum in relativistic quantum theory, computational algorithms, and electrodynamic effects (QED) predicts this to happen at techniques, very accurate calculations of properties of the element Z = 173, where the energy of the 1s electron falls into transactinide elements and their compounds are now possible. the negative energy continuum;17 that is, it becomes less than The experimental verification of these predictions is highly − 2 fi 2mec . desirable, but also very demanding. The chemical identi cation The place an element occupies in the Periodic Table is not of transactinide elements is, up until now, always accomplished only defined by its atomic number, i.e. the number of protons by the detection of the characteristic in the nucleus, but also by its electronic configuration, which properties of their isotopes. The knowledge about the nuclear defines its chemical properties. Strictly speaking, a new element decay properties of transactinide nuclei is by no means is assigned its proper place only after its chemical properties complete, and a great number of them have not even been have been sufficiently investigated. In some cases it has been discovered. The field of heavy element chemistry is therefore possible to experimentally investigate chemical properties of closely related to low energy nuclear physics. Actually, a transactinide elements and even synthesize simple compounds. chemical separation can be regarded as a somewhat slow (in Due to the predicted strong influence of relativistic effects, the terms of nuclear physics) but powerful Z separator. As will be experimental investigation of superheavy elements is especially discussed, the discovery of new elements and the identification fascinating. of new nuclides were not always straightforward. Especially the The study of the chemical properties of the heaviest known correct assignment of mass numbers proved difficult, since elements in the Periodic Table is an extremely challenging task nuclear isomeric states are frequent in the heaviest nuclides and and requires the development of unique methods but also the the experimental determination of masses has not progressed persistence to continuously improve all the processes and yet into the region of transactinide nuclides. components involved in order to achieve the ultimate goal of In the current article, recent advances in the synthesis, chemically isolating one single atom that lives for only a few chemical characterization, and theoretical studies of the heaviest seconds. At first sight, the study of the chemical properties of elements will be reviewed. Recent reviews on the topic were 18,19 the heaviest elements appears to be of purely academic interest. published by Schadel̈ and in a special edition of 20−28 Indeed, as of today, it is not conceivable that weighable Radiochimica Acta edited by Kratz. Books include The 29 quantities of any transactinide element will be produced in the Chemistry of Superheavy Elements and book sections by 30 31 32 near future, and their immediate practical use appears Münzenberg et al., Hoffman et al., and Kratz. Previous questionable. Nevertheless, chemical studies of the heaviest reviews summarize the theoretical chemistry of the heaviest 25,33−41 elements open up possibilities for a deeper insight into the elements. regularities of the Mendeleev Periodic System. Recent experi- ments have demonstrated that the chemical properties of the 2. HISTORY OF THE DISCOVERY OF THE heaviest elements can no longer be predicted from simple TRANSURANIUM ELEMENTS extrapolations of the regularities in the groups and periods of the Periodic Table. 2.1. Actinides Due to the low production rates of the heaviest elements, It is worthwhile to shed some light on the history of discovery chemical information obtained from experiments is limited to of new elements.42 Until World War II, the heaviest known the knowledge of very few properties. It mainly answers the elements, Th, Pa, and U, were thought to be homologs of Hf, question whether a new element behaves similarly to its lighter Ta, and W, respectively, and thus members of groups 4, 5, and congeners in a chemical , or whether some deviations 6. With the discovery of (Np, Z = 93)43,44 in 1940 from the trends occur due to very strong relativistic effects on and its chemical resemblance to U, and the subsequent its electron shells. Thus, in this area of the Periodic discovery of (Pu, Z = 94),45,46 serious doubts about Table, theory starts to play an extremely important role and is the placement of these elements in the Periodic Table surfaced. often the only source of useful chemical information. For The new elements were thought to be members of a new example, electronic configurations can only be calculated at the “uranide” series. A further significant change to the Periodic moment. Properties such as a chemical composition, stability Table came in 1944 with the recognition by Seaborg47 that and geometrical configuration, ionization potential (IP), actually all the elements heavier than were misplaced electron affinity (EA), etc. can also be obtained only and that this series was the series and not the uranide theoretically. Theoretical studies are also invaluable in series, resulting from the filling of the 5f shell. This concept had predicting and/or interpreting the outcome of sophisticated great predictive value and was instrumental in the discovery of and expensive experiments with single atoms. Moreover, it is many new actinide and transactinide elements.

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It is astounding that the rather innocent and misled (due to of the nucleus. This effect, generally referred to as or the wrong placement of Th, Pa, and U as members of groups 4, actinide contraction, is a consequence of the addition of 5, and 6 in the Periodic Table) search for transuranium successive electrons to the inner f shell. The diminishing elements by Fermi, and Hahn, Meitner, and Strassmann, shielding of the increasing nuclear charge by the f electrons ultimately started a spiral of monumental discoveries, which causes a contraction of the valence shells with each additional had a tremendous impact on humanity. Instead of identifying a proton in the nucleus and, hence, a decrease in the atomic or new element, Hahn, Meitner, and Strassmann discovered the ionic radius. nuclear fission process late in 1938.48 The spiral took its first The discovery of the last two actinide elements, turn with an experiment by McMillan, who studied the neutron (No, Z = 102) and (Lr, Z = 103), was not as induced fission process of 238U. Unexpectedly, he observed a straightforward as the discovery of elements Np through Md radionuclide with t1/2 = 2.3 days which did not recoil from the and involved scientists and institutions outside the United thin target.43 This radioactivity turned out to be the States. The name nobelium for element 102 was suggested by a new element Np.44 Surprisingly, the chemical behavior of Np team of scientists working at the Nobel Institute in Stockholm was very similar to that of U and not at all similar to that of a who claimed to have discovered the new element. This claim, group-7 element.49 The discovery of Np led to the subsequent however, has been proven wrong. Nevertheless, the element discovery of Pu. The β¯ decaying 238Np was produced by the name has been retained by the IUPAC and will remain in the bombardment of 238U with deuterons in 1941.45,46 Only a few Periodic Table. The discovery of the last actinide elements years after the synthesis of a few atoms of Pu, kilograms of Pu No54,55 and Lr56 bytheBerkeleyLaboratorywasnot were produced to build the first nuclear device using Pu as uncontested, and the discovery of new chemical elements fissile material. The elements (Cm, Z = 96) and became part of the cold war between the United States and the (Am, Z = 95) (in this sequence) were discovered at Soviet Union. In this context, the name of G. N. Flerov at the the Chicago Metallurgical Laboratory by irradiation of 239Pu now named Flerov Laboratory of Nuclear Reactions (FLNR) at 47 with α-particles and neutrons, respectively. The bombard- the Joint Institute for Nuclear Research (JINR) in Dubna, ment with α-particles took place in Berkeley at the 60-in. Russia, has to be mentioned. A paper by Flerov et al. “History cyclotron, after which the material was shipped to Chicago for of the transfermium elements Z = 101, 102, 103” presents the chemical processing. Shortly after the end of World War II, view by the involved Dubna scientists about the priority of 50 51 (Bk, Z = 97) and (Cf, Z = 98) were discovery of elements 101, 102, and 103.57 then discovered in Berkeley by irradiation of Am and Cm with 2.2. Transactinides α-particles. For the discovery of the actinide series and the synthesis of all the transuranium elements up to Cf, Seaborg All isotopes of the transactinide elements with atomic number and McMillan were awarded the Nobel price in 1951. Z ≥ 104 have artificially been synthesized in heavy ion fusion Again, very unexpectedly, the discovery of fission, which led reactions at accelerators and have only been studied in one to nuclear weapons and to nuclear energy, led to the discovery atom-at-a-time experiments. The discovery experiments of rapid multiple neutron capture in the explosion of a employed newly developed methods that relied on physics thermonuclear device and the isolation of the new elements rather than chemistry to identify a new element. The (Es, Z = 99) and (Fm, Z = 100) from the controversy on the priority of discovery between Berkeley debris in 1952.52 Transplutonium elements are being produced and Dubna also involved the first two transactinide elements, nowadays in weighable quantities, from kilograms of 241Am to a rutherfordium (Rf, Z = 104) and dubnium (Db, Z = 105). A few picograms of 257Fm in high flux reactors through successive detailed analysis of all the experiments made in Dubna and in neutron capture reactions.42 Berkeley was presented by Hyde et al.58 The authors concluded The synthesis and subsequent identification of that priority of discovery should be awarded to the work of the (Md, Z = 101) in 1955 marked the beginning of a new era,53 Berkeley team for both elements. A different view was since this was the first experiment where a new element was presented by Flerov and Ter-Akopyan from Dubna.59 Very produced on a “one atom at a time” level. Also, the recoil important points in the chain of evidence of the Russian claim − technique was invented, which takes advantage of the fact that, were the chemistry experiments conducted by Zvara et al.60 67 in the fusion process of the heavy ion beam with the target The fierce competition between Berkeley and Dubna, nucleus, enough momentum is transferred to the compound “frequently punctuated by acerbic comments” as noted by nucleus to eject it from the layer of target atoms. This way, the Seaborg,42 obscured to a great extent the advances that were reaction products are already separated from the target atoms. made in Berkeley and in Dubna in tackling the enormous In the discovery experiment of Md, only 109 target atoms of difficulties to produce and study few single atoms of very short- 253Es were bombarded with α-particles.53 In present day lived nuclides. In Berkeley, the development of the gas-jet experiments, targets with thicknesses of typically 1018 target technique and of sophisticated counting equipment allowed the atoms per cm2, such as 244Pu, 248Cm, 249Bk, and 249Cf, are used. registration of successive mother−daughter α-particle decay The availability of even heavier target nuclides, such as 250Cf, chains originating from the same sample, displaying the correct 251Cf, or 254Es, still is very limited. The high neutron flux from, sequence of decay energies and time differences. Thus, the for example, 252Cf presents enormous technical difficulties in genetic relationship could be established unequivocally. In handling milligram quantities of this material. At Md, an era Dubna, the development of ultrafast gas chemical separations ended where chemical identification of a new element played a was instrumental for later chemical studies of transactinide dominant role. The position at which a newly created heavy elements in the gas phase. actinide element was eluted from a cation-exchange (CIX) Also the discovery of seaborgium (Sg, Z = 106) was not column was indicative of its ionic radius and thus also of its uncontested. Almost simultaneously, scientists from Berkeley atomic number. Analogous to the , the radii of the and Dubna announced the discovery of element 106. In M3+ and M4+ ions are decreasing with increasing positive charge Berkeley the reaction 249Cf(18O, 4n)263Sg was used.68 The claim

1240 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review from Berkeley was firmly established by the observation of Table 1. Element Names and Symbols Adopted by IUPAC genetically linked α−α correlations from the decay of 263Sg and for Transfermium Elements with Atomic Numbers 101−118 259Rf. In the work of Oganessian et al.,69 the reaction 207Pb- 54 259 atomic ( Cr, 2n) Sg was utilized. The observation of a spontaneous- no. element name symbol other names suggested or prev in use fi ≅ − ssion (SF) radioactivity with t1/2 4 10 ms could later not be fi 259 260 101 mendelevium Md con rmed for Sg. However, the nuclide Sg decays by fl fi 102 nobelium No erovium joliotium, Jl ssion with about 50% branching and t1/2 = 3.6 ms. It is 103 lawrencium Lr therefore conceivable that the 1n-channel was observed. 69 “ ” 104 rutherfordium Rf kurchatovium, Ku; dubnium, Db Oganessian et al. realized that the so-called hot fusion 105 dubnium Db hahnium, Ha; nielsbohrium, Ns reactions using light projectiles and heavy actinide targets joliotium, Jl would at some point reach extremely low cross sections, since 106 seaborgium Sg rutherfordium, Rf the produced compound nucleus was formed with a high 107 bohrium Bh nielsbohrium, Ns excitation energy of the order of 40−50 MeV. The cooling of 108 hassium Hs hahnium, Hn the compound nucleus by the emission of neutrons always 109 meitnerium Mt competed with prompt fission. Therefore, the Dubna team 110 darmstadtium Ds 208 favored the so-called “cold” fusion reaction using Pb as target 111 roentgenium Rg material. Due to the heavier projectiles, the fusion cross section 112 copernicium Cn was diminished significantly, but due to the almost cold 113 ununtrium Uut formation of the compound nucleus, only one or two neutrons 114 flerovium Fl were evaporated. 115 ununpentium Uup The consequent exploitation of cold fusion reactions using 116 livermorium Lv Pb and Bi targets and the detection of genetically linked decay 117 ununseptium Uus chains by the group headed by Armbruster, Münzenberg, and 118 ununoctium Uuo Hofmann led to the discovery of 6 new elements70 at the Gesellschaft für Schwerionenforschung, GSI, in Darmstadt, discoverers and accepted by the IUPAC are also included in Germany. All new elements were discovered using the velocity Table 1. Three decay chains assigned to the nuclide 278113 filter SHIP (Separator for Heavy Ion Reaction Products), which synthesized in the reaction 209Bi(70Zn, 1n) have been reported − separates fusion reaction products in flight from the beam and by the same laboratory.85 87 The experiment required 553 days from transfer reaction products. Since all identified new of accelerator beam time, yielding a record low production α +20 87 elements decayed rapidly by successive -particle emissions cross section of 22−13 fb. to lighter already known nuclei, no ambiguities concerning the All the isotopes of the newly discovered elements were very fi ≈ 261 a identi cation of the new nuclides surfaced. The number of short-lived, and t1/2 decreased from 1 min for Rf to <1 ms emitted α-particles in the chain was indicative of the atomic for 277Cn.80,88 Only a few atoms were produced in month long number of the new nuclide by adding two units in Z for each experiments. With regard to chemical investigations, prospects emitted α-particle to the last securely identified decay product. for experimental chemical studies of elements beyond Sg were The SHIP velocity filter can identify reaction products with rather dim, with the exception of hassium, where, in the α- ≈ −39 2 277 269 production cross sections as low as 100 fb (1 fb = 10 cm ). decay chain of the nuclide Cn, the isotope Hs with t1/2 = With the reported discovery of elements up to Z = 109, the 10 s was observed.80 time had come to authenticate the different claims and to come Largely unnoticed by the community of chemists, the past 15 to conclusions about priority of discovery so that official years did bring another substantial extension of the Periodic element names could finally be adopted. In 1985 IUPAP and Table by six new elements through a series of physics IUPAC decided to establish a Transfermium Working Group experiments at the FLNR in Dubna, Russia. By using the (TWG) to consider questions of priority in the discovery of very tightly bound, doubly magic nucleus 48Ca and actinide elements with nuclear charge number Z > 100. The TWG target nuclei, Oganessian and co-workers89 were able to issued its report16 in 1992. The contents of the report were synthesize single atoms of elements 113 through 118 and accepted, in general, by the Dubna71 and Darmstadt72 observe their radioactive decay. So far, only elements 114 and laboratories, but heavily criticized by the Berkeley group.73 A 116 have been authenticated by IUPAC,90,91 and the element concise summary of the developments concerning the discovery names flerovium, Fl, and livermorium, Lv, suggested by the of elements 101−111 was published by Greenwood.74 Finally team of discoverers, have recently been made official by in 1997, a compromise was reached.75 The element names and IUPAC.4 symbols adopted in August 1997 by the general assembly of The addition of six new elements in the past decade is IUPAC for elements with atomic numbers 102 through 109 are remarkable in several ways. First, the maximum production listed in Table 1. A new joint working party of IUPAP and cross sections of elements 104−112 could be described rather IUPAC (JWP) took up its work in 1998 to assess the discovery well by an exponential decay law (see section 4, Figure 7), of elements 110−112. In its report,76 the JWP considered the where the cross sections dropped by roughly a factor of 10 work of Hofmann et al.77 as sufficient to claim discovery of when increasing the atomic number by 2 units. In the synthesis element 110, while confirmation by further results was of 277Cn, production cross sections of less than 1 pb (10−36 requested to assign priority of discovery of elements 111 and cm2) were determined. However, by using 48Ca projectiles, this 112. Priority of discovery of element 11178,79 and element trend was broken and rather constant maximum production 11279,80 was assigned to Hofmann et al.81,82 in 2003 and 2009, cross sections of several picobarns were measured for synthesis respectively, especially since in the same reactions both nuclides of elements 112−116, and even for elements 117 and 118, 272111 and 277112 were independently synthesized by Morita et values near or slightly below 1 pb were observed. Nevertheless, al.83,84 at RIKEN in Japan. The names proposed by the even under optimum conditions, a production cross section of

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1 pb translates into the synthesis of only 1 atom of a superheavy numbers” of nucleons, for example, 2, 8, 20, 28, 50, 82, and 126, nuclide every 36 h, on average. Second, of the more than 50 the liquid-drop model underestimated the stability of these new nuclides produced in these experiments, a number of them nuclei significantly. The shell model accounts for these regions “ ” have t1/2 > 1 s and, thus, live long enough for chemical of stability by showing that magic numbers of neutrons and investigations. This result is in strong contrast to those of the protons form stable shells similar to electrons in atoms and previously known, more neutron deficient isotopes of Mt, Ds, based on the same quantum mechanical laws. Nuclear shell Rg, and Cn in the range of a few milliseconds. effects can be so strong that nuclei exist beyond the Figure 2 shows the number of discovered transuranium macroscopic limit imposed by the liquid drop model of fission. elements as a function of time. The synthesis of new elements By introducing the macroscopic−microscopic method, Strutin- sky97 was able to introduce shell correction energies to the liquid-drop model. The prediction of the existence of long-lived ff (i.e., t1/2 > 1 s) super-heavy nuclei has been credited to di erent authors.92,98 Myers and Swiatecki99 first predicted an island of super-heavy elements around Z = 126 and N = 184, which was − later refined to mostly Z = 114 and N = 184.100 104 Modern theoretical descriptions of superheavy nuclei using a purely microscopic approach have been reviewed by Naza- rewicz et al.105 and Cwiok et al.106 These represent self- consistent Hartree−Fock-type calculations, which use effective -dependent interactions of both zero (Skyrme) and finite (Gogny) range, and also the relativistic mean field approach. These calculations tend to locate the center of the island at Z = 114, Z = 120, or Z = 126 while confirming the N = 184 neutron shell. The more traditional macroscopic−micro- scopic methods,21 which include higher orders of nuclear deformation are able to explain the increased stability observed around Z = 108 and N = 162, a region that was much closer to the region of then known nuclei. In Figure 3 the shell Figure 2. Growth of the number of transuranium elements as a function of time. was not a continuous process but occurred in phases. A new technical development or a new concept allowed the discovery of several elements before a limit was reached that could only be overcome by substantial improvements or a new concept. Such a period is evident between 1984 and 1994, where a number of improvements at the SHIP separator and the UNILAC accelerator allowed an increase in sensitivity of about 1 order of magnitude, resulting in the subsequent discovery of another three elements without abandoning the concept of cold fusion using Pb or Bi targets. A change of concept using 48Ca beams and actinide targets allowed the Dubna−Livermore collaboration the discovery of six new elements. Since no heavier actinides than Cf are available in sufficient quantities as target materials, this extremely successful path has been Figure 3. Contour map of calculated ground-state shell correction exhausted and new ideas are required to push past element energies. Figure adapted from ref 107. Copyright 2001 by The 118 and open the eighth period of the Periodic Table. American Physical Society. The locations of the doubly magic nuclei 208Pb, 270Hs, and 298Fl are indicated. The nucleus in the center of the 3. NUCLEAR PROPERTIES new subisland, 270Hs, can be reached, for example, in the reaction 248 26 109 48 The synthesis and observation of long-lived heavy nuclides is a Cm( Mg, 4n) and has recently been synthesized. Ca-induced triumph of the nuclear shell model that has predicted the reactions on actinide targets still fall short of reaching the predicted existence of such nuclei since the 1950s. These hypothetical center of spherical shell closure for superheavy elements around Z = 114. nuclei were called “super-heavy”.92 However, the term had appeared already in 1938 in a review by Quill93 on transuranium elements and was later used also for newly discovered elements.94 The limit of existence of heavy elements correction energies calculated by Sobiczewski et al.107 are is determined by the balance between the repulsive Coulomb shown. For barrel shaped nuclei (hexadecapole deformation) forces of the many protons and the attractive nuclear forces. around Z = 108 and N = 162, the shell-correction energy Immediately after the discovery of nuclear fission,48 Meitner reached almost the same magnitude as for the traditional, and Frisch95 located the limit of existence of heavy nuclei at spherical shell closure for superheavy elements around Z = 114. around Z ≈ 100, where the surface tension in the charged This meant that there exists not an isolated island far removed nuclear droplet model96 can no longer compensate the from the region of fairly well-known nuclides, but a subisland repulsive forces. Later, it was realized that at certain “magic reaching out to the island of superheavies.108

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Figure 4. Cut-out of a chart of nuclides showing all presently known transactinide nuclides (color coding according to the “Karlsruhe” chart of nuclides110).

This discovery had a tremendous impact on the synthesis of 4. NUCLEAR REACTIONS OF COLLIDING NUCLEI new elements, since now the region at or beyond Z = 114 could So far, transactinide elements can only be synthesized artificially be accessed in a step by step procedure, without risking in complete heavy ion fusion reactions. The fusion of two plunging into the “sea of instability”. This is especially 48 nuclei is a very complex process. Although there exist numerous important, since Ca-induced reactions on actinide targets, articles in the literature treating the collision and fusion of two which currently are the only experimentally feasible combina- nuclei, we restrict ourselves here to cite Zagrebaev et al.,111 tions to synthesize neutron-rich superheavy elements, still fall which gives an in-depth analysis of the individual steps short by a large margin of reaching the center of the predicted involving the synthesis of a heavy nucleus. First the repulsive spherical shell closure around Z = 114 and N = 184. Coulomb forces have to be overcome, so that the two colliding Due to the large number of nucleons in superheavy nuclides nuclei get into contact. When two nuclei are made to collide, and the resulting complicated nuclear structures and shapes, several different reaction pathways open, depending on the nuclear isomerism is a common phenomenon. This means that impact parameter of the approaching nuclei and the kinetic a nucleus may exist also in excited states, which are observable energy of the projectile. This is schematically shown in Figure and have measurable half-lives. For example, the observed 5. If the impact parameter is too large or the kinetic energy too decay properties of a nucleus may vary considerably depending low, the projectile is elastically scattered off the target nucleus, on whether it was produced directly in a nuclear reaction, usually with high spin, or as a decay product of a heavier nucleus. Isomeric states of nuclei are denoted with the suffixm (e.g., 211Pom). Often, it is not yet clear which one of the observed states is the ground state and which is the excited one. In this case, the suffix a or b is used to denote these states (e.g., 261Rfa and 261Rfb). Figure 4 displays an updated transactinide cut-out of the chart of nuclides. The color coding is that of the “Karlsruhe” chart of nuclides.110 One can see that the 48Ca induced reactions on actinide targets have led to the discovery of six new elements and their associated α-decay chains, always ending in SF. This region of >50 new nuclides is detached from the older region of previously known nuclei. The fact that there is no overlap of this new region with the region of previously known nuclei created some problems in unambiguously Figure 5. Schematic representation of projectile trajectories depending assigning the atomic number of the synthesized nuclei, since on impact parameter. Reprinted with permission from ref 112. all decay chains ended by SF in a previously uncharted region. Copyright 1997 Clarendon Press.

1243 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review leading to Coulomb excitation of the colliding nuclei. At lower prompt fission. Production cross sections for transactinide impact parameters, the nuclei may get close enough so that the nuclides range from nanobarns (10−33 cm2) to picobarns (10−36 exchange of few nucleons can occur (reaction pathway a). cm2); these very low cross sections have to be compared with These reactions are called quasi-elastic, since the contact time is the reaction channels a−c with cross sections of 1−100 very short and the projectile is scattered off the target similarly millibarns (10−27 cm2). as in elastic scattering. At impact parameters significantly In essence, a substantial part of the chart of nuclides (or the smaller than the sum of both radii, the contact of both nuclei is Periodic Table) is produced in these collisions. Thus, at typical deeper and the attractive nuclear forces come into play to the beam currents of 1012 projectiles/s and target thicknesses of point where the dinuclear system begins to fuse into a single 1018 target atoms/cm2, quickly isolating one single atom from large nucleus. Exchange reactions of many nucleons are called the plethora of reaction products is a formidable task. deep inelastic transfer (reaction pathway b). In rare cases the In Figure 7, experimental production cross sections of heavy two nuclei fuse (reaction pathway c). In most cases the fused and superheavy nuclei for “cold fusion” reactions (1n emission) system decays again in two fragments in a fission-like process. Reaction pathways a−c can be observed experimentally. In Figure 6 the total integrated mass yield of the reaction 288

Figure 7. Experimental cross sections for the formation of nuclei with Z ≥ 102 in (■) the 1n evaporation channel of cold fusion reactions, Figure 6. Total integrated mass yield of the reaction 288 MeV 40Ar + (○) the 5n channel of hot fusion reactions, and (△) the 3−4n channel 238U (dash-dotted line) and its decomposition into individual of warm fusion reactions with 48Ca + actinide targets. The curves are contributions: (a) quasi-elastic processes, (b) deep-inelastic multi- drawn to guide the eye. nucleon transfer, (c) fusion followed by fission, and (d) fission of 113,114 target-like reaction products. . Reprinted with permission from “ ” ref 114. Copyright Wiley VCH Verlag GmbH. and hot fusion reactions (5n emission) are shown as a function of atomic number. The “abnormal” behavior of 48Ca induced production cross sections, which stay rather constant at MeV 40Ar + 238U (dash-dotted line) and its decomposition into the level of a few pb, was associated with stabilizing shell effects, individual contributions is shown.113,114 Quasielastic transfer i.e. increasing fission barrier heights, when approaching the 89 reactions (a) to two narrow distributions with high region of Z = 114. maximum cross section near the masses of the projectile and the target nucleus. Deep-inelastic transfer reactions lead to 5. NUCLEAR EXPERIMENTAL TECHNIQUES wider distributions, where the identity of the projectile or the The synthesis of transactinide elements requires intense heavy target is not completely lost (b). The dashed line above mass ion beams of mostly neutron-rich, low-abundance isotopes and number 240 indicates that most of these transfer reaction often exotic, radioactive actinide target materials (see section products will disintegrate by fission due to the relatively high 7.1.1). The required projectile energies are those close to the excitation energy and angular momentum they are formed with, Coulomb barrier, i.e. about 5 MeV/nucleon. Accelerators used giving rise to the distribution of fission fragments denoted with for transactinide element synthesis are either cyclotrons such as (d). Central collisions lead to complete fusion. The Q value of the U400 at FLNR (Russia), the 88-Inch at LBNL (United the fusion reaction is positive, meaning that, even if the fusion States), the K-130 at JYFL (Finland), or various cyclotrons at occurs at the Coulomb barrier, the compound nucleus is GANIL (France), or linear accelerators such as the UNILAC at formed with an excitation energy between 30 and 50 MeV in GSI (Germany) or the RILAC at RIKEN (Japan). In order to so-called “hot fusion” reactions, i.e. reactions with light provide long-term stable beams for month-long experiments, projectiles (18O, 22Ne, 26Mg, 36S) and actinide targets, and highly efficient ion sources are required, which provide intense 10−20 MeV in “cold fusion” reactions, i.e. reactions with heavy beams with low material consumption (i.e., about 0.5 mg/h) of projectiles and 208Pb or 209Bi targets. In most cases, the formed expensive enriched stable isotopes. These requirements are best compound nucleus disintegrates by fission, giving rise to the met by modern electron cyclotron resonance (ECR) ion broad distribution denoted by (c). Only very rarely (≈10−9)is sources, which are installed at all the above-mentioned facilities. the high excitation energy dissipated by the evaporation of Since evaporation residues are recoiling out of the target particles, mostly neutrons, and by the emission of γ-rays. Each layer with the momentum of the beam, they can be separated in evaporated neutron removes ≈10 MeV of excitation energy, but flight from the projectile beam but also from all other types of each neutron evaporation process is in competition with reaction products (see section 4). Presently, kinematic

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Figure 8. Schematic view of the velocity filter SHIP. Reproduced with permission from ref 22. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH. separators such as velocity, mass, or energy filters and gas-filled decay sequence of a heavy nucleus consists of an implantation separators are in use.89,115,116 The flight time through these signal, followed by one or several α-particle decays. Often the separators is of the order of microseconds. Extremely successful sequence is terminated by SF decay. These events are not only separators in the discovery of new elements were the velocity correlated in time but also in position in the Si-detector. This fi fi lter SHIP at GSI and the Dubna gas- lled recoil separator way, the probability that such a sequence is consisting of purely (DGFRS) at FLNR. SHIP is a vacuum separator and consists of randomly correlated events is greatly diminished. Since the two symmetrically arranged Wien-filters with spatially separated implantation depth is rather shallow, α-particles and fission electrostatic and magnetic dipoles. Quadrupole triplets are installed before and after the velocity filter. An additional 7.5° fragments can escape the detector plane in the backward dipole bends the product beam out of a direct line of sight of (upstream) direction. Such escaping events still leave a the target position, which strongly reduces unwanted back- detectable signal in the focal plane detector, and often the ground. For 48Ca induced reactions, the efficiency of the full energy of these escaping events can be recovered by separator was reported to be about 20%.117 A schematic view of detecting them with side detectors. All these features are shown the velocity filter SHIP is shown in Figure 8. SHIP is required in the enlarged section of Figure 9. Since all events are to operate in vacuum, due to the high electric fields. In contrast, registered and stored in an event-by-event mode, month-long evaporation residues can spatially be separated in a gas-filled experiments create rather large amounts of data that has to be ff dipole magnet, making use of a charge focusing e ect. The analyzed off-line to reveal the presence of few heavy nuclei. In recoiling fusion products change their charge state when traveling through a dilute gas (usually H2 or He), which results in a trajectory through a dipole magnet that can be described by an average charge state, thus defining the magnetic rigidity. The magnetic rigidities of the beam and other reaction products are different so that a separation of evaporation residues can be accomplished. Quadrupole magnets are added to the dipole to focus the evaporation residues into a focal plane detector. A schematic of the DGFRS is shown in Figure 9. The transmission through the separator for 48Ca-induced reactions was estimated to be about 40%.89 One of the most important parts of the separator is the focal plane detector, where the decay of a transactinide nucleus is registered. Typically, the rate of particles reaching the focal plane detector is still few hundred hertz. In order to unambiguously detect decay events of transactinide nuclei, this background must be significantly reduced. This is accomplished in several ways. By adding so-called time-of- flight (TOF) transmission detectors, each particle transiting the separator leaves a signal in the TOF detectors, which can thus be separated from radioactive decay signals of implanted nuclei, which leave no TOF signal. Furthermore, the Si-detectors are divided into position sensitive strips, so each signal Figure 9. Schematic view of the DGFRS. The cut-out shows an (implantation, α-particle, and SF decay) is associated with a enlarged view of the focal plane Si-detector box. Reprinted with position coordinate in the detector. This way, the observed permission from ref 89. Copyright 2007 IOP Publishing Ltd.

1245 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review earlier times, the analysis of the data required a computing the 25% relativistic contraction and 5.8 eV stabilization of the center. 7s AO of Cn. New instruments, such as the gas-filled separator TASCA, The relativistic contraction and stabilization of the nsAO which are currently being used in the search for new elements reach their maximum in the seventh row of the Periodic Table 119 and 120, have a transmission of about 60% for 48Ca- at Cn (Figure 12).37 The shift of the maximum to Cn in the induced reactions on actinide targets.118 As focal plane seventh period in contrast to Au in the sixth period is due to detector, double-sided Si strip detectors are used, which allow the fact that in Rg and Cn the ground state electronic for a position resolution of 1 mm2. The focal plane consists configuration is d9s2 and d10s2, respectively, while the thus of more than 6900 pixels. In addition, new and faster corresponding electronic configurations in the sixth period ≥ 10 1 10 2 electronics is required in order to resolve nuclides with t1/2 1 are Au(d s ) and Hg(d s ). μs. The second (indirect) relativistic effect is the destabilization and expansion of outer d and f orbitals. The relativistic ffi 6. RELATIVISTIC EFFECTS ON CHEMICAL PROPERTIES contraction of the s and p1/2 shells results in a more e cient 6.1. Relativistic Effects on Atomic Electronic Shells screening of the nuclear charge so that the outer orbitals, which never come close to the core, become more expanded and With increasing Z of heavy elements, causing a stronger energetically destabilized. As an example, the expansion and attraction to the core, an electron is moving faster, so that its destabilization of the (n−1)d AOs with Z are shown in Figure mass increase is 11 for group-12 elements. While the direct relativistic effect 21/2 originates in the immediate vicinity of the nucleus, the indirect m =−mvc/[1 ( / ) ] (6.1.1) 0 relativistic effect is influenced by the outer core orbitals. ff − where m0 is the rest mass of the electron, v is the velocity of the The third relativistic e ect is the well-known spin orbit ± 1 electron, and c is the speed of light. The Bohr model for a (SO) splitting of levels with l > 0 (p, d, f, etc.) into j = l /2.It hydrogen-like species gives the following expressions for the also originates from the inner region in the vicinity of the velocity, energy, and orbital radius of an electron nucleus. The SO splitting for the same l decreases with 2 increasing number of subshells; that is, it is much stronger for v = (2πenhZ / ) (6.1.2) inner (core) shells than for outer shells. The SO splitting 24 2 2 2 decreases with increasing l for the same principal quantum EenhmZ=−(2π / ) (6.1.3) − number; that is, the np1/2 np3/2 splitting is larger than the − − 22 nd3/2 nd5/2 and both are larger than the nf5/2 nf7/2 one. This is r = Ze/ mv (6.1.4) explained by the decreasing orbital in the vicinity of where n is the principal quantum number, e is the charge of the the nucleus with increasing l. In transactinide compounds, the electron, and h is Planck’s constant. With increasing Z along the SO coupling becomes similar or even larger in size compared to Periodic Table, the m/m ratio gets larger. For H it is 1.000027. typical bond energies. 0 ff 2 From the sixth period onward, this ratio is exceeded by 10%, so All three e ects change approximately as Z for the valence that relativistic effects cannot be neglected anymore. For shells down a column of the Periodic Table. It was suggested that relativistic effects depend even on higher powers of Z, example, for Fl, m/m0 = 1.79, and for element 118, it is 1.95 123 (see also Pyykkö119 for other examples). The contraction (eq especially for the heaviest elements. ff 6.1.4) and stabilization (eq 6.1.3) of the hydrogen-like s and Breit e ects (accounting for magnetostatic interactions and ff retardation effects to the order of 1/c2) on energies of valence p1/2 electrons is a direct relativistic e ect and was shown to originate from the inner K and L shell regions.120 This effect orbitals and IPs are usually small, e.g., 0.02 eV for element 121, but can be as large as 0.1 eV for transition energies between the was found to also be large for the valence region due to the 124 direct action of the relativistic perturbation operator on the states including f orbitals. They can also reach a few percent 121 fi inner part of the valence density. Figure 10 shows the for the ne structure level splitting in the 7p elements and are ff relativistic contraction of the 7s atomic orbital (AO) of element of the order of correlation e ects there. Δ ⟨ ⟩ ⟨ ⟩ − ⟨ ⟩ ⟨ ⟩ QED such as vacuum polarization and electron self-energy 105, Db, R r ns = r nr r rel/ r nr = 21%. Figure 11 shows are known to be very important for inner-shells,125,126 for example, in accurate calculations of X-ray spectra127,128 for the heaviest elements. For highly charged few electron atoms, they were found to be of similar size as the Breit correction to the electron−electron interaction. It was shown that in the middle range (Z =30−80) both the Breit and Lamb-shift terms for the valence shells behave similarly to the kinetic relativistic effects, scaling as Z2.129 For the higher Z, the increase is even larger. The nuclear volume effect grows even faster with Z. Consequently, for the superheavy elements, its contribution to the orbital energy will be the second most important one after the relativistic contribution. QED corrections for the valence shells in heavy many-electron atoms of elements Rg through Fl, and 118 through 120 calculated using a perturbation theory are given in Thierfelder et al.130 Thus, − Figure 10. Relativistic (solid line) and nonrelativistic (dashed line) for example, QED on the DCB IP of element 120 is 0.013 eV, radial distribution of the 7s valence electrons in Db. Reprinted with while it is 0.023 eV for Cn. For element 118, QED effects on permission from ref 39. Copyright 2003 Kluwer Academic Publishers. the binding energy of the 8s electron cause a 9% reduction

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Figure 11. Relativistic (solid line) and nonrelativistic (dashed line) energies and the maximum of the radial charge density, Rmax, of the valence ns and (n−1)d AOs of group-12 elements25 with data from Desclaux.122 Reprinted with permssion from ref 25. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.

−−13 Brrrij =−1/2[(ααi⃗ j ⃗ ) ij+ ( α i⃗ ij⃗ )( α j⃗ ij⃗ )r ij ] (6.2.3)

The operators of the Dirac eq 6.2.1 are 4 × 4 matrix spinor operators, and the corresponding wave function is therefore a four-component (4c) vector function. The Vn includes the effect of the finite nuclear size, while some finer effects, such as QED, can be added to hDCB perturbatively. The DCB Hamiltonian in this form contains all effects through the second order in α, the fine-structure constant. Correlation effects are taken into account by the configuration interaction (CI), many-body perturbation theory (MBPT) and, presently at the highest level of theory, the coupled cluster single double (and perturbative triple) excitations (CCSD(T)) technique. The Fock−Space (FS) DCB CC method137,138 is presently Figure 12. Relativistic stabilization of the 6s and 7s orbitals in the 6th the most powerful method used for atomic calculations. It has and 7th rows of the Periodic Table.25 Redrawn from Schwerdtfeger et an accuracy of a few hundredths of an electronvolt for al.37 with Dirac−Fock (DF) data from Desclaux.122. Reprinted with excitation energies in heavy elements, since it takes into permission from ref 25. Copyright 2011 Oldenbourg Wissenschafts- account most of the dynamic correlation (states with high l). verlag GmbH. Due to the present limitation of the FS CCSD method in treating electronic configurations with no more than two (0.006 eV) of EA.131 Thus, the QED effects are not negligible: electrons (holes) beyond the closed shell, further developments they are of the order of 1−2% of the kinetic relativistic effects, are underway to remove this limitation.138 Thus, the high- which means that the existing studies of relativistic effects are sectors FSCC code is under development, which will allow for up to 99%129 (or 101%17) correct. treating systems with up to six valence electrons/holes in an 6.2. Current Relativistic Quantum-Chemical Methods open shell. The relativistic Hilbert space CC (HSCC) method The most appropriate quantum chemistry methods for the is also worked on, which could be used for systems with more heaviest elements are those which treat both relativity and than a couple of electrons/holes in the active valence shell. The − correlation at the highest level of theory.132 136 Presently, the mixed sector (MS) CC method will be a generalization of the highest theoretical level for many-body methods for molecules previous two (FSCC and HSCC) and will combine their is the Dirac−Coulomb−Breit (DCB) Hamiltonian advantages. A further improvement is the introduction of the intermediate Hamiltonian (IH). It is a generalization of the ff hhirBDCB =++∑∑D() (1/ij ij ) e ective Hamiltonian (EH) method and serves as a core of i ij< (6.2.1) most multiroot multireference approaches. The standard multireference FSCC and HSCC methods (described above) where the one-electron Dirac operator is are used in the effective Hamiltonian framework. The most 2 n problematic technical problem of the EH method is poor (or hiDi()= cpαβ⃗ ⃗ +−+ c ( 1) Vi () (6.2.2) i i no) convergence of iterations due to the presence of so-called Here, α⃗and β are the four-dimensional Dirac matrices, and Vn intruder states. Recently, many groups developed different is the nuclear attraction operator. The Breit term in the low forms of “intruder-free” intermediate Hamiltonian formulations photon frequency limit is of FSCC and HSCC. These formulations substantially extend

1247 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review the scope and applicability of the multiroot multireference CC were developed for some transactinides as well.157 As methods. The XIH (extrapolated intermediate Hamiltonian) mentioned, there are also ab initio model potentials method is a specific (very efficient) form of IH developed by (AIMP)158 that remove the drawbacks of the nodeless structure the Eliav−Kaldor group.138 of the PPs but have not yet been used for the very heavy For the heaviest elements, the electronic structures of Lr, Rf, systems. and Rg through element 122 were calculated with its use (see PPs are also used for 1D, 2D, and 3D infinite systems further). The DC FS CCSD method incorporated in the (polymers, surfaces, and the bulk). In the solid-state 139 DIRAC package has a slightly lower accuracy than the DCB calculations, PPs are constructed from Kohn−Sham rather FSCC one. It was also applied to various heavy element that Hartree−Fock equations. An overview of this class of the systems. PP methods is given elsewhere.151 A practical instrument for many-electron open-shell systems Relativistic density functional theory (DFT) is very popular fi − 140,141 is the multicon guration Dirac Fock (MCDF) method. in the area of the heaviest elements calculations. Due to the Based on the CI technique, it accounts for most of the high accuracy and efficiency, computational schemes based on ff correlation e ects while retaining a relatively small number of the DFT methods are among the most important in theoretical fi con gurations. It omits, however, dynamic correlation, which chemistry, especially for extended systems, i.e., large molecules, makes it less accurate than the DC(B) CCSD one. Elements Rf liquids, or solids. The modern DFT theory is exact,159 and the through Hs, Cn, and Fl were treated with its use (see further). accuracy depends on the adequate choice of the exchange- In the past, predictions of atomic properties of the heaviest fi correlation potential, Eex. The exact form of the latter is, elements were made using the single-con guration DF method however, unknown. In the past, the simplest local density and the Dirac−Slater (DS) one (see a review by Fricke,34 also 122 approximation, LDA, was used, which laid the basis of the tables of Desclaux ). Atomic calculations for the heaviest Dirac−Slater Discrete Variational (DS-DV) method.160,161 In elements were also performed using other approaches, such as, modern methods, the generalized gradient approximation for example, the relativistic complete active space MCSCF (GGA), also in the relativistic version, RGGA, is used for (CASMCSCF) CI method.142,143 E .162 There are quite a number of GGA potentials, and their QED are presently included perturbatively in the atomic ex choice is dependent on the system. Thus, PBE is usually used calculations on top of the self-consistent-field (SCF) solutions.17,130,131 An attempt to bridge the gap between for physics applications, PBE0, BLYP, B3LYP, B88/P86, etc. for chemical, and LDA for the solid state. The most accurate relativistic quantum chemistry and QED has been undertaken 163 144 noncollinear 4c-DFT methods are that of Anton et al. and by Liu. 164 ff Ab initio DF molecular methods are still at the stage of the Beijing one (BDF). They di er by the basis set development.134,135,145 The problems of electron correlation technique, though they give similar results. and proper basis sets make the use of these methods for the The 2c-DFT methods are a cheaper alternative of the 4c- 165 − heaviest elements very limited. Correlation effects are taken ones. Quasi-relativistic methods such as the spin orbit − zeroth-order regular approximation (SO ZORA),166,167 also into account there via the CI, MBPT (Møller Plesset, MP2), 168 or the CC techniques.146,147 One of the implementations of the implemented in the Amsterdam DFT code (ADF) and the 139 − − 169 DC method is a part of the DIRAC package. The methods Douglas Kroll Hess (DKH) one are also popular among are still too computer time intensive and not sufficiently theoretical chemists. economical to be applied to heaviest element systems in a 6.3. Relativistic Effects and the Future Periodic Table of the routine manner, especially to the complex systems studied Elements experimentally. The place of an element in the Periodic Table is determined by Due to the practical limitations of the 4c methods, the 2c its atomic number Z and its electronic ground state ones became very popular. In this approximation, the positronic configuration. The most complete and systematic Periodic and electronic solutions of the Dirac−Hartree−Fock (DHF) 135,145,148 Table up to Z = 172 is that of Mann, Fricke, Waber, and method are decoupled. This reduces the number of 170−172 interactions in the Hamiltonian to solely those among electrons Greiner, which is based on DF calculations of atomic ground states. DF calculations were also performed by and nuclei and, therefore, the number of computations. − Desclaux122 and Nefedov.173 Later on, MCDF174 182 and Another method of decoupling the large and small components 137,183 of the wave function is the Douglas−Kroll149 transformation, or DCB CC calculations provided more accurate values of − − 150 the energy terms, overall confirming the earlier DF predictions. the Douglas Kroll Hess (DKH) one. fi Effective core potentials (ECP) allow for more economical According to these calculations, lling of the 6d shell takes fi calculations within the DHF schemes by replacing inner place in the rst nine of the transactinide elements, Rf through orbitals, which do not take part in the bond formation, by a Cn, followed by the 7p elements 113 through 118, with special (effective core) potential. In this way, the number of element 118 falling into the group of noble gases. In elements fi basis functions and, therefore, two-electron integrals, is 119 and 120, the lling of the 8s shell takes place, so that these drastically diminished. elements will obviously be homologs of alkali and alkaline earth There are two main types of ECPs, as well as pseudo- elements in group 1 and 2, respectively. The next element, Z = potentials (PP) and model potentials (MP).151 The energy- 121, has a relativistically stabilized 8p electron in its ground adjusted PPs, known as the Stuttgart ones, have been developed state electronic configuration124 in contrast to predictions based for almost all elements of the Periodic Table,152,153 and on a simple extrapolation in the group. In the next element, Z = recently, also including QED effects, for the heaviest elements 122, a 7d electron is added to the ground state, so that it has a up to Z = 118.154 The shape-consistent relativistic ECPs 8s27d8p ground state configuration in contrast to the 7s26d2 (RECP)155 have also been developed for the heaviest state of Th. This is the last element for which accurate DCB elements.156 Generalized RECPs accounting for Breit effects CC calculations exist.184

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For heavier elements, calculations start to disagree on the ground states. The situation there becomes more complicated: 7d, 6f, and 5g levels, and further on 9s, 9p1/2, and 8p3/2 levels are located energetically so close to each other that clear structures of the pure p, d, f, and g blocks are no longer distinguishable. The usual classification on the basis of a simple electronic configuration and the placement of elements in this part of the Periodic Table becomes problematic. Thus, for example according to Fricke et al.,33,34,172 the Periodic Table has a very long eighth period starting from Z = 119 and counting 46 elements, so that the last element of this row is 164, while elements 167 through 172 are 8p3/2 ones (due to the very large SO effects on the 8p AOs) belonging to the ninth period. The 5g shell is being filled in elements 125 through 144. Elements 165 and 166 are then 9s ones belonging to group 1 and 2, respectively. Figure 13. Modern Periodic Table of the Elements. Seaborg and Keller have designed a different Periodic Table,185 even though they used the same DF calculations of Waber, Fricke, and Greiner.33,34,172 In their table, elements of detection of the nuclear decay require the development of the eighth period are from Z = 119 through 168, including unique separation methods. As already discussed in section 2, those from Z = 122 through 153, called superactinides. In the discovery of new elements up to Z = 101 was accomplished contrast to the results of Fricke et al.,33,34,172 the 8p elements by chemical means. Only from there on did physical methods are those from Z = 163 to 168, and the 9s elements are those prevail. Nevertheless, rapid gas-phase chemistry played an important role in the claim to discovery of elements 104 and from Z = 169 to 170. However, such an arrangement of the 58 elements is not reflecting the filling of the AOs obtained in the 105. In the following, a review of the instrumentation for original DF calculations.33,34,172 chemical separation procedures for transactinide elements will be given. Extensive reviews, covering instrumentation for single In a recent work based on MCDF (with average level, AL, 18,187 energy functional) calculations of highly charged states of some atom chemistry, can be found in the literature. elements of the eighth period, it was suggested that elements of 7.1. Prerequisites for a Chemical Isolation of Heaviest the 5g series are those from Z = 121 to Z = 138.182 Elements Elements

139 and 140 are assigned then to groups 13 and 14, Due to the very low production rates and short t1/2, respectively, denoting that they are 8p1/2 elements, while transactinide nuclei must be chemically processed “one-atom- those from Z = 141 to 155 are 6f elements. The eighth period at-a-time”188 on a very short time scale. A chemistry experiment finishes then at element 172. Thus, the Periodic Table of with a transactinide element can be divided into three basic 182 33,34,172 Pyykkö looks quite different from that of Fricke et al. steps: first, synthesis of the transactinide nuclide; second, rapid One should, however, note that it is difficult to decide about transport of the synthesized nuclide to the chemical apparatus; ground states of the elements on the basis of their highly third, fast chemical isolation of the desired nuclide, preparation charged states, so that those assignments are rather tentative. of a sample suitable for nuclear spectroscopy, and detection of Attempts to define ground states of these elements on the the nuclide via its characteristic nuclear decay properties. example of Z = 140 using the latest version of the MCDF 7.1.1. Synthesis of Heaviest Elements. The synthesis of 17 method (with the optimal level, OL, energy functional) failed. heavy and superheavy elements requires technologically The authors came to the conclusion that such calculations are advanced setups in order to allow the irradiation of exotic, presently restricted by computer limitations. It was also stated highly radioactive target nuclides such as 244Pu, 243Am, 248Cm, that, at the present level of the MCDF theory, which takes into 249Bk, 249,250Cf, or even 254Es with high intensity heavy ion ff account QED e ects, the Periodic Table ends at Z = 173, since beams such as 18O, 22Ne, 26Mg, 36S, 48Ca, or 50Ti. On the one − 2 the energy of the 1s electron dives below 2mec , and it is not hand, as intense beams as possible are to be used; on the other ff − a ected by the approximation used to evaluate the electron hand, the destruction of the very valuable and highly radioactive electron interaction. Diving into the negative continuum is also 186 targets has to be avoided. Production rates are proportional to discussed by Greiner et al. both the target thickness and the beam intensity. The rare Thus, at the modern level of relativistic electronic structure target nuclides are produced at high flux nuclear reactors such fi theory, the problem of de ning ground states of elements as the HIFR at Oak Ridge, United States, or the SM in heavier than 122 remains. Very accurate correlated calculations Dimitrovgrad, Russia, by successive neutron capture reactions ff of the ground states with inclusion of QED e ects at the SCF on Cm and Am starting materials. The valuable target nuclides level are needed in order to reliably predict the future shape of are chemically isolated in heavily shielded hot cells and have a the Periodic Table. At the time of writing, a more logical commercial value >100,000 USD/mg. From the isolated heavy version of the table is that shown in Figure 13, which is based 33,34,172 actinide nuclides, targets suitable for heavy ion irradiations have on the original DF calculations. to be prepared. For nuclear reactions involving the lighter projectiles, target thickness is limiting the compound nucleus 7. CHEMICAL TECHNIQUES TO INVESTIGATE recoil ranges (<1 mg/cm2), because the separation techniques TRANSACTINIDES used require the compound nucleus products to recoil out of The difficulties involved in the production and rapid chemical the target. With the higher-Z projectiles, the recoil ranges are isolation of a few single atoms of a transactinide element from larger, but because of the higher rate of projectile energy loss in numerous other reaction products and the subsequent the target material and the narrow projectile energy range

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Figure 14. Target segment of freshly deposited 249Bk on Ti backing. The target was prepared at Mainz University for a search experiment for the synthesis of element 119 in the reaction 50Ti + 249Bk at the gas-filled separator TASCA at GSI. The 249Bk target material was provided by Oak Ridge National Laboratory (photograph courtesy of Mainz University).

Figure 15. Left-hand side: cartoon of the water-cooled collimator−target−recoil chamber−beam stop assembly as used for experiments at − FLNR190 195 (red arrow: incoming beam). Right-hand side: Results of a finite element calculation of the thermal load in the Ti vacuum window. The scale indicates 2 cm in increments of 5 mm. effective for heavy element production, the effective target showed a radioactive decay rate of nearly 1012 Bq thicknesses are once again limited to ∼1 mg/cm2. These (disintegrations per second). thicknesses are too thin to allow the production of free- The projectile beam loses energy upon passing through the standing target foils. Therefore, thin backing foils are used on target backing and the target, resulting in deposition of heat in which the target materials are deposited as thin uniform layers. the target. The heat generated must be removed to prevent As backing materials, foils made of Be, Al, Ti, or Havar (a high damage to the target. To allow the highest beam intensities, strength nonmagnetic alloy) have been employed. Passage of highly efficient target cooling is necessary. For stationary the beams through the targets produces large amounts of heat, targets, this is usually accomplished by double-window systems which must be dissipated. In addition, the target material must and forced gas cooling (see upper part in Figure 19). In the be chemically stable in the highly ionizing environment created double window system, a vacuum isolation foil is placed by passage of the beam. In the majority of cases, the targets are upstream of the target backing foil (with the target material on being produced by the molecular plating technique.189 There, the downstream side of the backing foil). Cooling gas at the actinide compound, typically the nitrate, is dissolved in a pressures near 1 bar is forced at high velocity through a narrow small volume (5−10 μL) of and the aqueous phase is gap between the vacuum isolation foil and the target backing mixed with a surplus of an organic solvent (∼14 mL), usually foil, cooling both foils. Both the vacuum isolation foil and the isopropanol or isobutanol. A high voltage is applied between target backing foil must hold a pressure difference of greater the solution and the target backing foil. Under these conditions, than 1 bar. Because of the mechanical stresses associated with no electrolytic dissociation of the organic solvent occurs by the pressure differences across the foils, target areas have been applying an electric current. After molecular plating, the foils limited to <1 cm2. Heating of the target by passage of the beam are heated to ∼500 °C to complete the conversion of the is inversely proportional to the cross-sectional area of the beam; actinide materials to the . An example of a freshly therefore, spreading the beam over a larger area decreases the prepared target segment of a rotating 249Bk target of ∼500 μg/ thermal stress. This can be accomplished in two ways, either cm2 thickness on 2.2 μm Ti backing foil is shown in Figure 14. the target foil is placed between two supporting honeycomb The assembled target wheel consisting of four segments grid structures (see Figure 15), or the target is being rotated,

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Figure 16. Recoil transfer chamber installed at the BGS focal plane (left), and the support grid for the Mylar foil (right). Reprinted with permission from ref 187. Copyright 2003 Kluwer Academic Publishers. thus effectively spreading the beam heating over a larger target 106 particles/cm3 could be generated. The same technique area. could be applied to produce MoO3 aerosols. aerosol Rotating 248Cm targets have been used in heavy element particles of similar dimensions were generated by spark chemistry experiments at the GSI.189 The rotating target and discharge between two carbon electrodes. Transport efficiencies vacuum window assembly ARTESIA196 has allowed the use of of 50%−80% have routinely been achieved for transport modern high-intensity accelerated beams for heavy element capillary lengths over 20 m. However, the transport yields are chemistry experiments.197 heavily affected by the applied beam intensity.198 The passage 7.1.2. Rapid Transport. When a compound nucleus is of the beam through the gas in the recoil chamber creates a formed in the reaction of a projectile beam with a target plasma, and the harsh ionizing conditions significantly affect the nucleus, the compound nucleus is formed with the momentum mean mobility diameter and the number concentration of of the beam particle. If the target layer is thin enough, this particles and thus negatively affect the transport efficiency. recoil momentum is sufficient to eject the compound nucleus Furthermore, the transport by aerosols is unspecific. Trans- from the target. These recoiling reaction products can, in actinide atoms are produced at extremely low rates: atoms per principle, be stopped in a foil placed directly downstream minute for Rf and Db, down to atoms per day or week for of the target. However, with the recoil catcher foil technique, heavier elements. They are produced among much larger the time required for removal of the foil and dissolution and amounts of “background” activities (see section 4), which separation of the foil material results in chemical separation hinder the detection and identification of the decay of times longer than a few minutes. Therefore, this technique is transactinide atoms. For these reasons, there is a recognized not suitable for transactinide chemistry experiments. To achieve need for a physical preseparation of the transactinide atoms faster chemical separation times, the aerosol gas-jet transport before chemical separation. It was shown that kinematic technique has been used to deliver transactinide isotopes from separators, such as the BGS, DFGRS, GARIS, or TASCA, the target chamber to various chemical separation devices. could be coupled to a transactinide chemistry system with an − Transport times on the order of one second have been aerosol gas-jet device.28,199 205 Except for the most asymmetric achieved. The principles behind the aerosol gas-jet transport synthesis reactions, the recoil energy of the fusion reaction technique have been presented by Wollnik.198 Products of products imparted by the momentum of the beam is sufficient nuclear reactions recoiling out of a thin target are stopped in a to allow the separated nuclei to pass through a thin Mylar gas at a pressure usually above 1 bar. To allow for relatively window, which separates the low pressure in the separator (i.e., long recoil ranges, He gas is usually used. The He gas is seeded 1 mbar or vacuum) from the stopping gas cell (i.e., 1 bar). The with aerosol particles. After stopping in the gas, nonvolatile thin windows are supported by honeycomb grids, which, reaction products diffuse in the gas and attach themselves to the however, diminish the transmission (see Figure 16). The use of next available surface, which is provided by the aerosol particles. kinematic recoil separators as preseparator for chemistry By choosing appropriate particle sizes of about 100 nm experiments undoubtedly offers a number of advantages, but diameter, the aerosol particles can be transported rapidly and also some drawbacks have to be mentioned. Depending on the essentially without loss through long capillaries. In order to nuclear reaction, the transmission through the separator ranges limit the pressure in the recoil chamber, vacuum is applied to from about 10% to 60%, not including the losses of the window the downstream end of the capillary. At the end of the capillary, supporting grid. The useful target thicknesses (approximately the aerosol particles can be filtered off or collected on a foil by 500 μg/cm2) in recoil separators are only about half the impaction. The collected aerosol particles, containing the thickness of targets used without preseparation, which can be nuclear reaction products, can be made rapidly available for compensated to some extent with higher acceptable beam chemical separation. Many materials have been used to intensities. generate aerosol particles. They can be specifically chosen to Obviously, transactinide elements that are volatile in their minimize interference with the chemical separation being elemental state or can be converted easily to volatile molecules conducted. Widely used were KCl aerosols which can easily be do not need to be transported by the addition of aerosol generated by sublimation of KCl from a porcelain boat within a particles. For studies of Hs, Cn, and Fl, a new device named tube furnace. By choosing a temperature between 650 and 670 IVO (in situ volatilization and online detection) was ° 206 C, specially tailored aerosol particles with a mean mobility developed. By adding O2 to the He carrier gas, volatile diameter of about 100 nm and number concentrations of a few tetroxides of group-8 elements were formed in situ in the recoil

1251 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review chamber. A quartz column containing a quartz wool plug chemical procedures often work discontinuously, the chemical heated to 600 °C was mounted as close as possible to the recoil separation has to be repeated with a high repetition rate. Thus, chamber. The hot quartz wool served as a filter for aerosol thousands of experiments have to be performed. This inevitably particles and provided a surface to complete the oxidation led to the construction of highly automated chemistry setups. reaction. However, as with aerosol particles, the harsh ionizing Due to the fact that the studied transactinide elements as well conditions created by the passage of the beam through the as the interfering contaminants are radioactive and decay with a recoil chamber, do not allow the in situ synthesis of fragile certain t1/2, also continuously operating chromatography compounds, such as organometallic compounds.28,204,207 Again, systems were developed. this drawback can be circumvented by coupling an IVO type Detection: the unambiguous detection of the separated atom device not directly to the target station but behind a kinematic is the most essential part of the whole experiment. Even though recoil separator (see section 5). some techniques have reached the sensitivity to manipulate 7.1.3. Chemical Isolation, Sample Preparation, and single atoms or molecules, the detection of the characteristic Detection. The choice of the chemical separation system has nuclear decay signature of transactinide nuclei remains to be based on a number of prerequisites that have to be currently the only possibility to unambiguously detect the fulfilled simultaneously to reach the required sensitivity to presence of a transactinide element after chemical separation. chemically isolate a transactinide element. Thus, final samples must be suitable for high resolution α- Speed: due to the short t1/2 of even the longest-lived particle and SF-spectroscopy (coincident detection of both SF currently known transactinide nuclides, the time required fragments). Most transactinide nuclei show characteristic decay between the production of a nuclide and the start of the chains that involve the emission of α-particles or the SF of measurement is one of the main factors determining the overall daughter nuclei. The detection of such correlated decay chains yield. In contrast to physical separators, chemical instrumenta- requires the event-by-event recording of the data. In experi- tion currently only allows the investigation of transactinide ments with physical separators, the use of position sensitive ≈ nuclides with t1/2 1 s or longer. detectors further enhanced the discrimination against randomly Selectivity: due to the low production cross sections, ranging correlated events. from the level of a few nanobarns for the production of nuclides Speciation: due to the fact that transactinide nuclei are of elements Rf and Db, down to the level of a few picobarns or detected after chemical separation via their nuclear decay, the even femtobarns for the production of elements with Z > 108, speciation cannot be determined. Currently, the speciation in the selectivity of the chemical procedure for the specific all transactinide chemistry experiments has to be inferred by element must be very high. Two groups of elements are of carefully studying the behavior of lighter homolog elements. major concern as contaminants: due to the fact that many Po The chemical system must be chosen in such a manner that a α isotopes have similar t1/2 and/or -decay energies as trans- certain chemical state is probable and stabilized by the chemical actinide elements, the separation from these nuclides must be environment. particularly good. Some short-lived Po isotopes are observed as The chemistry of the early transactinide elements has been daughter nuclides of Pb or Bi precursors, also some At isotopes studied in the aqueous as well as in the gaseous phase. are of concern. All these elements are formed in multinucleon Techniques of varying complexity have been used: from the so- transfer reactions with Pb impurities in the target material and/ called “SRAFAP” technique209 (students running as fast as or the target assembly. Here, a chemical purification of the possible), consisting of simple manual separations that were actinide target materials and a careful selection of the materials repeated as often as possible to fully robotized setups allowing used in the target assembly can already reduce the production thousands of separations. Also, continuously working arrange- of unwanted nuclides by orders of magnitude. A second group ments were highly successful. of elements, which interfere with the detection of transactinide 7.2. Gas-Phase Chemistry: Isothermal Chromatography nuclei, are heavy actinides that decay by SF. These are and Thermochromatography inevitably produced with comparably large cross sections in multinucleon transfer reactions. The separation from heavy Despite the fact that only few inorganic compounds of the actinides must be particularly good if SF is the only registered transition elements exist, that are appreciably volatile below an decay mode and if no other information, such as t1/2 of the experimentally still easily manageable temperature of about nuclide can be derived from the measurement. In order to 1000 °C, gas-phase chemical separations played and still play an enhance the quality of separation from undesired reaction important role in chemical investigations of transactinide products, increasingly, kinematic separator systems are elements. A number of prerequisites that need to be fulfilled employed as preseparators in addition to chemical procedures. simultaneously to accomplish a successful chemical experiment Single atom chemistry: due to the very low production rates, with a transactinide element are almost ideally met by gas transactinide nuclei must be chemically processed on a “one- chromatography of volatile inorganic compounds. Since the atom-at-a-time” scale. Thus, the classical derivation of the law synthesis of transactinide nuclei usually implies a thermalization of mass action is no longer valid. Guillaumont et al.208 have of the reaction products in a gas volume, a recoil chamber can derived an expression equivalent of the law of mass action in be connected with a capillary directly to a gas chromatographic which concentrations are replaced by probabilities of finding system. Gas phase separation procedures are fast and efficient the species in a given state and a given phase. The consequence and can be performed continuously, which is highly desirable in for single atom chemistry is that the studied atom must be order to achieve high overall yields. Finally, nearly weightless subjected to a repetitive partition experiment to ensure a samples can be prepared on thin foils, which allow α-particle- statistically significant behavior. Here, chromatography experi- and SF-spectroscopy of the separated products with good ments are preferred. energy resolution and in high, nearly 4π, detection geometry. Repetition: Since the moment in time at which a single For the experimental investigation of volatile transactinide transactinide atom is synthesized cannot be determined and elements or compounds, two different types of gas chromato-

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Figure 17. Upper panel: temperature profiles employed in thermochromatography and isothermal chromatography. Lower panel: deposition peak and integral chromatogram resulting from thermochromatography and isothermal chromatography, respectively. Reprinted with permission from ref 187. Copyright 2003 Kluwer Academic Publishers.

Figure 18. Schematic of the gas chromatography arrangement used to chemically isolate Rf in the form of volatile chlorides. Reprinted with permission from ref 58. Copyright 1987 Oldenbourg Wissenschaftsverlag GmbH. graphic separations have been developed: isothermal chroma- The T50% temperature depends on various experimental tography (IC) and thermochromatography (TC). Sometimes, parameters. It can be shown that, for similar gas flow rates ≈ also combinations of the two have been applied. The basic and column dimensions, Ta T50%. By varying the isothermal principles of IC and TC are explained in Figure 17. temperature, an integral chromatogram is obtained. The yield 7.2.1. Isothermal Chromatography. In IC, a carrier gas is of the species at the exit of the column changes within a short flowing through a chromatography column of constant, interval of isothermal temperatures from zero to maximum isothermal temperature. Open or filled columns can be yield. A variant of IC using long-lived radionuclides is employed. Depending on the temperature and on the enthalpy temperature programmed chromatography. The yield of Δ 0(T) ff of adsorption, Ha , of the species on the column surface, the di erent species at the exit is measured as a function of the species travel slower through the length of the column than the continuously, but isothermally, increasing tempera- − carrier gas. This retention time can be determined either by ture.211,217 220 Online IC is ideally suited to rapidly and injecting a short pulse of the species into the carrier gas and continuously separate short-lived radionuclides in the form of measuring the time at which it emerges through the exit of the volatile species from less volatile ones. Since volatile species column210,211 or by continuously introducing a short-lived rapidly emerge at the exit of the column, they can be condensed nuclide into the column and detecting the fraction of nuclides and assayed with nuclear spectroscopic methods. Less volatile − that have decayed at the exit of the column.212 216 With this species are retained much longer, and the radionuclides respect, IC is a variant of frontal chromatography (FC), where eventually decay inside the column. A disadvantage of IC Δ 0(T) the sample is continuously injected into the column and a concerns the determination of Ha of transactinide nuclei on fi breakthrough pro le is measured at the exit. A characteristic the column surface. In order to determine the T50% temper- quantity is the temperature at which half of the introduced ature, a measurement sufficiently above and below this nuclides are detected at the exit (T50%). In this case, the temperature is required. Since for transactinide elements this retention time in the column is equal to t1/2 of the introduced temperature is a priori unknown, several measurements at nuclide, which is thus used as an internal clock of the system. different isothermal temperatures must be performed, which

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Figure 19. Schematic of OLGA(II) in combination with the tape detection system or the MG or ROMA wheel detection system.225 See text for details. Reprinted with permission from ref 225. Copyright 1992 Springer Science and Business Media.

62 means that long measurements are required below the T50% as carriers. It was found that carriers with a vapor pressure temperature that demonstrate that the transactinide compound similar to that of the investigated compound yielded optimum is retained long enough that most of the nuclei decayed in the transfer efficiencies.221 The addition of carriers was essential, column. Such an approach is very time-consuming. Further- since in this manner the most reactive adsorption sites could be more, it must be demonstrated that the experiment was passivated. Volatile reaction products were flushed into the performing as expected and the nonobservation of transactinide chromatographic section (section III), which consisted of a 4 m nuclei was not due to a malfunctioning of the apparatus. long column with an inner diameter of 3.5 mm. This section A schematic drawing of the chemical apparatus constructed consisted of an outer steel column into which tubular inserts of for the first chemical isolation of element 104 in Dubna is various materials (Teflon, glass) could be inserted.62 Section IV shown in Figure 18.62 In section I, a 242Pu target was consisted of a filter. This filter had a jacket into bombarded at the inner beam of the U-300 cyclotron at which, as a rule, crushed column material from section III was Dubna with 22Ne ions. The target was held between two plates filled.62 This filter was intended to trap large aggregate made of an aluminum alloy, into which a number of closely particles.222 Aerosol particles were apparently formed by the spaced holes were drilled. Both sides of the target were flushed interaction of the chloride vapors with present in the with at 250 or 300 °C. In the outer housing of the nitrogen carrier gas. Volatile products passing the filter in target chamber, a number of closely spaced holes were drilled, section IV now entered the detector in section V. This detector which matched the holes of the target holder. An Al foil served consisted of a narrow channel of mica plates, a silicate solid as vacuum window. The whole target was heated with a state detector for registering latent SF tracks of the SF decay of heater. The space behind the target was limited with an Al foil. a Rf nuclide. The mica plates were removed after completion of Reaction products recoiling from the target were thermalized in the experiment, etched, and analyzed for tracks. The filter in fl a rapidly owing stream of N2 and transferred to section II, section VI served for the chemisorption of Hf nuclides in where the chlorination of the reaction products took place. The ancillary work with long-lived nuclides. In section VII the transfer efficiency was >75% and the transfer time only about chloride carriers were condensed. −2 10 s. In the reaction chamber of section II, vapors of NbCl5 The whole apparatus has been built to chemically identify an and ZrCl4 were continuously added as chlorinating agents and isotope of Rf decaying by SF with t1/2 = 0.3 s, that has

1254 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review previously been synthesized and identified by a team of much better. OLGA(III) has very successfully been applied to physicists at Dubna. In a number of experiments, Zvara and co- study volatile halides and/or oxyhalides of Rf,230 Db,231 − workers identified multiple SF tracks in the mica detectors Sg,232 234 and Bh.235 In all these experiments the separated when they used glass surfaces and temperatures of 300 °C.62 transactinide nuclides were unambiguously identified via their They had shown in preparatory experiments with Hf that nuclear decay properties. An improved version of OLGA(I), indeed the transfer of Hf through the apparatus occurred within named HITGAS (high-temperature online gas chromatography less than 0.3 s, and, thus, that the experimental setup was suited apparatus), has been developed at Forschungszentrum to study the short-lived Rf isotope.221 The distribution of SF Rossendorf, Germany, and successfully applied to study 236−239 tracks along the mica detectors appeared consistent with t1/2 = hydroxides of group-6 elements including Sg. 0.3 s.62 A number of possible sources of SF tracks in the mica In order to further reduce the background of unwanted α- detectors other than the SF decay of a Rf isotope were decaying nuclides, the so-called parent−daughter recoil discussed and ruled out. Further experiments with a slightly counting modus had to be implemented at the rotating wheel modified apparatus63 were conducted immediately after the detection systems. Since the investigated transactinide nuclei experiments described here. A total of 63 SF events were decay with characteristic decay sequences involving α-particle attributed to the decay of Rf nuclide. The team in Dubna decay and/or SF of daughter nuclei, the significance of the regarded these experiments as further proof of the claim of observed decay sequence can be enhanced by observing the discovery of element 104. daughter decays in a nearly background free counting regime. One of the most successful approaches to the study of This can be accomplished in the manner shown in Figure 20 volatile transactinide compounds is the so-called OLGA (online on the example of 267Bh. In the parent mode, a 267Bh atom gas chromatography apparatus) technique. Contrary to the technique in Dubna, reaction products are rapidly transported through a thin capillary to the chromatography setup with the aid of an aerosol gas-jet transport system. Transport times of less than 10 s are easily achieved. This way, the chromatography system and also the detection equipment can be set up in a fully equipped chemistry laboratory accessible during irradiation and close to the shielded irradiation vault. A first version of OLGA (I) was developed and built by Gaggeler̈ and co-workers214 for the search of volatile superheavy elements, and it was tested with 25 s 211Pom. Volatile elements were separated in a stream of He and hydrogen gas at 1000 °C from nonvolatile actinides and other elements. The separated nuclei were condensed on thin metal foils mounted on a rotating wheel (ROMA, rotating 187 multidetector apparatus223,224) and periodically moved in front Figure 20. Parent−daughter mode for rotating wheel systems. See of solid state detectors, i.e. PIPS (passivated implanted planar text for detailed description. Reprinted with permission from ref 187. ) detectors, where α-particles and SF events were Copyright 2003 Kluwer Academic Publishers. registered in an event-by-event mode. For the study of volatile halides and/or oxyhalides of Rf225 and Db225,226 and their attached to the aerosol transport material is deposited on the lighter homologs,227 OLGA(II) was built.215 Instead of surface of a thin foil. The wheel is double-stepped at preset condensing the separated molecules on metal foils, they were time intervals to position the collected samples successively attached to new aerosol particles and transported through a between pairs of α-particle detectors. When the 267Bh α-decay thin capillary to the detection system. This so-called is detected in the bottom of a detector pair, it is assumed that reclustering process was very effective and allowed collection the 263Db daughter has recoiled into the face of the top of the aerosol particles on thin (≈40 μg/cm2) polypropylene detector. The wheel is single-stepped to remove the sources foils in the counting system (ROMA or MG, merry-go- from between the detector pairs, and a search for the 263Db and round228). Thus, samples could be assayed from both sides in a 259Lr daughters is made for a second preset time interval, before 4π geometry, which doubled the counting efficiency. At the single-stepping the wheel again to resume the search for decays same time, the PSI tape system was developed,215 which of 267Bh. The measurement of daughter nuclides at significantly allowed significant reduction of the background of long-lived reduced background conditions comes at the expense of a SF activities, that accumulated on the wheel systems. However, reduced overall efficiency of the apparatus. only a 2π counting geometry could be realized. An improved 7.2.2. Thermochromatography. In TC,240 a carrier gas is version of OLGA(II) was built at Berkeley and nicknamed flowing through a chromatography column, to which a negative HEVI (heavy element volatility instrument).216 With both longitudinal temperature gradient has been applied. Open or instruments, the time needed for separation and transport to filled columns can be employed. Species that are volatile at the detection was about 20 s, with the time-consuming process starting point are transported downstream of the column by the being the reclustering. A schematic of the applied experimental carrier gas flow. Due to the decreasing temperature in the equipment is shown in Figure 19. column, the time the species spend in the adsorbed state In order to improve the chromatographic resolution and increases exponentially. Different species form distinct 229 Δ 0(T) increase the speed of separation, OLGA(III) was developed. deposition peaks, depending on their Ha on the column Using a commercial gas chromatography oven and a 2 m long surface, and are thus separated from each other. A characteristic quartz column, which ended in a much smaller, redesigned quantity is the deposition temperature (Ta), which depends on recluster unit, the overall separation time could be reduced by 1 various experimental parameters. The mixture of species to be order of magnitude, while the chromatographic resolution was separated can be injected continuously into the col-

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− umn,62,241 243 or the experiment can be performed discontin- facing each other at a distance between 0.5 and 1.5 mm. A first uously by inserting the mixture of species through the hot end cryo-thermochromatographic separator (CTS) was constructed of the chromatography column and removing the column at LBNL.201 In the Hs experiment,197 the cryo online detector through the cold end after completion of the separation. The (COLD) was used, which was constructed at PSI. A schematic two variants (continuous or discontinuous) result in slightly of the experimental setup used in the first successful chemical different peak shapes. The chromatographic resolution is identification of Hs as volatile tetroxide is shown in Figure 21. somewhat worse for the continuous variant. Thermochromato- graphic separations are the method of choice to investigate species containing long-lived nuclides that decay either by γ- emission, by EC- or β+ decay, or by the emission of highly − energetic β− particles.244 249 Thus, the emitted radiation can easily be detected by scanning the length of the column with a detector. The detection of nuclides decaying by α-particle emission or SF decay is more complicated. By inserting SF track detectors into the column, SF decays of short- and long- lived nuclides can be registered throughout the duration of the experiment. After completion of the experiment, the track detectors are removed and etched to reveal latent SF tracks. Columns made from fused silica have also been used as SF track detectors.250 However, the temperature range for which SF track detectors can be applied is limited, due to the annealing of tracks with time. It should also be noted that in TC all information about t1/2 of the deposited nuclide is lost, Figure 21. The 26Mg-beam passed through the rotating vacuum which is a serious disadvantage in experiments with trans- 248 269,270 actinide nuclides, since SF is a nonspecific decay mode of many window and Cm-target assembly. In the fusion reaction, Hs nuclei were formed which recoiled out of the target into a gas volume actinide and transactinide nuclides. However, TC experiments fl and were ushed with a He/O2 mixture to a quartz column containing with transactinides decaying by SF have an unsurpassed a quartz wool plug heated to 600 °C by an oven. There, Hs was sensitivity (provided that the chromatographic separation converted to HsO4, which is volatile at room temperature and from actinides is sufficient), since all species are eventually transported with the gas flow through a perfluoroalkoxy (PFA) adsorbed in the column and the decay of each nuclide is capillary to the thermochromatography detector array registering the registered. Thus, the position of each single decay in the nuclear decay (α-decay and SF) of the Hs nuclides. The array Δ 0(T) consisted of 36 detectors arranged in 12 pairs, with each detector pair column contributes chemical information about Ha of the investigated species. TC, as a nonanalytical tool to study the consisting of 3 PIN (positive implanted N-type silicon) diode behavior of compounds at a tracer scale, was developed and sandwiches. Always, 3 individual PIN diodes (top and bottom) were − electrically coupled. A thermostat kept the entrance of the array at −20 appliedtothegassolid chromatographic separation of °C; the exit was cooled to −170 °C by means of liquid nitrogen. transactinide elements mainly by the group of Zvara and co- Depending on the volatility of HsO4, the molecules adsorbed at a workers at FLNR Dubna, Russia. Zvara et al. reported the 262 − − characteristic temperature. chemical identification of elements Rf,61 64 Db,65 67 and − Sg,250 253 whereas experiments to chemically identify Bh254 255−257 The geometrical efficiency for detecting a single α-particle and Hs yielded negative results. However, due to the fact emitted by a species adsorbed inside the detector array was that in all these experiments the separated nuclides were 77%. The detectors of the COLD array were calibrated online identified by the noncharacteristic SF decay, and no further with α-decaying 219Rn and its daughters 215Po and 211Bi using a information such as t of the investigated nuclide could be 1/2 227Ac source. A further improved TC detector nicknamed measured, most of the experiments fell short of fully convincing the scientific community that indeed a transactinide element COMPACT (cryo online multidetector for physics and was chemically isolated.32,58,258,259 chemistry of transactinides) was constructed at Technical TC experienced a renaissance in the chemical investigations University Munich (TUM). By reducing the gap between the 197 192,194 195,260 detectors to only 0.5 mm and integrating four detectors on one of Hs, as volatile HsO4, Cn, and Fl in the elemental state, which all are volatile at room temperature. chip, the active area inside the detector channel could be increased to 93%. The COMPACT detector was used in Since it was no longer necessary to introduce highly corrosive 109,261 halogenating chemicals to synthesize volatile compounds, the experiments to discover new Hs isotopes. Furthermore, the detectors were of the PIPS type, which have a more rugged simple quartz tubes that served as chromatography columns fi could be replaced by narrow channels formed by silicon surface and also allow surface modi cations, e.g. the deposition detectors, which are able to record high resolution α-particle of a thin layer of noble such as Au or Pd. This became spectra and register SF fragments in a time-resolved mode. very important in experiments where the interaction of a single Along this channel a longitudinal negative temperature gradient atom of a superheavy element such as Cn or Fl was investigated is established. Due to the close proximity of the silicon diodes with a Au surface. facing each other, the probability to register a complete decay 7.3. Liquid-Phase Chemistry chain of a superheavy nucleus, consisting of a series of α-decays Liquid-phase chemical separations are standard; thus, their and often terminated by SF correlated in time and position, is utility for separation and isolation of the chemical elements has rather high. A position resolution of 1−3 cm was sufficient to been demonstrated. Usually, liquid−liquid extractions or also extract chemical information. Usually, the channels are column based separations are performed. Adaptations of formed by silicon detectors of 1 × 1cm2 dimension, which are these well-understood separation techniques have been

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Figure 22. Schematic drawing of AIDA. Reprinted with permission from ref 280. Copyright 2005 Oldenbourg Wissenschaftsverlag GmbH. extensively applied to the transactinide elements. These The first liquid-phase transactinide chemical separations were adaptations have been developed to overcome the single- manually performed Rf cation exchange separations performed 268 α α atom and short t1/2 limitations inherent in the study of by Silva et al. in 1970 using -hydroxyisobutyrate ( -HIB) as transactinide element chemical properties. eluent. The then newly discovered 65-s 261Rfa was produced in 7.3.1. Manual Liquid−Liquid Extractions and Column the 248Cm (18O,5n)261Rfa reaction, and the recoils were stopped Chromatography. Manually performed liquid−liquid extrac- on NH4Cl-coated Pt foils which were transported to the tions have been used for the study of chemical properties of chemical separation area with a rabbit system. The 261Rfa and − Rf263 266 and Db.267 The microscale liquid−liquid extraction other products from the nuclear reaction, along with the α technique used in these studies allowed minimizing the NH4Cl, were collected from the Pt disk in a small volume of - separation and sample preparation times; phase volumes were HIB and were run through a small cation exchange resin kept to ∼20 μL. Usually, a KCl aerosol gas-jet system was used column. Under these conditions, all cations with charge states α for rapid transport and samples were collected on a special of 4+ or higher were complexed with the -HIB and eluted turntable by impaction on a Teflon disk. At the end of from the column. These experiments showed that Rf had a charge state of 4+ (or higher) and that its chemical properties collection, the sample was dissolved with a few microliters of ff liquid. The time from end of collection to the beginning of are distinctly di erent from those of the actinides. counting of the transactinide chemical fractions was as short as 7.3.2. Automated Column Separations. With expected 50 s, and the collection-separation-counting cycle could be detection rates of only a few atoms per day or week, manually repeated every 60−90 s. performed chemical separations become impractical. With Because of interference from the radioactive decay of other automated liquid-phase chemical separation systems, faster chemical separation and sample preparation times can be nuclides (which are typically formed with much higher yields), achieved and the precision and reproducibility of the chemical extraction systems with relatively high decontamination factors separations has been improved over that obtainable via manual from actinides, Bi, and Po must be chosen. The presence of the separations. An early, already rather sophisticated apparatus for transactinide in the selectively extracting organic phase was automated extraction chromatographic studies of Rf−chloride determined by evaporating the fraction on a hot Ta foil and 269 α complexes was described by Hulet et al. in 1980. The placing the sample in a -particle spectroscopy system. With experimental apparatus and the data storage were fully this technique, the measurement of distribution coefficients is ffi automated and computer controlled. A total of six decays somewhat di cult. By comparing the Rf or Db detection rate attributable to 261Rfa and its daughter 257No were registered. under a certain set of chemical conditions to the rate observed Later, to improve the speed and reduce cross-contamination, under chemical control conditions known to give near 100% the ARCA II (automated rapid chemistry apparatus) was built ffi yield, distribution coe cients between about 0.2 and 5 can be by the GSI-Mainz collaboration, featuring two magazines of 20 determined. If the control experiments are performed nearly miniaturized chromatography columns.270 With the large ffi concurrently, many systematic errors, such as gas-jet e ciency number of columns, cross-contamination between samples and experimenter technique, are canceled out. However, it must can be prevented by using each column only once. By be ensured that during the chemical operations the trans- miniaturizing the columns, the elution volume, and therefore, actinide element is not adsorbed to surfaces of the used the time needed to dry the final sample to produce a source for equipment. For example, it was observed that Hf did adsorb on α-particle spectroscopy, is much reduced. The radioactivity is the used Teflon surfaces265 and that Pt-foils and polypropylene delivered from the site of production at the accelerator to vials and pipet tips were better suited to conduct the ARCA by an aerosol gas-jet system. Aerosol particles are experiments. Additionally, extraction systems which come to collected on a frit or by impaction on a small spot on a slider equilibrium in the 5−10 s phase contact time must be chosen. (seen at the center of Figure 22). At the end of the collection

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Figure 23. Schematic of the ion-exchange part in AIDA. Reprinted with permission from ref 281. Copyright 2004 Elsevier. time, the slider is moved to position the collection site above forward movement of the column magazine to avoid cross- one of the miniature chromatography columns. A suitable contamination at the collection site. aqueous solution is used to dissolve the aerosol particles and The robotic sample preparation and counting technology, load the activities onto the column below. Selective elutions of together with mechanical improvements in the chemical transactinide elements are carried out by passing appropriate separation system, resulted in an automated column solutions through the column. A slider below the ion exchange chromatography system that runs almost autonomously. Each column is moved at the appropriate time to collect the chemical separation in columns is accomplished within 20 s and the α- fraction of interest on a hot Ta disk. A sample suitable for α- particle measurement can be started within 80 s after the particle spectroscopy is prepared by rapid evaporation of the collection of the products at the AIDA collection site. To chemical fraction on the Ta disk, which is heated from below by shorten the time for the sample preparation of α sources, the a hot-plate and from above by a flow of hot He gas and a high- newly developed rapid ion-exchange apparatus AIDA-II was intensity infrared lamp. The final samples are then manually introduced; the apparatus is based on continuous sample placed in a detector chamber. The two magazines of collection and evaporation of effluents, and successive α- chromatography columns can be moved independently. During particle measurement. The ion-exchange part is the same as a chemical separation on the left column, the right column is that of AIDA. The AIDA-II was successfully applied for the conditioned by passing an appropriate solution through it. After chemical experiments with Db.282 The effluent is collected as the separation on the left column is finished, the magazine is fraction 1 on a 15 mm × 300 mm sheet which was moved forward; placing a new column in the left position, the continuously moved toward an α-particle detection chamber at next separation is performed on the right column while the left 2.0 cm s−1. The sample on the sheet is automatically evaporated column is being prepared for the subsequent separation. In this to dryness with a heat lamp and then subjected to α- way, up to 40 separations can be carried out, at time intervals of particle spectroscopy in a chamber equipped with an array of 12 less than 1 min, with each separation performed on a freshly silicon PIN photodiode detectors.282 Products remaining on prepared, unused column. Although the column separations are the resin were eluted with a strip solution. The eluate was fully microprocessor controlled and performed automatically, collected on a second Ta sheet as fraction 2, followed by the still several people were required to operate the ARCA II same procedures for sample preparation and measurement. The system for a transactinide chemistry experiment. Transactinide measurements were started 14 and 38 s after the end of product chemical separations with ARCA II have been performed with collection. − Rf,271,272 Db,273 277 and Sg.278,279 7.3.3. Automated Liquid−Liquid Extractions. Liquid− Building upon the design of ARCA II, an automated column liquid extractions allow for very fast, continuously working separation apparatus, AIDA (automated ion-exchange separa- arrangements, especially if detection of the separated nuclides tion apparatus coupled with the detection system for α- occurs with highly efficient, continuous liquid scintillation spectroscopy), has been developed at JAERI (now JAEA).280 counting (LSC). A system that has successfully been applied to Even though the apparatus for collection of aerosol particles transactinide chemistry is the so-called SISAK (short-lived and performing multiple chemical separations on magazines of isotopes studied by the AKUFVE-technique, where AKUFVE is miniaturized ion-exchange chromatography columns is very a Swedish acronym for an arrangement of continuous similar to that in ARCA II, AIDA has automated the tasks of investigations of distribution ratios in liquid extraction) system. sample preparation and placing the samples in the detector This system performs continuous liquid−liquid extractions chambers. Using robotic technology, the selected fractions are using small-volume separator centrifuges.283 Nuclear reaction dried on metal Ta disks and are then placed in vacuum products are delivered to the apparatus by an aerosol gas-jet. chambers containing large-area PIPS α-particle detectors. A The gas-jet is mixed with the aqueous solution to dissolve the schematic drawing of AIDA is shown in Figure 22. radioactivity-bearing aerosol particles, and the carrier gas is In the ion-exchange process as shown in Figure 23, two removed in a degasser centrifuge. The aqueous solution is then different paths to supply solutions are available; the first eluent mixed with an organic solution, and the two liquid phases are goes through the collection site to the microcolumn, while the separated in a separator centrifuge. A scintillation cocktail is second strip solution is directed to the column after one-step then mixed with the organic solution, and this is passed through

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Figure 24. Schematic of the SISAK liquid−liquid extraction system using the BGS as a preseparator. Reprinted with permission from ref 200. Copyright 2002 The Japan Society of Nuclear and Radiochemical Sciences. a detector system to perform liquid scintillation α pulse-height molecule with its surroundings in order to ensure a statistically spectroscopy on the flowing solution. An electronic suppression significant behavior. Here, chromatographic methods are of signals originating from β-decays is applied. preferred. This modular separation and detection system allows the use 8.1. Volatility of well-understood liquid−liquid extraction separations on time scales of a few seconds, with detection efficiencies near 100%, In experiments with single molecules, the behavior of a species and it has been used to study the subsecond α-active nuclide in a gas adsorption chromatographic experiment is determined − 224Pa.284 286 However, the detection of α-active transactinide by its molecular properties and the nature of the interaction isotopes failed despite pulse-height discrimination against with the adsorbent. The state of the adsorbent (column interfering β-activities, which are produced with much larger material) should be known, if possible. It is assumed that the yield. Preseparation with a kinematic recoil separator, as investigated molecules are rather stable and/or stabilized by the described in section 7.1.2, allowed chemical separation and chemical environment, which usually is characterized by detection of 4-s 257Rf using the SISAK technique.200 A extreme surpluses of the reaction partners in the carrier gas. schematic of the BGS-RTC-SISAK apparatus is presented in Also, the state of zero surface coverage can be assumed for the Figure 24. These proof-of-principle experiments have paved the stationary phase. way for detailed liquid−liquid extraction experiments with As described in section 7, in gas-phase chromatography short-lived transactinide element isotopes. By isolating a experiments, a measure of volatility is either the deposition counting cell from the stream of flowing liquid as soon as a temperature in a TC column, Ta, or the temperature at which potentially interesting signal occurred, genetically correlated 50% of the investigated species emerge from an isothermal decay chains could be detected. column, T50%. From these temperatures, the enthalpy of Δ 0(T) 287 adsorption, Ha , is deduced using adsorption models, 8. METHODS TO PREDICT EXPERIMENTALLY or Monte Carlo simulations are applied using a microscopic MEASURABLE PROPERTIES OF TRANSACTINIDES model developed by Zvara.288,289 The adsorption enthalpy, Δ 0(T) Due to the very small production rates on the “one atom at a Ha , can be empirically related to the sublimation enthalpy, Δ 0(298) time” level, only very few physicochemical quantities of HS , of the macroamount. However, the usage of a Δ 0(T) Δ 0(298) transactinide elements and their compounds can be determined correlation between Ha and HS is restricted to some experimentally. A further difficulty arises from the fact that, groups and types of compounds, while not generally even in experiments with lighter homolog elements at the tracer permissible (see below). In macrochemistry, a measure of scale (i.e., 106 to 109 atoms), there currently exist no analytical volatility is an equilibrium vapor pressure over a substance, Pmm. Δ instruments with the required sensitivity that would allow Boiling points, Tb, and enthalpy of evaporation, Hevap, determining the speciation of the investigated elements. basically correlate with Pmm. ffi Therefore, the measured microscopic properties have to be Besides the low statistics of single events, a di culty arises directly predicted by theory. Those theoretical predictions with respect to the interpretation of results, since the surface of should be based on the best possible relativistic electronic the chromatographic column is not well-defined. Usually, it is structure calculations, since relativistic effects influence not only modified by the evaporated aerosol transport particles and/or properties of volatile atoms and molecules but also the halogenating agents, so that the mechanisms of adsorption and, adsorption phenomenon. Some empirical correlation can also associated with it, the nature of chemical or physical 240 be useful. Such correlations between micro- and macroamounts interactions can only be assumed. Thus, available exper- may be valid for selected groups of elements and types of imental data are often difficult to interpret and do not correlate compounds, but they cannot necessarily be extrapolated to with a single property or electronic structure parameter of the transactinide elements. adsorbate. Several constraints have to be fulfilled simultaneously to 8.1.1. Physisorption and Chemisorption Models. serve the purpose of a meaningful chemical study of Quantum-mechanical calculations of adsorptionboth phys- transactinide compounds. First, a class of compounds should isorption and chemisorptionof atoms and molecules on be studied, where relativistic effects are expressed in an (poly)crystalline and even amorphous substrates are nowadays experimentally detectable parameter. Second, fast, simple, and performed using modern periodic DFT codes. Within this slab/ efficient procedures must be available to isolate the trans- supercell-approach, the surface of usually a single crystal is actinide compound from interfering components. Third, the modeled by a slab of finite thickness due to the application of studied class of compounds should be thermodynamically periodic boundary conditions, which introduce the semi-infinite stable and allow multiple interactions of the transactinide character of the system. While in these codes the relativistic

1259 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review ff ⎛ ⎞ α e ects of the core electrons are treated on the (relativistic) 3 ⎜⎟ε − 1 mol core-potential or pseudopotential level, the valence electrons Ex()=− ⎝ ⎠ 16 ε + 2 11+ x3 are treated in nonrelativistic or outermost in a scalar-relativistic ()IPslab IP mol (8.1.1) manner. Unfortunately, there are no full-relativistic, all-electron periodic implementations available yet. where IPslab and IPmol are ionization potentials of the molecule For adsorption of atoms or small molecules on metal and slab, respectively, ε is the dielectric constant of the surface surfaces, straightforward fully relativistic calculations of the material, and x is the molecule−surface distance. In a adsorption energy are possible using relativistic 4c-DFT comparative study, x for a lighter element is deduced from Δ 0(T) methods and the cluster approach. In this case, a surface of the known Ha , while that for a heaviest element is fi ff an adsorbent is modeled by a cluster Mn of a speci c number n estimated using the di erence in their molecular size. of metal atoms. The size of the cluster is then steadily enlarged Thermodynamic equations to predict Ta of a heaviest until convergence of the binding energy of the ad-atom-metal element with respect to Ta of a homolog in a comparative cluster is reached with cluster size n. Such direct calculations are study using the knowledge of the electronic structure of the 295 now possible up to more than a hundred atoms of the adsorbate are also presented. One of those equations is given cluster.290a A possibility to treat an even larger number of atoms below for the case of mobile adsorption of molecules with one economically is foreseen via an embedded cluster procedure rotational degree of freedom: (see schematic in Figure 25).290b −Δ 11−Δ ERTAB/ = ERT/ e A 1/2 1/2 e B 1/2 1/2 trdTm1/2 AA A A trdTm1/2 BB B B (8.1.2)

where t1/2 is the half-life of the central nuclide, r the molecular radius, d the metal−ligand distance, R the gas constant, T the adsorption temperature, m the mass, and ΔE the adsorption energy of a heaviest molecule A and of its lighter homolog B. In the same work, various measures of volatility were critically compared. The most adequate one in a comparative study (in macrochemistry) was shown to be the ratio of adsorption/ desorption constants, Kads/Kdes. For predictions of adsorption of molecules with nonzero dipole moments, equations taking into account long-range interactions, such as molecular dipole−surface charge, dipole− ′− induced dipole, and van der Waals one, were used. Thus, for Figure 25. Embedded M Mn system. Reprinted with permission from ref 25. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH. example, the interaction energy of a molecule with a surface having a charge is as follows:291

2 2Qeμ Qe22ααα3 =−mol −mol − mol slab Δ 0(T) Ex() 2 4 As was mentioned, to determine Ha of a heavy molecule x 2x 2 11+ on a complex surface is still a formidable task for quantum ()IPmol IP slab chemical calculations. Especially difficult is the prediction of (8.1.3) physisorption phenomena caused by weak interactions, where μ α where mol,IPmol, and mol belong to the molecule and those the DFT generally fails. with index “slab” to the surface; Q is the charge of the surface In the past, DS-DV calculations were helpful in establishing atom and x is the molecule−surface distance. some correlations between electronic structure parameters and With the use of those models (eqs 8.1.1−8.1.3), adsorption volatilities of halides, oxyhalides, and oxides known from 35,39 of various group-4 through group-8 species, including the macrochemistry. For example, it was established that heaviest ones, on various () surfaces was predicted covalent compounds having high overlap populations (OP) (see below). are more volatile than ionic ones, that molecules with dipole 8.1.2. Empirical Correlations and Extrapolations. The μ moments ( ) interact more strongly with surfaces than those concept that bond energies between identical molecules or without, and that the sequence in the adsorption energy is atoms in the crystal lattice should be proportional to the fi μ de ned by the sequence in . adsorptive bond energies between the same single molecules or Lately, predictions of interaction energies of heaviest element atoms and a surface was empirically demonstrated by plotting molecules with inert surfaces (quartz, silicon , also Δ 0(T) Δ 0(298) Ha versus HS for certain gas chemical systems, for modified) were made with the use of physisorption example, for chlorides and oxychlorides in Cl , HCl, and CCl 291−294 2 4 models. These models are based on the principle of (O2) on quartz, and for oxides and oxyhydroxides in O2 (H2O) intermolecular interactions subdivided into usual types for long- on quartz.296 In other words, it is assumed that the molar range forces: dipole−dipole, dipole−polarizability, and van der binding energy of an adsorbed single molecule to the surface Waals (dispersion) ones. The molecular properties required by approximately equals its partial molar adsorption enthalpy at those models are then calculated using the most accurate zero surface coverage. As an example, the empirical correlation −Δ 0(T) relativistic methods. observed for chlorides and oxychlorides between Ha Thus, for a molecule with zero dipole moment adsorbed on a measured for single molecules on the chromatographic surface dielectric surface by van der Waals forces, the following model (quartz, statically or dynamically modified by the chlorinating 294 Δ 0(298) of the molecule-slab interaction is used: reagents) and the macroscopic HS is shown in Figure 26.

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Δ 0(298) enthalpy HS of these elements. Those linear extrapola- tions should be used with care, as there is no solid theoretical basis for them. 8.2. Complex Formation in Aqueous Solutions In liquid-phase chemistry, single atoms of transactinide elements (or of their lighter homologs) are transported by an aerosol gas-jet technique from the site of synthesis behind the target or behind a kinematic recoil separator to a filter or impaction device, where samples are collected and dissolved in an aqueous solution after an appropriate collection time. Usually, salt aerosol particles, such as KCl, are used that are easily generated by heating the salt in a gas stream to a temperature slightly below the . The KCl particles readily dissolve in contact with an aqueous solution. The aqueous solution contains suitable ligands for complex formation. The complexes are then chemically investigated Figure 26. Empirical correlation observed for chlorides and oxy- −Δ 0(T) using a partition method by studying, for example, their chlorides between Ha measured for single molecules on the chromatographic surface (quartz, statically or dynamically modified by extraction into an organic solvent, or by anion- or cation- Δ 0(298) 234 exchange chromatography, or by reversed-phase extraction the chlorinating reagents) and the macroscopic HS . chromatography. The ultimate goal of partition experiments is ffi to determine the distribution coe cient Kd as a function of Unfortunately, there are no generally valid correlations that ligand concentration. Δ 0(298) relate HS to other molecular properties. Attempts were For a simple complex MLn, the cumulative complex made to correlate the inverse (Tb) with the formation constant geometric structure of molecules. For the halides of the metals β = [ML ][M]−−1 [L] n of the type MXn (n = 4, 5, 6; X = F, Cl, Br, I), Zvara empirically n n (8.2.1) derived a formula which, for a given stoichiometry (at is a measure of its stability. For stepwise processes, consecutive maximum symmetry), correlated Tb with the ionic radius of z−n 289,297 constants Ki are used. If various MLn complexes exist in the the central metal ion. For certain classes of compounds, p− aqueous phase, but only one MLi complex in the organic such as the tetrachlorides, an empirical correlation exists ffi 0(298) phase, the distribution coe cient, Kd, for the anion exchange is between ΔH and the orbital radii or crystal ionic radii of 300 S given by the following equation the group-4 elements Ti, Zr, and Hf, but also U and Th.298 It fl ff +−p −ip − should be noted that due to the in uence of relativistic e ects KDM[RB L ] org βi [L ] the extrapolative power of such empirical relationships is Kd = ∑N β − n limited. 0 n[L ] (8.2.2) A simple and very early extrapolation, which will be referred where K is the association constant with the organic cation. to later, is a prediction of the standard enthalpies of monatomic DM Δ * Thus, a sequence in the Kd values for a studied series, for gaseous elements, H 298(E(g)), to the heavy transactinides Cn fl 299 example, for elements of one group, re ects the sequence in the through element 120 by Eichler by extrapolating over the stability of their complexes. atomic number Z (Figure 27). The standard enthalpies of Complex formation is known to increase in the transition monatomic gases are mostly equal to the standard sublimation element groups. In aqueous solutions, it is, however, competing with hydrolysis. This may change trends in the stabilities of complexes and, finally, in their extraction into an organic phase. One should distinguish between hydrolysis of cations and hydrolysis of complexes.301 The former is described as a process of a successive loss of protons z+ (1)z−+ + M(H221 O)n ⇄+ MOH(H O)n− H (8.2.3) In acidic solutions, hydrolysis involves either the cation, anion, or both, and is competing with complex formation described by the following equilibrium z+−− xM(H2 O)w° ++yi OH L ()xz−− y i + ⇄ Mxu O (OH) z−22 u (H O) wi L

+°+−()HOxw u w 2 (8.2.4)

In order to predict a sequence in Kd (eq 8.2.4), one should predict a sequence in the formation constants of a series of species of interest. For a reaction such as, for example, eq 8.2.3 Figure 27. Extrapolation of the standard enthalpies of monatomic or 8.2.4, Δ * gaseous elements H 298(E(g)) along the groups of the Periodic Table r based on the atomic number Z.299 log KGRi =−Δ /2.3 T (8.2.5)

1261 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review where ΔGr is the free energy change of the complex formation difference to some elements of the sixth row.). The relativistic reaction. To obtain it in a straightforward way, binding or total stabilization of the 7p1/2 electron was explained to be energies of species in the left-hand and right-hand parts of the responsible for the unusual ground state in comparison with − 2 2 3 reaction should be calculated. Such relativistic calculations, with the (n 1)d ns ( F2) state of the lighter homologs Zr and Hf. full geometry optimization for the heaviest elements, are More accurate DCB FSCC calculations,312 however, corrected 175,176 2 2 3 extremely computer time intensive. In addition, the obtained the MCDF result, giving 7s 6d ( F2) as ground state accuracy might be insufficient to predict stabilities of similar configuration. A very high level of correlation with l = 6 was species of homologs. Therefore, the following economical required to reach this accuracy. model was suggested by Pershina. The IP of Rf of 6.01 eV was calculated at best using the DCB In a fashion analogous to that of Kossiakoff and Harker,302 FSCC method.312 It is smaller than the IP of Hf (6.83 eV313) the following expression for the free energy of formation of the due to the relativistic destabilization of the 6d AOs. The first (xz−2u−v)+ MxOu(OH)v(H2O)w species from the elements was ionized electron in Rf is the 6d one, like the 5d electron in Hf. adopted MCDF multiple IPs of M through M4+ states were also 176 → 4+ f calculated. There is a steady decrease in the IPs(M M ) −ΔGuvw(, , )/2.3 RT in group 4 due to the same reason, i.e., the destabilization of the w − =+∑∑aaPiij +−!!!log log( uvw 2 ) (n 1)d AOs with increasing Z (Figure 28).

+++(2uv 1)log 55.5 (8.2.6) fi ∑ The rst term on the right-hand side of eq 8.2.6, ai, is the nonelectrostatic contribution from M, O, OH, and H2O, which is related to the overlap population. For a reaction, OP Δ ∑ aEki =Δ = ΔOP (8.2.7) ffi ∑ where k is an empirical coe cient. The next term, aij,isa sum of each pairwise electrostatic (Coulomb) interaction: C EaBQQd==−∑∑ij ij/ ij ij (8.2.8) where dij is the distance between moieties i and j; Qi and Qj are their effective charges, and B = 2.3RTe2/ε, where ε is a Δ C ff C Figure 28. Ionization potentials to the maximum dielectric constant. For a reaction, E is the di erence in E (IP ) and ionic radii (IR) for Rf through Hs obtained from the max − for the species in the left and right parts. P in eq 8.2.6 is the MCDF calculations.175 179 Reprinted with permission from ref 39. partition function representing the contribution of structural Copyright 2003 Kluwer Academic Publishers. isomers if there are any. The last two terms are statistical: one is a correction for the indistinguishable configurations of the species, and the other is a conversion to the molar scale of This results in an increase in the stability of the maximum ∑ ∑ oxidation state in this group, which was shown by the values of concentration for the entropy. aij and ai for each 314,315 compound are then calculated directly via a Mulliken analysis the respective redox potentials. The (AR) fi implemented in most of the quantum-chemical methods (e.g., de ned by the outer 7s AO of Rf is 1.49 Å, as derived from the 163 β earlier DF atomic calculations.35 It is smaller than the AR(Hf) 4c-DFT ). To predict log Ki or log i for transactinide 316 complexes, coefficients k and B should then be defined by of 1.55 Å due to the relativistic contraction of the 7s(Rf)AO. fi Ionic radii (IR) obtained via a correlation with the outer (n− tting log Ki to experimental values for the lighter homologs. 1)p AOs of the M4+ ions show an increase in the group, so that Using this model, hydrolysis and complex formation constants 4+ 176 4+ were predicted for a large number of aqueous compounds of the IR(Rf ) is 0.79 Å, which is larger than the IR(Hf )of 303−311 0.71 Å317 (Figure 28). A better value of the IR(Rf4+) derived group-4 through group-6 elements in very good 318−320 agreement with experimental results. The results of these from molecular calculations is, however, 0.76 Å. A set of calculations and a comparison with experimental data reveal atomic single and triple bond covalent radii (CR) for most of − the elements of the Periodic Table, including the heaviest ones that a change in the electrostatic metal ligand interaction 319,320 energy (ΔEC) of a complex formation reaction contributes till Z = 118 and Cn, respectively, was suggested. They are predominantly in the change in ΔGf, i.e., in ΔGr. Thus, only by deduced from the calculated molecular (equilibrium) bond calculating ΔEC can trends in the complex formation be reliably lengths (Re) of various covalent compounds. The CR of the − predicted. group-4 to group-8 6d elements are about 0.5 0.8 Å larger than those of the 5d elements. An important finding of these − ff 9. RUTHERFORDIUM (Z = 104) works is a decrease in the R6d R5d di erence starting from group 9, reaching negative values in groups 11 and 12, as a 9.1. Theoretical Predictions result of the relativistic bond contraction (Figure 29). This is The chemistry of Rf is expected to be similar to that of Zr and called a “transactinide break”. Hf and defined by the four valence electrons, thus favoring the There are several other calculations of molecular compounds 175,176 stable 4+ oxidation state. The MCDF calculations have of Rf. The hydrides MH4 (M = Ti through Rf) were calculated 2 3 321−323 given 7s 7p6d ( D2) as ground state for this element (The using the DF one-center expansion method. Relativistic 2 ff stabilization of the 7s pair was found for the entire seventh e ects were shown to decrease the Re in RfH4, so that it is only period as a result of the relativistic stabilization of the 7s AO, in 0.03 Å larger than Re(HfH4). The relativistic contractions of

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The binding energy of RfCl4 and homologs was calculated using the 4c-DFT method.318 The compound was shown to be stable with De of 19.5 eV, though less stable than ZrCl4 (21.7 eV) and HfCl4 (21.1 eV). The lower stability was explained via a smaller contribution of the ionic contribution to the binding ff 315 energy due to a decreasing e ective metal charge, QM. The 327 RECP calculations gave 18.8 eV for De(RfCl4), in overall agreement with ref 318. A decrease in De(RfCl4) in comparison ff with De(HfCl4) was shown to be due to the larger SO e ects on the 6d AOs. Re(RfCl4) was shown to be larger by about 0.05 Å 327 319,320 than Re of HfCl4, in agreement with other data. Solid-state calculations were performed on Rf metal.329 The structural and electronic properties were evaluated by first principles DFT in the scalar relativistic formalism with and without SO coupling and were compared with those of its 5d homolog, Hf. It is found that Rf should crystallize in the ff hexagonal close packed structure, as does Hf. However, under Figure 29. The di erence in the lengths of the single (open circles) ff and triple (filled triangles) bonds between the 6d and 5d metals.319,320 pressure, it should have a di erent sequence of phase transitions than Hf: hcp → bcc instead of hcp → ω → bcc. An explanation is offered for this difference in terms of the orbitals and bond lengths were shown to be two parallel but ff competition between the band structure and the Ewald energy largely independent e ects. The calculations showed a decrease contributions. in the atomization energy, De, of RfH4 as compared to that of The aqueous chemistry of Rf has also been studied HfH4. Results of ab initio noncorrelated DF calculations were 307,310 324 theoretically. As do other group-4 elements, Rf undergoes reported for RfCl4. 315,325 hydrolysis and complex formation in acidic solutions. The DS-DV calculations were performed for MCl4. The following reactions are important: calculations agreed on the fact that the electronic structure of the first hydrolysis step RfCl4 is similar to that of ZrCl4 and HfCl4 and that RfCl4 is a typical d-element compound. Covalency judged by the 4+ 3+ M(H O)⇄ MOH(H O) (9.1.1) Mulliken OP was shown to increase down group 4.315 This is 28 27 due to an increase in an overlap of the relativistic ns and the stepwise fluorination − (n 1)d AOs of the central ion with AOs of the ligand. 4++3 + Nonrelativistically, the trend is just opposite. Due to its highest M(H28 O)⇄⇄ MF(H27 O) ...... MF(H325 O) ... ⇄ covalency, RfCl4 is therefore expected to be the most volatile (9.1.2) 315 among group-4 MCl4. This is in contrast to simple − 2− extrapolation procedures229,298,326 using a radius−volatility MF422 (H O)⇄⇄ ... MF 52 (H O) MF 6 (9.1.3) correlation of tetrachlorides that predict the opposite (Figure and the total chlorination 30). So experimentally measuring the volatility of RfCl4 in 4+−2 comparison to its lighter homologs HfCl4 and ZrCl4 provides M(H28 O)+⇄ 6HCl MCl6 (9.1.4) an ideal test case. Thefreeenergychangeofreactions9.1.1−9.1.4 was calculated using the model described in section 8.2 and 4c- DFT calculations of the electronic structure of the com- plexes.307 The obtained data suggest the following trend in hydrolysis of the group-4 elements Zr > Hf > Rf. For cation exchange separations (CIX) performed at <0.1 M HF (no hydrolysis), that is, for extraction of the positively charged complexes, the Kd values will change in the following way in group-4: Zr ≤ Hf < Rf. This is caused by the decreasing trend in the formation of positively charged complexes according to eq 9.1.2: Zr ≥ Hf > Rf. In the case of formation of complexes with a lower positive charge from complexes with a higher positive charge, the sequence in the Kd values is opposite to the sequence in complex formation, since complexes with a higher charge are better sorbed on the CIX resin than those with a lower charge. For the formation of anionic complexes sorbed by anion- exchange (AIX) resins, the trend becomes more complicated Figure 30. Vapor pressure of group-4 chlorides over their respective depending on pH, that is, depending on whether the solids as a function of temperature. Literature data for ZrCl4 and HfCl4 328 “ fluorination starts from hydrated or hydrolyzed species. Thus, from Knacke et al. The vapor pressure curve labeled RfCl4 −3 −1 ” 315 for experiments conducted in 10 to 10 M HF (where some relativistic resulted from using the QM data from Pershina et al. and applying the procedure outlined in section 8.1.1, whereas the hydrolyzed or partially fluorinated species are present), the “ ” 2− shaded area labeled RfCl4 extrapolated indicates the predicted vapor trend for the formation of MF6 (eq 9.1.3) should be reversed ff 326 ≥ pressure of RfCl4 using two di erent extrapolation procedures. in group 4: Rf Zr > Hf.

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4− Figure 31. M(SO4)2(H2O)4 and M(SO4)4 complexes of Zr, Hf, and Rf. Reprinted with permission from ref 310. Copyright 2006 Oldenbourg Wissenschaftsverlag GmbH.

For the AIX separations at 4−8 M HCl, where no hydrolysis that group-4 elements form rather volatile halides was should occur at such high acidities, the theoretical data suggest exploited. A good measure for the volatility of a molecule is that the trend in the complex formation and Kd values should its vapor pressure over its respective solid (see Figure 33) (It be continued with Rf: Zr > Hf > Rf. Complex formation of group-4 elements in H2SO4 solutions was also studied theoretically.310 In this work, relative values of the free energy change of the M(SO4)2(H2O)4, 2− 4− M(SO4)3(H2O)2 , and M(SO4)4 (M = Zr, Hf, and Rf) formation reactions from hydrated and partially hydrolyzed cations were calculated using the 4c-DFT method. Geometrical configurations of two of the considered complexes are shown in Figure 31. The results indicate the following trend in complex ≫ formation, Zr > Hf Rf. The obtained log Kd values for the extraction of Zr, Hf, and Rf from H2SO4 solutions by amines are shown in Figure 32. Figure 33. Vapor pressure curves for Zr and Hf halides over their 9.2. Experimental Results respective solids.334. Reprinted with permission from ref 334. Copyright 1994 Springer Science and Business Media. Early on, two different chemical strategies were chosen to demonstrate that the chemical properties of Rf are distinctly 330,331 different from those of the actinide elements. In Dubna, the fact was, therefore, originally suggested by Pershina et al. to use those vapor pressure curves also in the chemical studies on the transactinides in comparison with their lighter homologs). The volatility decreases according to MCl4 > MBr4 >MI4 > MF4 with M = Zr, Hf. While the iodides show poor thermal stability and the fluorides are the least volatile, chlorides and bromides are a good choice for a transactinide chemistry experiment. Clearly, the heavy actinides do not form volatile chlorides or bromides. Therefore, an experiment that isolates Rf in the form of volatile chlorides or bromides also demonstrates that this element does not belong to the actinide series. Immediately after the discovery of a short-lived spontaneously fissioning radioactivity at Dubna in a physics experiment, a series of gas-phase chemistry experiments were conducted by − Zvara et al.60 67 to prove that this radioactivity was indeed not due to an actinide element. Later, the volatility of group-4 chlorides and bromides was reinvestigated by Türler,225,230 Kadkhodayan,332 and Sylwester333 using online IC with identification of 261Rfa through α-particle spectroscopy, α−α 261 a correlations, and determinations of t1/2 of Rf and its daughter 257No. Enthalpies of adsorption of the investigated volatile compounds on the chromatographic surface (−ΔH0(T)) Figure 32. Predicted log Kd for the extraction of Hf and Rf from a H SO solutions by amines with respect to the ones measured for Zr. were deduced. 2 4 88 Reprinted with permission from ref 310. Copyright 2006 Oldenbourg In Berkeley, aqueous phase chemistry experiments were Wissenschaftsverlag GmbH. conducted with the same isotope 261Rfa. Tetravalent Rf (Rf4+)

1264 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review was eluted with the complexing agent α- were interpreted that the SF-tracks observed were due to SF of hydroxyisobutyrate (α-HIB) from a cation exchange column, 259Rf behaving like the lighter homolog element Hf, and that way ahead of the trivalent actinides or divalent No. By also in earlier experiments the SF decay of this Rf isotope has preparing samples suitable for α-particle spectroscopy, Silva et been observed. al.268 could unambiguously identify 261Rfa in the eluate This work and also the earlier experiments by Zvara et al. containing the tetravalent elements by observing time were criticized by the members of the Berkeley group.258 The correlated α−α mother daughter correlations attributed to three points of criticism concerned the unknown magnitude of 261 a α 259 the decay of Rf followed within 1 min by the -particle the SF-branch in Rf, the inability to determine t1/2 of the decay of the daughter 257No. source of SF, which would have allowed excluding SF from 9.2.1. Gas-Phase Chemistry of Rutherfordium. When it actinide nuclei such as 256Fm, and the absence of many SF 260 became obvious that t1/2( Rf) < 0.3 s and a newly detected tracks in the isothermal section due to the decay of shorter- 335 4.5-s SF radioactivity was due to a possible SF (or EC, lived 260Rf. 336,337 leading to SF in 259Lr) branch in 259Rf (which decays Concerning the measurement of the magnitude of the SF- α 338 64 259 primarily by -emission with t1/2 = 3.2 s ), Zvara et al. branch of Rf not too much progress has been accomplished. repeated their experiments with a new apparatus, which The only direct measurement was performed by Bemis et al.,336 combined IC with TC. A schematic of the apparatus and the who reported a SF branching ratio of 6.3 ± 3.7%. In the work obtained results is shown in Figure 34. of Bemis et al.,336 a total of 22 SF events were observed. Of these, 8 ± 2 events were ascribed to long-lived 256Md/256Fm. Another source of SF was identified in the presence of 256No. Here, a SF branch of 0.25% was used to estimate an additional contribution of 4.8 ± 1.2 SF events not related to 259Rf, leaving 9.2 ± 5.2 SF events. However, in a recent experiment, the SF/α 256 339 +0.0006 ratio in No was measured as 0.0053−0.0003. With this new value, the additional contribution due to 256No increases to 10.2 SF events, leaving now only 3.8 of the observed 22 SF events attributable to 259Rf. Thus, the SF branch in 259Rf reduces to 2.6%. Gates et al.337 have observed an EC branch of 15 ± 4% in 259Rf, leading to 259Lr, which decays by SF with 25 ± 3% branching. This would result in an apparent SF-branch of 259Rf of 3.8 ± 1.1%. Ascribing some of the observed SF events to 259Lr after EC decay of 259Rf would not alter the conclusions drawn on the chemical properties of Rf, since, under the conditions of the experiment by Zvara et al.,61,63,64 Lr is expected to form nonvolatile compounds which would deposit essentially at the same position in the column as Rf. In addition, a SF-branch of >2% (or an EC-branch >8%) in 259Rf would be sufficient to explain the number of SF events by Zvara et al.61,63,64 Figure 34. (a) Schematic of the experimental apparatus to investigate Interestingly, the chemical aspects of the experiment were 259 170,171 fi the volatility of Rf and Hf chlorides; (b) temperature pro le in not discussed. The fact that Rf nuclides with t1/2 = 3 s deposited 170,171 the column (isothermal combined with a gradient); (c) distribution of in the column at the same position as did Hf (t1/2 = 16.0 SF tracks (open and closed circles) for 44mSc and 170,171Hf. Reprinted and 12.2 h, respectively) does not at all mean that HfCl4 and with permission from ref 58. Copyright 1987 Oldenbourg RfCl exhibit the same volatility. With the development of a Wissenschaftsverlag GmbH. 4 microscopic description of the chromatographic process,288 the migration of a molecule through the column can be simulated As in previous experiments, the nuclides recoiling from the using a Monte Carlo technique. With this new technique, a target were stopped in a stream of N gas and flushed into the quantitative analysis of this experiment is possible and will 2 Δ 0(T) 259 fl allow a comparison of the obtained Ha (RfCl4) with data of chromatography section. Due to the longer t1/2( Rf), the ow newer, IC experiments. The results of a Monte Carlo rate could be reduced by about 1 order of magnitude, allowing 288 higher separation efficiency from short-lived SF-isomers in the simulation with the microscopic model of Zvara are shown actinides and better chromatographic resolution. SF track in Figure 35. In the upper panel, the experimental data are fi detectors were inserted into sections II and III. As reactive shown together with the simulated deposition zone pro les for 259RfCl and 170,171HfCl . The only adjustable parameter was agents a mixture of SOCl2 and TiCl4 (which also served as 4 4 Δ 0(T) fi carrier) was used. In order to simultaneously produce Hf Ha ; all other parameters were xed at their experimental nuclides, the 242Pu target contained an admixture of Sm. This values. In the lower panel the integrated yields are shown in 170,171 259 way, the deposition peak of long-lived Hf and Rf could comparison with the simulation. The deposition zone of HfCl4 be measured simultaneously in the same experiment. Indeed, can be reproduced very accurately. The experimental after completion of the experiment, the isothermal section II distribution of SF events of 259Rf is somewhat narrow compared contained only a few fission tracks at the very beginning and to the simulated zone profile, but in the light of the poor one SF track further downstream, whereas, in the gradient statistics, the two distributions are in reasonable agreement. −Δ 0(T) · −1 section, 15 SF tracks were observed at about the same position The determined Ha (RfCl4) is 110 kJ mol . For the −Δ 0(T) · −1 as long-lived Hf was deposited. The results of this experiment lighter homolog Hf, Ha (HfCl4) = 146 kJ mol resulted.

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pure quartz surface but on a surface covered at least partially with KCl. These first experiments were repeated with the significantly improved HEVI setup.216,332,333 Instead of KCl, MoO3 was used as aerosol particle forming material, which is converted to the volatile MoO2Cl2 in the chloride experi- ments.332 In the bromide experiments, using KBr aerosol particles, significantly smaller deposits inside the chromatog- raphy column were observed compared to the cases of the chloride experiments with KCl aerosol particles.333 As in earlier experiments, RfBr4 was more volatile than its lighter homolog HfBr4. The volatility of ZrBr4 was in-between that of HfBr4 and RfBr4. A similar picture emerged for the chlorides. Again RfCl4 was more volatile than HfCl4, while ZrCl4 turned out to be similarly volatile as RfCl4. The volatility of RfCl4 in comparison to its lighter homologs was once again investigated using the OLGA(III) setup.230 In this work, carbon aerosol particles were used, which were converted to CO2 in the hot reaction oven. In these experiments it was shown that the concentration of O2 is an important parameter, which has to be carefully controlled. fi The addition of O2 led to a signi cant shift of the volatility of both Hf and Rf to higher temperatures by about 100 to 200 °C, respectively, as shown in Figure 36, but this confirmed the 259 Figure 35. Measured and simulated deposition peaks of RfCl4 and 170,171 HfCl4. The modeled deposition peaks were obtained using the microscopic model of Zvara288 and a Monte Carlo simulation technique, with the only adjustable parameter being the adsorption enthalpy (for details see text).326

Further TC experiments with the nuclide 259Rf involving also the study of the tetrabromides were conducted much later (1991) and reported only in the form of a contribution to the Joint Institute for Nuclear Research, Laboratory of Nuclear Reactions annual report.340 A quantitative analysis of these experiments was reported.229 For the adsorption on quartz Δ 0(T) − · −1 Δ 0(T) surfaces, Ha (RfBr4)= 68 kJ mol and Ha (HfBr4)= − · −1 86 kJ mol were obtained. Again, RfBr4 seemed to be more volatile than the homologous HfBr4. A first experiment using IC of group-4 chlorides and bromides with direct identification of the reaction products α 225 with -particle and SF-spectrometry was published in 1992 Figure 36. Yields of 261Rfa and 165Hf as a function of isothermal using the OLGA(II) setup in combination with the MG temperature. Reactive gases were 200−300 mL/min HCl purified from 261 a □ ● rotating wheel detector. For these studies, the nuclide Rf traces of O2 ( , ) and 150 mL/min Cl2 with SOCl2 and 20 mL/min ≈ 88 α−α △ ◆ 230 (t1/2 1 m) was used, and altogether 14 correlations O2 ( , ), respectively. . Reprinted with permission from ref 230. attributed to the decay sequence Copyright 1998 Elsevier. αα 261Rfa →→ 257 No 253 Fm higher volatility of RfCl4 compared to HfCl4. In a more recent 341 were detected, yielding the correct t1/2 of the mother and the publication, the volatility of Zr, Hf, and Rf chlorides was daughter nuclides. So, this experiment yielded unambiguous investigated simultaneously. In this work, no significant proof that volatile complexes of Rf were isolated using gas- differences in the volatility of the three group-4 chlorides phase chromatography. In addition, the behavior of Rf could be were observed. Nevertheless, the T50% temperature of RfCl4 was 162 ° directly compared with that of Hf (t1/2 = 41 s), a nuclide with about 25 C lower compared to that of the longer-lived Hf at 261 a a very similar t1/2 compared to that of Rf . It was shown that the same isothermal temperature, yielding some indications of a group-4 chlorides RfCl4 and HfCl4 were more volatile than more volatile RfCl4. their respective bromides. In addition, RfBr4 was more volatile In the following, the available results on group-4 halides, than HfBr4 in the same experiment. Note that in earlier TC including Rf, have been analyzed in a meta analysis using the experiments the behavior of 3.1-s 259Rf was compared with that microscopic model of Zvara288 and a standard set of of 16-h 170Hf. Although the isothermal temperature profiles parameters. This was necessary since, in the original were far from being ideal and the column was too short in this publications, differing parameters were used and thus apparent first experiment, it was possible to extract thermochemical deviations occurred. In Table 2, the published and the 326 288 Δ 0(T) information by applying the microscopic model by Zvara. evaluated Ha values measured for group-4 tetrachlorides A further drawback was the usage of an aerosol particle gas-jet, and tetrabromides of Zr, Hf, and Rf in various experiments are Δ 0(T) which lead to visible deposits of KCl in the chromatography summarized. The table shows Ha values evaluated with a column, and thus the adsorption behavior was not studied on a consistent characteristic period of oscillation of SiO2 in the

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0(T) Table 2. Comparison of Published and Evaluated ΔHa Values Measured for Group-4 Tetrachlorides and Tetrabromides in Various Experiments

−Δ 0(T) · −1 −Δ 0(T) · −1 Ha publ (kJ mol ) Ha eval (kJ mol )

technique ref year aerosol or carrier gas ZrCl4 HfCl4 RfCl4 ZrCl4 HfCl4 RfCl4 − − a FC 62 1969 N2 84 84 84 96 84 84 100 109 c c b b TC 64 1971 N2 155 104 146 110 TC 340 1991 Ar 105c 83c cc c IC (OLGA II) 225 1992 He/KCl ≤70d ≤80d ≤75 ≤85 ± ± ± ± ± ± IC (HEVI) 332 1996 He/MoO3 74 5965775795 103 5825 IC (OLGA III) 230 1998 He/C 110c 92c 97e 103 87

ZrBr4 HfBr4 RfBr4 ZrBr4 HfBr4 RfBr4 TC 340 1991 Ar 82c 63c 86f 68f IC (OLGA II) 225 1992 He/KCl 125d 105d 130 111 IC (HEVI) 333 1996 He/KBr 91 ± 6 113 ± 5g 89 ± 5g 95 ± 6 117 ± 593± 5 aData from ref 343. bSee Figure 35. cData from ref 297; evaluation not possible due to missing experimental details. dData from ref 344. eUnpublished data by Türler et al. fSee also ref 229. gData from ref 345.

τ × −13 297 Monte Carlo model of 0 =2 10 s. Also shown are the halogenating agents, as was speculated by Zvara, but only Δ 0(T) ff Ha values originally published in the literature or the due to di erences in the analysis procedures. Actually, the Δ 0(T) 297 evaluated Ha values of Zvara. After analyzing the agreement between all experiments conducted on quartz 332 ̈ 230 230,332,340 experiments of Kadkhodayan et al. and Turler et al. surfaces is extraordinary. In all experiments, RfCl4 τ using a consistent 0, both experiments are in very good was found to be more volatile than HfCl4. The volatility of Δ 0(T) agreement concerning the Ha values of HfCl4 and RfCl4, RfCl4 seems to be more similar to that of ZrCl4, so that the ff Δ 0(T) ≈ while there are di erences in the Ha values for ZrCl4. The sequence in volatility is ZrCl4 RfCl4 > HfCl4. This result is −Δ 0(T) · −1 unpublished value of Ha (ZrCl4) = 97 kJ mol , which was surprising and was interpreted as a manifestation of relativistic evaluated in measurements with the OLGA(III) setup in test effects.230 Extrapolation procedures that rely on the regularities experiments with 98Zr under very similar conditions, is in very that govern the trends in the physicochemical properties within ff −Δ 0(T) good agreement with o -line TC experiments, where Ha the groups and periods of the Periodic Table clearly indicated · −1 342 (ZrCl4) = 98 kJ mol was determined. The use of MoO3 that RfCl4 should be less volatile than HfCl4. A high volatility of 332 ̈ 315,325 aerosol particles (Kadkhodayan et al. ) or C aerosols (Turler RfCl4 was only discussed after relativistic MO calculations et al.230) did not influence the adsorption characteristics of pointed to a low ionic and a high covalent contribution to the fi group-4 chlorides on the quartz surface. In the rst IC bonding in RfCl4. experiments with Rf of Türler et al.,225 only upper limits could For group-4 bromides, the agreement of the evaluated −Δ 0(T) −Δ 0(T) Δ 0(T) ff be established for Ha (HfCl4) and Ha (RfCl4). Due Ha values of di erent experiments is poor (see Table 2). to the very short isothermal length of the chromatography While all experiments consistently found RfBr4 to be more −Δ 0(T) columns, no decrease of the yield at low temperatures was volatile than HfBr4, the evaluated Ha values range from −Δ 0(T) · −1 observed. While the upper limit deduced for Ha (RfCl4)is 68 to 111 kJ mol . The fact that in OLGA(II) experiments in agreement with the values obtained in later experiments, about 15 kJ·mol−1 less volatile Hf- and Rf-bromides were HfCl4 was found to be much more volatile. The reason for the observed compared to experiments with HEVI could be observed behavior is not clear, and the experiment was not attributed to the use of a KCl aerosol gas-jet to transport the Δ 0(T) repeated at the time. The Ha values for HfCl4 and RfCl4 reaction products to the chemistry setup. Thus, the quartz evaluated from a TC experiment of Zvara et al.340 in a quartz column was at least partially covered with KCl. Generally, lower chromatography column are in very good agreement with the volatilities have been observed on KCl surfaces compared to Δ 0(T) 246 results of IC experiments. The Ha values of RfCl4 evaluated pure quartz. The two experiments using IC found RfBr4 and from the very early experiments at Dubna are in good HfBr4 to be less volatile than the corresponding tetrachlorides, agreement with each other, but compared to later experiments, while in the TC experiment RfBr4 and HfBr4 were considerably Rf-chlorides were about 25 kJ·mol−1 less volatile. However, more volatile. Considering macroscopic properties such as the these experiments were performed on different chromato- vapor pressure over the respective solid or the boiling point, graphic surfaces. The columns were made from glass into which ZrBr4 and HfBr4 are expected to be slightly less volatile than Δ 0(T) mica sheets were inserted. Also, the Ha values for HfCl4 are their corresponding tetrachlorides, as observed in both IC quite different in the two experiments, which may point to a experiments. In the TC experiment the possibility of two close varying content of residual O2 in the two experiments. The and thus unresolved deposition peaks was discussed, since the addition of O2 reduced the volatility of both Hf- and Rf- distribution of SF tracks was much wider than that of the Hf chlorides considerably in isothermal experiments of Türler et deposition peak.340 However, the Monte Carlo analysis of the al.230 (see Figure 36). experiment clearly showed229 that almost all data points are The introduction of a consistent characteristic period of contained in a 3σ error interval if only one deposition peak is τ ff oscillation 0 has reduced the apparent di erences in the assumed. The presence of two deposition zones for Rf- Δ 0(T) fi Ha values signi cantly. The reason for the disagreeing bromides is possible but is not corroborated by the measured Δ 0(T) Ha values originally published in the literature is therefore data due to the low statistics. In conclusion, the amount of data not mainly due to the varying degree of the chemical accumulated for RfCl4 and RfBr4 is impressive. In Figure 37 the modification of the column surface by the different evaluated adsorption enthalpy values for group-4 tetrachlorides

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appropriate oxycarboxylates, such as α-HIB, which appears to be the most selective. In the experiment by Silva et al.,268 261Rfa was produced by irradiation of only 47 μgof248Cm with 92 MeV 18O ions. Rf atoms recoiling from the target were swept with He from the recoil chamber through a nozzle and deposited on the surface of a rabbit which was coated with thin layer of NH4Cl. Periodically, the rabbit was transported to the chemistry apparatus, where the surface was washed with 50 μLofα-HIB solution (0.1 M, pH 4.0) on the top of a small, heated (80 °C) Dowex 50-X12 cation exchange column (2 mm i.d., 20 mm long). The solution was forced into the resin, and more eluent was added. The first two drops corresponding to the free column volume contained no or little radioactivity and were discarded. The next four drops (in 2 drop fractions) were collected on Pt disks, evaporated to dryness, and heated to 500 Figure 37. Δ 0(T) Evaluated adsorption enthalpies ( Ha ) of group-4 °C. The disks were assayed by α-particle spectroscopy. Group-4 tetrachlorides and tetrabromides from isothermal gas adsorption α chromatographic experiments with HEVI. elements Zr, Hf, and Rf were strongly complexed with -HIB and were thus not retained on the column and eluted far ahead of the actinides in the +3 or +2 oxidation state. A large part of and tetrabromides are shown.332,333 It is evident that the the chemical system was automated so that the average time from beam off to counting was reduced to about 60 s. Several adsorption properties of Rf halides do not follow simple linear α trends, which seem to be valid for the lighter homologs. hundred separations were performed, but only 17 -particles in the energy range from 8.2−8.4 MeV were registered; about half Whether the surprisingly high volatility of RfCl4 and RfBr4 is 257 definitely due to relativistic effects requires detailed calculations of these events are due to the decay of the 26-s No daughter. α−α of the interaction of the molecule with the adsorbent. This is In two experiments, a correlated pair was registered which 261 a 257 not an easy task as modifications of the surface by the reactive was attributed to the decay of Rf and its No daughter. π agents occur and the structure of real surfaces is very hard to Due to the limited 2 detector geometry, this number of integrate into the model calculations. Also, it is not clear if the correlated events is consistent with the total number of adsorption process on real surfaces is sufficiently approximated registered α-particles in the energy range 8.2−8.4 MeV. Silva 268 by the model of mobile adsorption. Since a detailed discussion and co-workers concluded that “for the particular cation of all these aspects is beyond the scope of this review, we refer exchange conditions used in these experiments, the behavior of here to the book by Zvara.240 the radioactivity assigned to element 104 with mass 261 is 9.2.2. Liquid-Phase Chemistry of Rutherfordium. The entirely different from trivalent and divalent actinide elements first investigations of Rf in the liquid phase were performed as but is similar to Hf and Zr as one would predict for the next early as in 1970 by Silva et al.268 The technique employed built member of the Periodic System following the actinide series of on the very successful application of cation-exchange elements”. However, the experiment was not yet efficient chromatography using the chelating agent α-HIB in discovering enough to determine Kd values. actinide elements. The elution position was indicative of the 9.2.2.1. Fluoride Complexation of Group-4 Elements Zr, ionic radius of the heavy actinides that were all present in the Hf, and Rf. The fluoride complexation of group-4 elements was − +3 oxidation state (except for No). The largest hydrated ions studied by various authors.266,272,346 350 HF is one of the best are eluted first. This effect can be enhanced by using media on an anion-exchange resin, because hydrolysis need not

ff Figure 38. (a) Sorption of Zr, Hf, Th, and Rf on the cation exchange resin Aminex A6 in 0.1 M HNO3 at various HF concentrations. O -line data are shown for Zr, Hf, and Th (open symbols) and online data for Hf and Rf (closed symbols), re-evaluated data from Strub et al.272 Reprinted with permission from ref 26. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH. (b) Sorption of Zr, Hf, Th, and Rf on the anion-exchange resin ̈ ff (Riedel-de Haen) in 0.1 M HNO3 at various HF concentrations. O -line data are indicated by lines, online data for Hf and Rf by symbols. Reprinted with permission from ref 272. Copyright 2000 Oldenbourg Wissenschaftsverlag GmbH.

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− be taken into account due to the high complexing strength of values of Hf and Rf were rising with decreasing NO3 fl 347,351 − the uoride ion. In early studies, Rf nuclides were not concentration. Apparently, the counterion NO3 is much directly detected. In the so-called multicolumn technique more effective in competing for binding sites on the anion (MCT), the reaction products, including 261Rfa, were trans- exchanger to remove Rf compared to Zr and Hf.272 ported to the chemistry setup with the aid of a KCl aerosol gas- In order to better understand the anion-exchange behavior of fi jet and continuously dissolved in 0.2 M HF. The solution was group-4 elements in mixed HF/HNO3 solutions, rst the passed through 3 ion-exchange columns. In the first cation- behavior in pure HF solutions of varying concentrations of exchange column, heavy actinide elements were adsorbed while 1.9−13.9 M HF was investigated by Haba et al.348 As it was 261 a 2− − Rf in the form of anionic RfF6 passed through and was observed that NO3 can compete as counterion for binding adsorbed on an anion-exchange column. A third cation- sites for Rf on the anion-exchanger and HF is a weak acid, the 253 253 + − − exchange column sorbed the descendants Fm and Es, equilibrium among HF, H ,F , and HF2 in aqueous solution which were eluted at the end of each experiment and measured has to be considered: off-line by α-particle spectroscopy. Detection of the descend- +− HF+⇄ HF (9.2.1) ants was regarded as proof of the formation of anionic fluoride 351,352 − − complexes. Pfrepper et al. refined the method and studied HF+⇄ F HF (9.2.2) fl 2 group-4 elements uoride complexation in mixed HF/HNO3 solutions and determined first K values. These were At an initial concentration of 0.4 M [HF]ini,the d concentration of the HF − anion starts to dominate compared indistinguishable for Hf and Rf. However, these results are at − 2 variance with recent results of Toyoshima et al.350 that will be to that of F and becomes by far the dominating anionic species discussed later. at concentrations >1 M [HF]ini. Indeed, decreasing Kd values The fluoride complexation of group-4 elements Zr, Hf, and are observed with increasing [HF]ini, which can be explained as the displacement of the metal complex from the binding sites of Rf, and of the pseudohomolog Th, in mixed 0.1 M HNO3/HF − 272 the resin by HF2 . Ignoring the knowledge of the activities of solutions was investigated by Strub et al. studying Kd values on both cation exchange resins and anion exchange resins using the chemical species involved, the adsorption equilibrium of an anionic complex An− with the charge state n− to the counterion ARCA. As expected, in cation exchange experiments below − − 3 3 HF2 between the resin phase R and the solution can be 10 M HF, the Kd values for Zr, Hf, and Th are >10 , indicating the presence of cations. At higher concentrations, represented by the equation −3 −2 fl n− − between 10 M and 10 M HF, neutral or anionic uoride nRHF22+⇄ A Rn A +n HF (9.2.3) species are formed and the Kd values are decreasing. The behaviors of Zr and Hf were very similar. Also, off-line and The equilibrium constant of the exchange reaction Dn can be expressed as follows: online data for Hf were consistent. Later, the Kd values measured for 261Rfa were slightly adjusted26,29,32 compared to ‐ n 272 [Rn A] [HF2 ] the original publication by applying a more accurate analysis Dn = n− n procedure. The separation of Zr, Hf, Th, and Rf on the cation- [A ] [RHF2 ] (9.2.4) exchange resin Aminex A6 in 0.1 M HNO3 at various HF For anionic complexes being present as single chemical concentrations is shown in Figure 38a. Much later, Ishii et species, the Kd value can be written as follows: al.353,354 reexamined K values for Zr, Hf, Th, and Rf on a d [R A] [RHF ]n cation-exchange resin in mixed HF/0.1 M HNO3 solutions as KD==n 2 fl d n−−n n function of the uoride ion concentration, fully validating the [A ] [HF2 ] (9.2.5) results of Strub et al.272 The observed behavior was 310 Taking the logarithm on both sides yields: theoretically predicted (see section 9.1). The Kd values should change in the following way in group-4: Zr ≤ Hf < Rf. [HF− ] 272,353 logKDn=− log log 2 This trend was indeed observed in the experiments. dn [RHF ] Investigations of the anion-exchange behavior showed some 2 (9.2.6) surprises. While the off-line data, as expected, clearly indicated The concentration of the counterion in the resin phase can the formation of anionic fluoride complexes for Zr and Hf be regarded as a constant value that is equal to the exchange −3 − ≈ above 10 M HF, the online data for Hf as well as Rf were capacity of the anion-exchange resin. Since log [HF2 ] log ff − di erent. There was almost no sorption of Rf on the resin [HF]ini 1.3, the charge of the anionic complex n can thus be −2 between 10 and 1 M HF, and the sorption of Hf was lower evaluated from a plot of log [HF]ini versus log Kd as shown in compared to the case of off-line experiments, as displayed in Figure 39. The adsorption of Rf−fluoride complexes on the Figure 38b. anion-exchanger is clearly weaker compared to that of the The unusual result concerning the anion-exchange behavior lighter homologs Zr and Hf. Also, the slope of the curve of −2.0 of Rf-fluorides, which would be in contradiction to earlier ± 0.3 for Rf clearly differs from the slope of −3.0 ± 0.1 for Zr fl fl 2− results, called for a detailed investigation of the in uence of the and Hf. The associated anionic uoride complexes are RfF6 − 2− 3− 3− fi counterion NO3 . The trend for the formation of MF6 (eq and ZrF7 or HfF7 , respectively. For the rst time, a 9.1.3) should be reversed in group 4: Rf ≥ Zr > Hf310 (see also significant deviation of the chemical behavior of a transactinide discussion at the end of this section). Such a trend was, indeed, element compared to its lighter homologs was observed. observed in the experiments on the AIX separations of group-4 Building on the work of Strub et al.,272 Toyoshima et al.350 elements from 0.02 M HF.355 A similar result was obtained have investigated the fluoride complexation of group-4 350 2− later, where the formation constant of RfF6 was reported elements in mixed HF/HNO3 solutions in the concentration 2− − − to be at least 1 order of magnitude smaller than those of ZrF6 ranges of 0.0054 0.74 M HF and of 0.010 0.030 M HNO3.In 2− ff and HfF6 . When the HNO3 concentration was lowered to o -line static distribution experiments, the behavior of Zr and 0.01 M and the HF concentration was chosen as 0.05 M, the Kd Hf was studied as a function of the equilibrated concentration

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348 formed. In Figure 40b, Kd values of Zr, Hf, and Rf are shown as a function of [F−] . The increasing K values of Rf, Zr, and eq − d − Hf with increasing [F ]eq under constant [NO3 ]eq indicate that the anionic fluoride complexes are increasingly formed. The formation reactions are represented as follows: 4(−−n 1) − 4 −n MFFMn−1 +⇄F(16n n =−)(9.2.7) where M indicates Zr or Hf and n denotes the coordination number in the products. In anion exchange, only anionic fl − 2− uoride complexes of MF5 and MF6 participate in the − 2− adsorption desorption process, with MF6 being the domi- nant species, as confirmed by the slope analysis. The sudden drop of the K values above [F−] >5× 10−3 M was associated d eq− with the abrupt increase of [HF2 ]eq. The solid, dashed, and ffi dotted curves in Figure 40b are results of calculations where Kd Figure 39. Variation of the distribution coe cient, Kd, of Rf, Zr, and is related to the consecutive formation of the fluoride Hf on the anion-exchange resin CA08Y as a function of the initial HF complexes of Zr and Hf in mixed HF/HNO3 solutions and concentration, [HF]ini. The Kd values of Rf, Zr, and Hf are shown by − diamonds, squares, and circles, respectively. Open and closed symbols the onset of the additional counterion HF2 . depict different column dimensions. Linear relationships with slopes of As was mentioned in section 9.1, the trend in the extraction − ± − ± 2.0 0.3 for Rf and 3.0 0.1 for Zr and Hf in the log [HF]ini of group-4 species becomes complicated depending on pH and − ··· versus log Kd are indicated by solid ( ) and dotted ( ) lines, other experimental conditions. For very dilute HF solutions, 348 − ≥ respectively. the trend in the extraction of MF6 was predicted as Rf Zr > Hf.310 Such a trend was observed in the experiments on the − − − 355 of free F ([F ]eq) at concentrations of [NO3 ]eq of 0.01, 0.03, AIX separations of group-4 elements from 0.02 M HF. The 0.1, and 0.3 M. In column experiments using AIDA, K values assumed formation and extraction of the ZrF 3− or HfF 3 − d 7 7 for Rf were measured at [NO3 ]eq of 0.01 and 0.015 M. It was complexes at higher HF concentrations in comparison with shown that, at concentrations of [F−] <5× 10−3 M, the RfF − studied experimentally353 have not yet been considered eq − − 6 contribution of the additional counterions F and HF2 present theoretically. However, the suggested lower coordination fl in the HF/HNO3 solutions is negligible. A slope analysis in the number (CN) of Rf equal to 6 in the uorine complexes in − fi − × −3 log [NO3 ]eq versus log Kd plot at a xed [F ]eq =3 10 M comparison with CN of 7 in complexes of Zr and Hf can hardly consistently resulted in an anionic charge of −2 for Zr, Hf, and be expected, since the Rf ion is larger than those of Hf and Zr: 2− 4+ 321−323 176 Rf, indicating the presence of MF6 complexes (see Figure that is, the IR (CN = 6) of Rf (0.76 Å or 0.79 Å )is 40a). However, as already observed in the pure fluoride system, larger than the IR of Zr4+ (0.72 Å) and Hf 4+ (0.71 Å)317 (see − − the complexation of Rf was again much weaker compared to section 9.1). Also, the competition between the NO3 and F ff that of Zr and Hf, as the Kd values di ered by almost 3 orders ions in the complex formation has not been treated of magnitude. This result is in contrast to the experiments by theoretically. Thus, this complex aqueous and extraction Pfrepper et al.351,352 The difference was attributed to incorrect behavior of group-4 species at various experimental conditions assumptions about the breakthrough volume of trivalent needs further theoretical considerations. actinides,351,352 which was probably a factor of 10 lower than 9.2.2.2. Chloride Complexation of Group-4 Elements Zr, anticipated,26,32 based on newer experimental data.356 Ob- Hf, and Rf. The chloride complexation and hydrolysis of group- fl 2− viously, at low uoride concentrations, Zr and Hf form MF6 4 elements including Rf was studied with a variety of methods. complexes, whereas, at higher concentrations of HFini, where The formation of group-4 anionic-chloride species of the form − 3− 2− HF2 becomes the dominant species, MF7 complexes are MCl6 (M = Zr, Hf, Rf) was observed on the one hand by the

ffi Figure 40. (a) Distribution coe cients, Kd, of Zr and Hf under static conditions (open symbols) and those of Zr, Hf, and Rf from column − − × −3 chromatography (closed symbols) as a function of [NO3 ]eq and [F ]eq = 3.0 10 M. Reproduced with permission from ref 350. Copyright 2008 Oldenbourg Wissenschaftsverlag GmbH. (b) Variation of the Kd values of Zr and Hf under static conditions and of Rf in column chromatography as − − − a function of [F ]eq. Values for Zr and Hf are shown for [NO3 ] = 0.01, 0.03, 0.1, and 0.3 M. Values for Rf are shown for [NO3 ] = 0.01 and 0.015 M. The solid, dashed, and dotted curves are theoretical calculations of Kd values. Reproduced with permission from ref 350. Copyright 2008 Oldenbourg Wissenschaftsverlag GmbH.

1270 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review adsorption on anion-exchange chromatography columns from investigated in 4.0−11.5 M HCl. The results of these >7 M HCl solutions357 and by their extraction into a quaternary experiments clearly showed that at concentrations below 8 M 269 amine in reversed phase chromatography and into TiOA HCl the Kd values of Zr and Hf are similar, indicating the (triisooctylamine)263 in liquid−liquid extractions. formation of cationic or neutral species of the type + Using a fully automated solvent extraction chromatography M(OH)2Cl , M(OH)2Cl2, and M(OH)Cl3 (M = Zr, Hf). At 269 apparatus, Hulet et al. investigated the chloride complexation concentrations above 8 M HCl, the Kd values of both Zr and Hf of Rf in comparison to Hf, Cm, and Fm. On the basis of their increase steeply with increasing HCl concentration, indicating 2− 2− tendency to form strong anionic-chloride complexes in 12 M that anionic species such as M(OH)Cl5 and MCl6 are HCl, group-4 elements were separated from group-1, 2, 3, and formed. The behavior of the pseudohomolog Th is similar to actinide elements by extraction chromatography into a that of Zr and Hf at HCl concentrations below 8 M; at higher quaternary amine (Aliquat-336 on an inert support). Anionic- concentrations, the Kd values strongly deviate, as Th does not chloride complexes were thus extracted and retained on the form anionic chloride species. In online experiments at 6 column while i.e. actinide elements passed through. Sub- different HCl concentrations between 4.0 and 11.5 M HCl, the sequently, group-4 elements were eluted with 6 M HCl. extraction behavior of Rf was studied. At each measured Altogether, in 44 chemistry runs, only 6 α-particle decays of concentration, up to 400 separations were performed, resulting 261Rfa and its daughter 257No were registered, including one in a total of 186 α-particles attributed to 261Rfa and its daughter correlated mother−daughter pair. Therefore, Hulet et al.269 257No including 35 α−α correlations. The resulting adsorption concluded that “in 12 M HCI solutions the chloride curve as a function of HCl concentration is shown in Figure 41. complexation of element 104 is clearly stronger than that of the trivalent actinides and is quite similar to that of Hf, which is expected to be its homolog in the Periodic Table”. More than 10 years later, liquid-phase chemical separations of Rf were picked up again at LBNL.263,265,358 These were manually performed separations, as described in section 7.3.1. In the beginning only the organic phase was assayed, which turned out to be a source of systematical errors.26,32,265,359 Liquid−liquid extractions from 12 M HCl into TiOA263 corroborated earlier results of Hulet et al.269 Cationic species were extracted into TTA (thenoyltrifluoroacetone).360 The measured distribution coefficient for Rf in comparison to those of the pseudohomologs Th and Pu indicated that Rf is less affected by hydrolysis than Zr, Hf, and Pu.360 fi ≈− The rst hydrolysis constant log K11(Rf) 4 was predicted theoretically on the basis of the 4c-DFT calculations for the group-4 hydrated and hydrolyzed complexes,307 in good agreement with the experimental value of −2.6 ± 0.7.360 The predicted trend Zr > Hf > Rf is also in agreement with the Figure 41. Variation of %ads values of carrier-free radionuclides of Zr, experimental data for Zr and Hf, giving log K11(Zr) = 0.3 and Hf, and Rf on the anion-exchanger CA08Y as a function of HCl − 301 log K11(Hf) = 0.25. One should note here that a simple concentration. Reproduced with permission from ref 357. Copyright model of hydrolysis301 based on the ratio of a cation charge to 2002 The Japan Society of Nuclear and Radiochemical Sciences. its size would give an opposite and, hence, a wrong trend from 317 Zr to Hf, since IR(Zr4+) > IR(Hf4+). The %adsorption values of Rf increase rapidly with increasing 358 Bilewicz et al. studied the onset of hydrolysis at decreasing HCl concentration from 7.0 to 9.5 M, very similar to Zr and Hf, HCl concentrations by sorption of Zr, Hf, Th, and Rf on indicating that anionic chloride complexes such as 2− 2− ferrocyanide surfaces, which are known to be selective sorbents Rf(OH)Cl5 or RfCl6 are formed. Between 7 and 9 M for singly charged cations such as Rb+ or Cs+, but also for HCl, the extraction sequence is Rf > Zr > Hf, probably tetravalent metal ions such as Zr4+,Hf4+, and Th4+. This result indicating a decreasing chloride complexing strength in the was in contrast to the findings of Czerwinski et al.360 and same order.357 This result cannot find its theoretical attributed by Bilewicz et al.358 to relativistic effects which explanation, as the theory claims that the complex formation predict that Rf4+ would be more prone to having a CN of 6 at 4−8 M HCl should be continued with Rf, e.g. having the rather than 8 in aqueous solutions due to a destabilization of trend Rf: Zr > Hf > Rf.307 the 6d5/2 shell. The latter supposition can, however, not be In order to elucidate possible differences in the chemistry of supported by the relativistic calculations that show that both Rf compared to its lighter homologs, a series of liquid−liquid the 6d3/2 and 6d5/2 AOs take part in the bond formation of the extractions into TBP (tributylphosphate) in benzene to study Rf compounds. the effect of HCl, Cl−, and H+ concentration between 8 and 12 The chloride complexation and hydrolysis of group-4 M on the extraction of Zr4+,Hf4+, and Rf4+, as well as the elements using anion-exchange chromatography was studied pseudohomologs Pu4+ and Th4+, was conducted.264 TBP is one in much more detail by Haba et al.357 First, in batch of the most important organic extractants and widely used in 88 experiments Kd values of carrier-free radiotracers of Zr and reprocessing of spent , and it extracts the metal in 175Hf and the pseudohomolog 234Th were determined on the the form of a neutral species. The phosphoryl oxygen anion-exchange resin CA08Y in 1.0−11.5 M HCl. Second, in coordinates with the metal ion by forming an adduct. The online experiments using the fully automated AIDA apparatus, TBP extraction process of group-4 elements in HCl can be the anion-exchange behavior of 85Zr, 169Hf, and 261Rfa was expressed as follows:

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4+− M(H28 O)+⇄ 4Cl MCl4(aq) + 8H 2O (9.2.8)

MCl4(aq)+⇄ 2TBP (aq) MCl 4 (TBP) 2(org) (9.2.9) The extraction of group-4 elements into TBP depends on the strength of the chloride complexation and the stability of the TBP complex. In the experiments by Czerwinski et al.,264 an extraction sequence Zr > Rf > Hf for the group-4 chlorides was established. In additional experiments by Kacher et al.,265 it was observed that significant adsorption of Hf on the Teflon surfaces occurred. Subsequently, all equipment was changed to polypropylene equipment. The extraction sequence from 8 M HCl into TBP/benzene was revised to Zr > Hf > Rf > Ti. A critical evaluation of these experiments is discussed in detail by Kratz.26,32,359 On the one hand, the observed sorption of group-4 elements on Teflon that went unnoticed in the work Figure 42. Percent extractions (%Ext) of Rf, Zr, and Hf on the 20 wt of Czerwinski et al.264 and the suspected slow kinetics in the % TBP/CHP20Y resin as a function of HCl concentration. work of Bilewicz et al.358 may have contributed to partly Reproduced with permission from ref 361. Copyright 2007 Old- conflicting results and warranted improved experiments to enbourg Wissenschaftsverlag GmbH. determine Kd values of group-4 elements by measuring the distribution in both liquid and organic phases. ions, and the sequence of complexing strength, one would ̈ 271 Gunther et al. have investigated the Kd values of the expect that also in the sulfate system Rf forms significantly liquid−liquid extraction of group-4 elements, including Rf, into weaker complexes compared to those of Zr and Hf under ffi undiluted TBP using reversed phase column chromatography identical conditions. First, distribution coe cients, Kd, of Zr, and the ARCA II apparatus. Group-4 elements were fed onto Hf, and Th between a cation-exchange resin and a 0.0018 − + undiluted TBP/Voltalef (an inert support) columns in 12 M 0.69 M H2SO4/HNO3 mixed solution ([H ] = 1.0 M) by a HCl and quantitatively adsorbed. While 75% Hf and no Zr batch method using the long-lived carrier-free radiotracers 88Zr, were eluted in a first fraction with 8 M HCl, the remaining Hf 175Hf, and 234Th were measured. The online chromatographic and 93% Zr were stripped in a second elution in 2 M HCl. The behavior of Zr and Hf was then studied to obtain the distribution of Rf in both fractions was measured. In the Hf adsorption probability on the resin as well as elution curves of α−α 261 a fraction, two correlated pairs of Rf were observed, and these elements with the simultaneously produced, short-lived three were observed in the Zr fraction, indicating an extraction isotopes 85Zr and 169Hf. Finally, cation-exchange experiments of Rf in-between Hf and Zr. The extraction trend of group-4 with 261Rfa were performed together with 169Hf in 0.15−0.69 M elements from undiluted TBP was established as Zr > Rf > Hf + 362 H2SO4/HNO3 mixed solutions ([H ] = 1.0 M) using AIDA. at 8 M HCl. The determined Kd values and the extraction As can be seen in Figure 43, adsorption of Rf, Zr, and Hf on the sequence were at variance with earlier results and conclusions − 264,265 cation-exchange resin decreases with an increase of [HSO4 ], concerning the extraction of Hf and Rf into TBP. Such an representing the successive formation of sulfate complexes of inversion of the trend is consistent with the theoretical trend 307 these elements. While Rf shows the same functional shape as for the formation of the MCl4 species. Hf, the decrease for Rf is shifted to about 0.15−0.20 M higher Recent, very detailed experiments by Haba et al.361 − [HSO4 ]. This indicates that the sulfate complex formation of concerning the extraction behavior of group-4 elements into Rf is significantly weaker than that of the lighter homolog TBP at different HCl concentrations using the AIDA apparatus largely confirmed the results obtained by Günther et al.271 The %extraction values (%Ext) of Zr, Hf, and Rf are plotted in Figure 42 as a function of HCl concentration. Within the error limits, the extraction sequence between 7.2 and 8.0 M HCl into TBP is Zr > Hf ≈ Rf. Although Günter et al.271 determined at 8 M HCl an extraction sequence Zr > Rf > +64 Hf with Kd values of 1180, 150−46, and 64, respectively, also in this work Rf and Hf are much closer compared to Zr. More interesting than these small differences is the fact that Rf is not extracted ahead of Zr. The fact that Zr is extracted at lower HCl concentrations than Hf is corroborated by the larger complex formation strength of Zr as observed in anion-exchange experiments357 and also EXAFS studies.281 Some further calculations for the MCl4(TBP)2 complexes should be performed to study this case in more detail. 9.2.2.3. Sulfate Complexation of Group-4 Elements Zr, Hf, 261 a and Rf. 2− Figure 43. Variation of Kd values of Rf on a cation-exchange resin The sulfate ion SO4 is a strong complexing ligand for − derived from its adsorption probabilities as a function of [HSO4 ]in group-4 elements. Its strength to form complexes with Zr and − + − − − 0.0018 0.69 M H2SO4/HNO3 mixed solutions ([H ] = 1.0 M), Hf is intermediate between those of F and Cl ions, i.e. F > 88 175 234 − − − together with those of Zr, Hf, and Th obtained in batch SO 2 >Cl ≥ NO . Taking into account the experimentally experiments. Reproduced with permission from ref 362. Copyright 4 3 − − − studied complexing behavior of Rf with F ,Cl, and NO3 2012 Oldenbourg Wissenschaftsverlag GmbH.

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Hf,362 very much similar to the fluoride complexation reduction of the metal, so that an increase in this energy means behavior272,353,354 (see also Figure 38a). The observed trend an increase in the stability of the 5+ oxidation state in the is in good agreement with the predicted one from the 4c-DFT group. This is also shown by the reduction potentials E°(V− calculations of various sulfate complexes (see section 9.1 and IV) for MCl5 (M = V, Nb, Ta, and Db) estimated using a Figure 32).310 correlation with the energies of the charge-transfer transitions SISAK experiments on liquid−liquid extractions of Zr, Hf, (Figure 44).38,366 Thus, nonrelativistically, Db5+ would have 363 5+ and Rf from H2SO4 solutions by trioctylamines (TOA) been even less stable than Nb . (Similar correlations can be confirmed the predicted trend in the complex formation, Zr > shown for compounds of group-4 to group-8 elements). Hf > Rf, and have given Kd(Rf) values close to the predicted ones.310

10. DUBNIUM (Z = 105) 10.1. Theoretical Predictions The predicted ground state electronic configuration of Db is 6d37s2 based on the results of the DF122 and MCDF177 calculations. This suggests that the element will have the maximum oxidation state 5+ as its lighter homologs Nb and Ta. The MCDF calculations have also given the first IP as 7.37 eV (corrected value obtained by an extrapolation procedure177), which is smaller than the IP of Ta (7.89 eV)313 with the 6s ionized electron. However, the energies of the 6d(Db) and 6s(Ta) AO are very similar.122 All multiple IPs(M → MZ+) were also calculated there.177 The IPs(M → M5+) were shown to decrease from V to Db, due to the destabilization of the − Figure 44. Correlation between reduction potentials E°(V−IV) and (n 1)d AOs, so that the stability of the maximum oxidation → 5+ energies of the lowest charge transfer transitions E[3p(Cl) state should increase toward Db (see Figure 28). The IR(M ) − 317 (n 1)d(M)] in MCl5 (M = V, Nb, Ta, and Db). The nonrelativistic increase in group 5 with Z, as was shown earlier by the fi 5+ value for Db is shown as a lled circle. Reprinted with permission from MCDF calculations (0.74 Å for Db in comparison with 0.64 Å ref 38. Copyright 1999 World Scientific. 5+ 5+ 177 − for Nb and Ta ), because Rmax values of the outer (n 1)p 5+ AOs in M increase in this direction (see Figure 28). Later on, Relativistic effects were also shown to be responsible for a molecular calculations confirmed such an increase in IR and 5+ 327,364 trend to a decrease in ionicity (QM) and an increase in gave a more accurate value for IR of Db of 0.69 Å. Such covalency (OP) (Figure 45). A partial OP analysis (Figure 46) an increase is also in line with an increase in CR.319,320 On the basis of the MCDF calculated multiple IPs, redox potentials were estimated for Db and its homologs in group 5.177,365 It was indeed shown that the stability of the pentavalent state increases from V to Db, while that of the tetra- and trivalent states should decrease. Thus, for example, the stability of the M3+ should decrease from V3+ to Db3+, because the ground state of the Db3+ ion is not 7s2, as was assumed earlier, but 6d2. MO calculations were performed mostly for chemically interesting stable compounds of Db and its group-5 homologs in the highest oxidation states. Thus, the electronic structures of the following molecular compounds were studied with the use of the relativistic DFT method: MCl5, MOCl3, MBr5, and 330,331,364,366,367 MOBr3 (M = Nb, Ta, and Db). RECPs were 327 applied to TaBr5 and DbBr5. Various properties, such as optimal geometries, bonding, IPs, polarizabilities, dipole Figure 45. Relativistic (rel) and nonrelativistic (nr) effective charges, QM, and overlap populations, OP, in MCl5 (M = V, Nb, Ta, and moments, and charge density distribution, were predicted. 367 According to the calculations, Db should be a typical d-element Db). L denotes the ligand. Reprinted with permission from ref 367. where bonding is defined by the participation of the 6d AO and Copyright 1993 American Institute of Physics. 7s AO. There is also a slight admixture of the relativistically stabilized 7p1/2 AO. The influence of relativistic effects on the electronic structure shows that such an increase in covalency (total OP) is due to of group-5 d-element compounds was studied on the example the increasing contribution of the relativistic ns1/2, np1/2, and 367 ff − ff of MCl5 (M = Nb, Ta, and Db). Relativistic e ects were (n 1)d AOs. Thus, relativistic e ects are responsible for the shown to increase the HOMO-LUMO gap, ΔE, due to the continuation of trends in IP, EA, and stabilities of oxidation relativistic destabilization of the (n−1)d AOs. This results in a states in the groups in going over to the 6d elements. (The SO fi ff decrease in the EAs, de ned by the LUMO, and an increase in e ects are, however, responsible for a trend reversal in De; see the energies of the electron charge-transfer transitions, below.) The nonrelativistic description of these properties E[3p(Cl)→(n−1)d(M)]. The latter is associated with the would give opposite and, therefore, wrong trends.

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Figure 46. Relativistic (rel) and nonrelativistic (nr) partial overlap populations in MCl5 (M = Nb, Ta, and Db). L denotes valence orbitals of the ligand. Reprinted with permission from ref 367. Copyright 1993 American Institute of Physics.

The De in the Db molecules turned out to be lower than De of the Ta homologs due to a decrease of the ionic contribution to bonding, although the covalent contribution increases with Z 366 ff from Ta to Db. Also, SO e ects decrease De, as was shown 327 by the RECP calculations for TaBr5 and DbBr5. The bond lengths of the Db molecules are typically about 0.05 Å larger 327,364 than the Re of Nb and Ta homologs, in agreement with the IR and CR.319,320 Dipole moments should increase from the Nb to Db oxyhalides due to the increase in the interatomic distances.331 Using the calculated properties of the group-5 molecules, predictions of the volatility of halides and oxyhalides were attempted.315,330,331,364,366,367 Volatility of the oxyhalides, as either a sublimation process or adsorption on a surface, should decrease in group 5 with increasing Z due to an increase in the dipole moments of MOCl3 causing their stronger attraction to Figure 47. Formation of MBr − on the KBr surface (M = group-5 the surface.331 Volatility of the pure pentahalides is, however, a 6 fi elements). Reprinted with permission from ref 364. Copyright 2012 more complex process and depends on the de nition of this American Institute of Physics. phenomenon and the nature of the interactions. Thus, for 5+−+ example, as a sublimation process, volatility should increase in M(H O)⇄+ M(OH) 6H (10.1.1) group 5, as was shown by the calculations of intermolecular 26 6 330 interactions. It was shown that DbBr5 should have a higher Hydrolysis of the Nb, Ta, Db, and Pa (for comparison 330 Pmm over the solid than its lighter homologs. The same purposes) cations was studied theoretically on the basis of 4c- 303 higher volatility of DbBr5 in comparison with the homologs was DFT calculations of the components of reaction 10.1.1. The predicted on the basis of calculations of the molecule−inert calculated relative ΔEC values for this reaction are indicative of 364 surface interaction energies (eq 8.1.1). Such a decrease in the the following trend in hydrolysis of group-5 cations: Nb > Ta > molecule−surface binding energy is caused by the increasing Db ≫ Pa. This sequence is in agreement with experiments on molecular size related to the separation distance x with Z (even hydrolysis of Nb, Ta, and Pa.301 A simple model of 301 though polarizabilities are about the same for TaBr5 and hydrolysis does not reproduce the difference between Nb Δ 0(298) DbBr5). This trend is in agreement with HS of and Ta having equal IR. The present model (section 8.2) based Δ 0(298) macroamounts of NbBr5 and TaBr5, so that HS of on the real (relativistic) distribution of the electronic density DbBr5 obtained on the basis of the correlation with E(x) should correctly describes the experimental observations. be smaller than those of the lighter homologs.364 In solution, for example HCl, a large variety of complexes, ff − − 2− − The trend in volatility of MBr5 should, however, be di erent such as M(OH)2Cl4 , MOCl4 , MOCl5 , and MCl6 (M = if adsorption takes place via a chemical bond formation on a Nb, Ta, Db, and Pa) can be formed with different degrees of surface, for example, on quartz modified with KCl or KBr. The hydrolysis according to the following equilibrium 4c-DFT calculations364 have shown that, in this case, complexes − −− (6−−i ) of the type MBr5L (L = Br and Cl) are formed on the KCl/ M(OH)66+⇄i L MOuui (OH)−2 L (10.1.2) KBr surface (Figure 47) and their strength changes as Nb < Ta < Db. This means that volatility should change in an opposite Their stability and energy change of the complex formation way in group 5, e.g., Nb > Ta > Db. reactions were predicted theoretically on the basis of the 4c- A number of studies were devoted to predict the extraction DFT calculations.304,305 The obtained data suggest the and ion exchange behavior of Db as well as other group-5 following trend in complex formation in group 5: Pa ≫ Nb homologs from HF, HCl, and HBr solutions.304,305 Like group- > Db > Ta. Taking into account the association with an organic 4 cations, also group-5 ones undergo hydrolysis according to cation, the following trend was predicted for the sorption of the reaction group-5 complexes by an anion exchanger

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Figure 48. Distribution of SF tracks attributed to the SF decay of a Db isotope. These events were registered in three separate experiments (a, b, and c). The deposition zones of other elements are also indicated. Reprinted with permission from ref 58. Copyright 1987 Oldenbourg Wissenschaftsverlag GmbH. Figure adapted from Zvara et al.65

Pa≫≥> NbDbTa (10.1.3) the temperature gradually decreased to 50 °C. The reaction 22Ne + 243Am was used to produce 261Db, a nuclide decaying Thus, complexes of Pa should be formed in much more α mainly by -particle emission with a t1/2 = 1.8 s. The branching dilute HCl solutions, while much higher acid concentrations are ratio for SF was determined to be <18%.373 A total of four needed to form those of Ta. The calculations also predicted the individual experiments were performed. In the first experiment following sequence in the formation of various types of at 114 MeV beam energy using SOCl as chlorinating agent, no complexes as a function of the acid concentration: 2 − − − SF tracks were observed in the column. In the next three M(OH)2Cl4 >MOCl4 >MCl6 , in agreement with experiments the beam energy was raised to 118−119 MeV, and experimental data for Nb, Ta, and Pa. The calculations also − − − in addition to SOCl2 also TiCl4 was added as reactive agent. In reproduced the MF6 > MCl6 > MBr6 sequence as a function these experiments a total of 18 SF tracks were observed in the of the type of ligand. 304,305 column, which were attributed to the decay of a Db isotope Theoretical investigations have shown that the trend in (Figure 48). Later studies were conducted in a brominating gas complex formation and extraction (sequence 10.1.3) known for medium and again yielded evidence that Db bromide is less Nb, Ta, and Pa turned out to be reversed in going to Db. This volatile compared to the homolog compound with Nb.67 could not have been predicted by any extrapolation of this A quantitative analysis of these experiments was not made at property within the group, which would have given a the time. The location of the SF tracks coincided roughly with continuous and, hence, wrong trend, but this came as a result the position where 170Hf was deposited, whereas 90Nb was of the relativistic electronic structure calculations for the real deposited further downstream, at the very end of the column. chemical equilibrium. As will be shown below, the first Applying the microscopic model of Zvara,288 the distribution of experiments on the AIX separation of the group-5 elements −Δ 0(T) ff SF tracks can best be described with Ha (Db-chloride) = from mixed HF/HCl solutions have given a di erent trend than · −1 −Δ 0(T) 100 kJ mol . For Nb resulted Ha (Nb-chloride) = 96 theoretically predicted, where Db was found in the Nb/Pa −1 368 kJ·mol . The influence of the different t values on the fraction. In view of this disagreement, a recommendation 1/2 304 deposition temperature is clearly apparent. Since both the was made to repeat the AIX separations in either pure HCl or HF solutions. Accordingly, amine separations of the group-5 pentachlorides as well as the oxytrichlorides of group-5 elements were systematically redone by Paulus et al.277 A elements Nb and Ta are thermodynamically stable compounds, reversed extraction sequence Pa > Nb ≥ Db > Ta, as that no conclusions about the speciation of the observed compounds can be drawn. predicted theoretically (sequence 10.1.3), was then observed Δ 0(T) (see below). The fact that nearly identical Ha values were derived for compounds of Nb and, presumably, Db cannot be interpreted 10.2. Experimental Results as positive proof that indeed a Db isotope was chemically As homolog of the group-5 elements Nb and Ta, also Db is isolated. Due to the absence of a long isothermal section, a very expected to behave chemically like a typical transition element. similar distribution of SF tracks can, in principle, result from the Early chemical investigations of single atoms of Db were deposition of a long-lived SF radioactivity such as 256Fm. restricted to rapid gas-phase chemical investigations due to the Indeed, the separation from long-lived actinides was not 261 246 relatively short t1/2( Db) = 1.8 s that was accessible in the particularly good; between 2.2 and 8.5% of Cf was found in 22Ne + 243Am reaction. Only when sufficient amounts of the the gradient section (detectors 1−27).65 That the separation 249 ffi very rare and short-lived target material Bk (t1/2 = 320 d) from actinide nuclides could have been insu cient is also became available was the longer-lived 34-s 262Db synthesized in apparent from later experiments with 262,263Db produced in the the reaction 249Bk(18O, 5n).369,370 Therefore, the first aqueous reaction 18O+249Bk.374,375 In these experiments, the deposition phase chemistry experiments were only conducted in 1988. peaks of 262,263Db were observed on top of a background of Indeed, 262Db can also be produced in the reaction 248Cm(19F, hundreds of SF tracks in the column. Again 5−10% of all Fm 5n), however with lower production rates.371,372 nuclei were flushed into the gradient section of the column. 10.2.1. Gas-Phase Chemistry. A similar apparatus as Isothermal experiments with Db-halides were conducted 262 263 shown in Figure 34 was used as early as 1970 to study volatile with the longer-lived Db-nuclides Db and Db (t1/2 =34 chlorides of element 105, Db.65 Isothermal section II was and 27 s, respectively). In a first experiment, employing the 18O missing: the chromatography column consisted of sections I + 249Bk synthesis reaction and the OLGA(II) setup in (30 cm) and a longer temperature gradient section III (120− conjunction with the moving tape detection system,225,226 130 cm). Section I was heated to 300 °C, whereas in section III only SF decays could be used as signal for the decay of a Db

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0(T) Table 3. Comparison of −ΔHa Values Evaluated for Group-5 Halides and Oxyhalides in Various Experiments

−Δ 0(T) · −1 Ha (kJ mol )

technique ref year aerosol material NbCl5 NbOCl3 TaCl5 TaOCl3 DbCl5 DbOCl3 TC 378 1973 85a 88a TC 374 1991 95a 118a ± b ± b ± b IC (HEVI) 376 1993 MoO3 75 5 157 12 76 10 IC (OLGA III) 231 1996 C 80 ± 199± 1 ≤97 117 ± 3

NbBr5 NbOBr3 TaBr5 TaOBr3 DbBr5 DbOBr3 TC 378 1973 87a 82a TC 374 1991 83a 108a IC (OLGA II) 226 1992 KCl 93 ± 4c 101 ± 4c 121 ± 11c IC (OLGA III) 377 2012 KBr 89 ± 5 155 ± 5 103 ± 571± 5 a 297 b τ × −13 376 c ̈ 226 τ × −13 231 Evaluated data. Reanalyzed data with 0 =2 10 s. Data from Gaggeler et al. reanalyzed with 0 =2 10 s. nuclide, since these experiments were severely hampered by concluded that while the pure bromides are observed α comparably large -particle emitting Po-activities, which experimentally, also DbBr5 should be formed, since Db showed − ffi 364 Δ 0(T) obscured the interesting energy range 8.40 8.60 MeV, where the lowest a nity toward oxygen. The Ha values for Nb- α-decays of 262,263Db and their 258,259Lr daughters were and Db-chlorides evaluated from TC experiments on a quartz 374 Δ 0(T) expected. The volatility of group-5 bromides decreased in the surface are in very good agreement with the Ha values sequence Nb ≈ Ta < Db. Later, the HEVI setup and the MG- measured for oxytrichlorides in OLGA experiments.231 Also, Δ 0(T) rotating wheel detector were used to investigate the volatility of Ha (NbCl5) evaluated from HEVI experiments of Kadkho- group-5 chlorides.376 This part of the PhD-thesis of dayan376 is in agreement with the value measured by Türler et 231 Kadkhodayan is unpublished and has only appeared as an al. In experiments with Ta, only TaOCl3 was obtained. The internal Lawrence Berkeley Laboratory report. In this work the nuclide 262Db was positively identified after chemical separation nuclide 262Db was unambiguously identified after chemical in experiments with HEVI376 at isothermal temperatures as low separation by observing seven correlated α−α correlations. The as 250 °C. A measurement at 100 °C isothermal temperature α 262 fi ° -particle energies as well as the deduced t1/2 for Db and its resulted in a signi cantly lower yield compared to 250 C and daughter 258Lr were in good agreement with literature values. higher temperatures. If this data point is included into the −Δ 0(T) ± · −1 The observed high volatility of Db-chlorides was comparable to analysis, Ha (DbCl5)=76 10 kJ mol (in Kadkho- 376 −Δ 0(T) ± · −1 that of Nb under similar conditions. For Ta-chlorides only a dayan Ha (DbCl5)=73 10 kJ mol ) results. significantly less volatile compound was observed, which was A similar situation as for group-5 chlorides was observed for attributed to the compound TaOCl3. Group-5 chlorides were the bromides. Here, Db-bromides seemed to be less volatile 231 262,263 ̈ 226 reinvestigated using the OLGA(III) setup. Again, Db than NbBr5 or TaBr5 in experiments by Gaggeler et al. fi α−α Δ 0(T) were identi ed via correlations and SF decays. This time, However, since the measured Ha value for Db-bromide is two species were observed for Nb- and Db-chlorides, which very similar to the one determined for DbOCl3, it is likely that were attributed to the pentachlorides and the oxytrichlorides. the volatility of DbOBr3 was measured. Interestingly, TaBr5 No data was measured for Ta-chlorides. could only be formed when a mixture of HBr and BBr3 was Δ 0(T) In Table 3 the evaluated Ha values measured for group-5 used as brominating agent, whereas NbBr5 was formed with halides and oxyhalides of Nb, Ta, and Db in various HBr only. Again, TC experiments on quartz surface374 yielded experiments are summarized. In cases where the assignment similar results. Nb- and Db-bromides are about 10 kJ·mol−1 to either the pure pentahalide or the oxytrihalide was uncertain, more volatile as compared to the data of Gaggeler̈ et al.226 This Δ 0(T) ̈ the obtained Ha value was listed in a separate column could be attributed to the fact that, in experiments of Gaggeler between the two species. In the very first TC experiments with et al.226, a KCl aerosol was used, which may have covered the group-5 chlorides on glass and/or mica surfaces,65 similar quartz surface to some extent. Generally, a lower volatility was Δ 0(T) Ha values were measured for Nb- and Db-chlorides, but the observed on KCl surfaces compared to a pure quartz surface. In 297 assignment to either the pentachloride or the oxytrichloride a meta analysis by Zvara, Db was listed as DbBr5; however, cannot be made. In experiments with Nb- and Db-chlorides of also DbOBr3 must be considered, as in the experiments of Türler et al.,231 two species of different volatility were observed Gaggeler̈ et al.226 More recently, the experimental study of Db- 377 fi and attributed to MCl5 and MOCl3 (M = Nb, Db). While bromide was repeated with an improved puri cation of the DbOCl3 is less volatile than NbOCl3, only an upper limit could He carrier gas. Under these conditions, Db-bromide is more −Δ 0(T) be established for Ha (DbCl5), which allowed no volatile than observed previously. The volatility sequence conclusions about the relative volatility of NbCl5 and DbCl5. deduced from this experiment together with independent Also, experiments investigating the volatility of TaCl5 are still studies on the behavior of Nb and Ta under identical chemical missing. The strong tendency of group-5 elements to form conditions was Db > Nb > Ta.377 This experimental oxyhalides, even with traces of oxygen, requires experiments observation is in conflict with previous studies, but only with a very low oxygen partial pressure, which is hard to realize concerning the behavior of Db (see Figure 49). The deduced in online gas chromatography experiments, where typically 1 L/ adsorption enthalpies for NbBr5 and TaBr5 were in excellent min of, for example, He is used as carrier gas. Earlier relativistic agreement. It was concluded that indeed the previous 67,226 calculations predicted an increasing tendency down group 5 to studies were performed with DbOBr3 rather than DbBr5. preferentially form the oxyhalide.331 However, in a recent Adifferent explanation is offered by Pershina and Anton,364 − theoretical publication concerning group-5 bromides, it was where the formation of MBr5L (L = Br or Cl) complexes is

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behavior of Db was indicative of the formation of oxo-halide or − − hydroxohalide complexes such as NbOCl4 and PaOCl4 or − Pa(OH)2Cl4 at intermediate HCl concentrations, in contrast − to the pure halide complexes of Ta, like TaCl6 . The observation that Db might show some chemical resemblance with Pa prompted experiments where the extraction of Db into 2,6-dimethyl-4-heptanol (diisobutyl carbinol, DIBC), a secondary alcohol, was studied.275 DIBC is a specific extractant for Pa. In online experiments with Db, the KBr aerosol particles delivering the reaction products from the production site were dissolved in concentrated HBr and fed onto a reversed phase chromatography column (DIBC/ Voltalef). The extraction was followed by the elution of an Nb fraction in 6 M HCl/0.0002 M HF and a Pa fraction in 0.5 M HCl. The number of 262Db decays observed in the Nb fraction indicated that <45% of the Db was extracted into Δ 0(T) Figure 49. Evaluated adsorption enthalpies ( Ha ) of group-5 DIBC, in qualitative agreement with experiments where the halides and oxyhalides from isothermal gas adsorption chromato- columns where striped directly with acetone after the extraction graphic experiments. step. An extraction sequence Db < Nb < Pa was established.275 A possible explanation might be the increasing tendency to discussed on the KCl or KBr covered surface, which would form nonextractable polynegative complexes in the sequence Pa indeed suggest a less volatile Db compared to Nb and Ta. < Nb < Db. 10.2.2. Liquid-Phase Chemistry of Dubnium. The first In elutions from cation-exchange columns with α-HIB, the studies of aqueous phase properties of Db were performed by elution sequence strongly depends on the charge of the metal Gregorich et al.267 in a manually performed experiment making ion. As was shown in experiments by Schadel̈ et al.274 Db is use of the typical property of group-5 elements to strongly eluted rapidly from a cation-exchange column with unbuffered adsorb on glass surfaces from strong nitric acid solutions. The 0.05 M α-HIB together with group-5 elements Nb and Ta and relatively long-lived 34-s 262Db was produced in the reaction the pseudohomolog Pa, while group-4 elements and trivalent 249Bk(18O, 5n) and transported with the aid of a KCl seeded actinides are strongly retained on the resin. These experiments aerosol gas-jet to the chemistry laboratory. The KCl aerosol demonstrated that the pentavalent state is the most stable one particles were collected by impaction on a glass plate. After the in aqueous solution and also provided the cleanest separation of end of collection, the glass plate was fumed twice with 15 M Db so far and allowed discovery of the new isotope 263Db 249 18 273,274 HNO3 and then rinsed with 1.5 M HNO3 and acetone. After produced in the reaction Bk( O, 4n). drying in hot air, the plate was assayed by α-particle The competition between hydrolysis and halide complex spectrometry. In 801 collection and separation cycles, 26 α- formation of group-5 elements was systematically investigated 277 particles in the pertinent energy range 8.42−8.70 MeV, of by Paulus et al. by studying the extraction from pure HF, which 5 were time correlated mother−daughter pairs, as well as HCl, and HBr solutions by the quaternary aliphatic amine 26 SF events were observed. Therefore, Db was shown to (Aliquat-336 on an inert support). In the pure chloride system, behave like a typical group-5 element. In a second experiment, the following separation procedure was established for ARCA the extraction of anionic fluoride species into methyl isobutyl II. The activities were fed onto the reversed-phase chromato- ketone (MIBK) was investigated. The KCl aerosol particles graphy column (Aliquat-336/Voltalef) in 10 M HCl, whereby were collected on Pt disks and dissolved in 3.8 M HNO3/1.1 M the trivalent actinides passed through. Subsequently, a Ta- HF. This solution was transferred to a centrifuge cone fraction was eluted with 6 M HCl, and a combined Nb, Pa- containing the MBIK. After mixing and phase separation, the fraction was stripped with 6 M HNO3/0.015 M HF. MIBK phase was dried on a Ni foil and subsequently assayed by Theoretical considerations involving complex formation and α-particle spectroscopy. Under the conditions of the experi- hydrolysis predicted an extraction sequence Pa ≫ Nb ≥ Db > ment, Ta is extracted nearly quantitatively, while Nb is Ta above 4 M HCl.304 As can be seen in Figure 50, the extracted only to a small extent. In 335 extraction experiments, experimental extraction sequence is in perfect agreement with neither α-particles nor SF events of Db were observed, predictions concerning the behavior in a pure chloride system. indicating a non Ta-like behavior of Db. Interestingly, the extraction sequence from 4 M HCl/0.03 M Using the ARCA II setup, the extraction of group-5 elements HF into TiOA, also an aliphatic amine, was exactly reversed, i.e. into TiOA from HCl solutions typically containing 0.02 M of Ta > Db ≥ Nb ≥ Pa, which in retrospect must probably be HF to prevent hydrolysis was studied in reversed phase attributed to the influence of the fluoride ions. This is extraction chromatography.368 From 12 M HCl/0.02 M HF corroborated by extraction studies from pure HF solutions and from 10 M HCl, 262Db was extracted into TiOA, like the into Aliquat-336.277 Even in dilute 0.5 M HF solutions, group-5 elements Nb and Ta and the pseudohomolog Pa and extractable fluoride complexes are formed by group-5 elements − separated from actinide elements. In elutions of a Pa Nb and Pa. In 4 M HF, the Kd value of Db is high, of the order of fraction with 4 M HCl/0.02 M HF and a Ta fraction with 6 M the ones of Nb and Ta, but it differs markedly from that of Pa. 262 − HNO3/0.015 M HF, Db was found in the Pa Nb fraction. In pure HCl and HBr, extractable chloride and bromide In separate elutions with 10 M HCl/0.025 M HF (Pa-fraction) complexes are only formed above 3 M HCl and above 6 M 262 277 and 6 M HNO3/0.015 M HF (Nb-fraction), Db was equally HBr, respectively. distributed among the Pa- and Nb-fractions. Kratz and co- Investigations of the fluoride complexation and the anion- workers368 concluded that the non Ta-like halide complexation exchange behavior of Db have just been initiated. Using the

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states decrease. The calculations have shown that the stepwise ionization process results in the 6d2 state of Sg4+ and not in the 7s2 one.380 Since the 6d AOs of the 6d elements are more destabilized than the (n−1)d AOs of the 4d and 5d elements, the Sg4+ will even be less stable than W4+. Nonrelativistically, the trend would be opposite. Using IPs, redox potentials for Sg and its homologs in acidic solutions were estimated.380 They can be used as a guiding tool for reduction experiments on Sg probing the stability of the lower oxidation states. IR(M6+) values of group-6 elements were estimated using a − correlation with Rmax of the outer (n 1)p AOs obtained in the MCDF calculations.178 They also show a typical increase of 0.05 Å from W (0.60 Å) to Sg (0.65 Å). This is in agreement 319,320 Figure 50. K values of Pa, Nb, and Ta in the system Aliquat-336/HCl with the increase in CR. d Like Mo and W, Sg should form volatile halides, oxyhalides, (left-hand side) and Aliquat-336/HF (right-hand side). The Kd value determined for Db in 6 M HCl (right-hand side) and 4 M HF (left- oxide hydroxides, or carbonyls. In order to predict the stability hand side) is also indicated. Reproduced with permission from ref 277. of these complexes for Sg, electronic structures of MCl6, Copyright 1999 Oldenbourg Wissenschaftsverlag GmbH. MOCl4,MO2Cl2, and MO3 (M = Mo, W, and Sg) were calculated using the 4c-DFT and RECP CCSD(T) meth- 315,327,381,382 248 19 ods. Optimized geometries (Re and bond angles), Cm( F, α μ 262 379 De, IP, , and were predicted for the Sg compounds. Trends 5n) Db reaction, Tsukada et al. investigated the anion- in these properties in group 6 turned out to be similar to those exchange behavior of group-5 elements Nb and Ta and the of group-4 and group-5 halides and oxyhalides. pseudohomolog Pa. The Kd value of Db was determined only at Both DFT and RECP calculations predicted an increase in one [HF]ini concentration of 13.9 M. The sorption of Db on fi ff the stability of compounds of the 6d elements with increasing the resin was signi cantly di erent from that of the homologs number of oxygen atoms, e.g., from SgCl to SgOCl and to Nb and Ta. Sorption on the resin decreases in the sequence Ta 6 4 SgO2Cl2, as is experimentally known for the lighter homologs ≈ Nb > Db > Pa. Theoretical investigations have not been 382 Mo and W. Thus, SgO Cl was recommended as the most offered for complex formation at such high HF concentrations. 2 2 stable type of oxychloride for high-temperature gas phase It is clear only that the lowest sorption of Pa by the AIX resin 379 2− 3− experiments. SgCl6 was shown to be unstable with respect to found in the experiments is due to the PaF or PaF 315,381 7 8 the loss of Cl transforming into compounds of Sg5+. formation. Further determinations of K values at different HF d RECP CCSD(T) calculations for the group-6 oxyhalides, concentrations are, therefore, required to determine the charge 327 with and without SO coupling, have shown that larger SO of the anionic species. effects on the 6d AOs result in a decrease in D of the 6d In solutions with more dilute fluoride ion concentration e − − compounds by 1−1.5 eV in comparison with the 5d ones. The [F ], Nb is known to form fluoro−oxo complexes NbOF , − 4 effects are larger for the Sg compounds than for the Rf ones due whereas Ta forms pure fluoro complexes TaF , which was 6 to an increasing 6d −6d splitting. confirmed in anion-exchange experiments in mixed HF/HNO 3/2 5/2 3 As in groups 4 and 5, covalency increases down group 6. The solutions.282 The K value of Db was measured at 0.31 M HF/ d − dipole moments of the oxyhalides also increase due to the 0.10 M HNO , corresponding to a concentration of [F ] = 3 eq increase in the metal−ligand separation. Such an increase in μ 0.003 M. The measured K value of Db was similar to that of d of the MO Cl molecules (1.04 D for Mo, 1.35 D for W and Nb but also to that of Pa. Therefore, it was speculated that Db 2 2 − 2− 1.83 D for Sg) should result in a decrease in the volatility of forms either DbOF4 complexes, as does Nb, or even DbOF5 2− these compounds, so that the trend is MoO2Cl2 >WO2Cl2 > or DbF7 complexes, as does, Pa. However, theory says that 382 5+ SgO2Cl2. the latter is less probable, as Db has a much smaller IR (0.69 μ 5+ The importance of electron correlation for QM, OP, , and Å) than Pa (0.78 Å) and is not its homolog. Again, further 327 De was demonstrated on the example of group-6 MO2Cl2. experiments are required to elucidate the speciation of Db. ff fi Correlation e ects were shown to signi cantly decrease QM and μ 11. SEABORGIUM (Z = 106) , and increase De, accounting, for example, for about 65% in ff De(SgO2Cl2). The e ects on De were found to be larger in the 11.1. Theoretical Predictions W compounds than in the Sg ones, and they become more The ground state of Sg is 6d47s2, and it is a homolog of Mo and significant as the number of oxygen atoms increases. 178 W. The first IP is 7.85 eV according to MCDF calculations DS-DV calculations were performed for M(CO)6 (M = Mo, 383 (corrected value obtained by an extrapolation procedure, while W, Sg, and U). Sg(CO)6 was found to be very similar to fi ff the calculated one is 7.03 eV), where the rst ionized electron is W(CO)6 and di erent from U(CO)6. A typical bond length 6d. This is almost equal to the IP(W) of 7.864 eV, where the increase was found for the Sg compound in comparison with first ionized electron is 6s.313 The energies of the 6d(Sg) AO that of W, as for the other types of species. 122 and 6s(W) AO are indeed very close to each other. Multiple Group-6 hydrides, MH6 (M = Cr, Mo, W, and Sg) were used IPs(M → MZ+) were also calculated within the MCDF to study the influence of relativistic effects on the molecular approach.178 The IPs(M → M6+) of group-6 elements reveal properties of Sg. The DF one-center expansion calcula- − the same decreasing trend as the IPs in the maximum oxidation tions321 323 showed relativistic effects to decrease the bond state for group-4 and group-5 elements (see Figure 28). Thus, length of SgH6, so that Re(SgH6) is 0.06 Å larger than as in groups 4 and 5, the stability of the maximum oxidation Re(WH6). The calculations revealed a slight increase in De of state increases in group 6, while those of the lower oxidation SgH6 as compared to that of WH6.

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The aqueous chemistry of Sg and its homologs has also received detailed theoretical consideration. In aqueous solutions, Sg should undergo hydrolysis similarly to its lighter homologs Mo and W. To predict hydrolysis of Sg at various pH of solutions, the free energy change of the following protonation reactions was calculated using the 4c-DFT method:306

2−− MO4 ⇄⇄ MO32222 (OH) MO (OH) (H O) ++2 ⇄⇄⇄MO(OH)322 (H O) M(OH) 422 (H O) ... 6+ ⇄ M(H26 O) (11.1.1) The results indicate that for the first two protonation steps, the trend in group 6 is reversed: Mo < Sg < W. For the further protonation process, the trend is continued with Sg: Mo < W < Sg. Thus, the same reversal of the trend is predicted for the protonation of oxyanions of the group-6 elements as that for the complex formation of the group-4 and group-5 elements. Figure 51. Predicted relative values of log Kd of W (triangles) and Sg The predicted trends in the complex formation are in (squares) with respect to those of Mo (rhomboids) by AIX separations from HF solutions as a function of the acid concentration. Points 1 agreement with experiments for Mo and W at various pH − 301 through 5 correspond to the following extracted complexes: MO3F , values. log K values were determined for Sg, as given in − 2‑ − 306 MO2F2(H2O)2,MO2F3(H2O) ,MO2F4 , and MOF5 . Reproduced Table 4. with permission from ref 308. Copyright 2004 Oldenbourg Wissenschaftsverlag GmbH. K 2− Table 4. log Values for the Stepwise Protonation of MO4 (M = Mo, W, and Sg)306 a detailed account of all Sg TC experiments was published by 250 263 log Kn Zvara et al. In these experiments the nuclide Sg (t1/2 = 0.9 249 18 reaction Mo W Sg s) was produced in the reaction Cf( O, 4n). A very similar 2− + ⇆ − setup, as already shown in Figure 33, was used in these MO4 +H MO3(OH) 3.7 3.8 3.74 − + ⇆ ± experiments. Reaction products were thermalized behind the MO3(OH) +H +2H2O 3.8 4.3 4.1 0.2 target setup in a rapidly flowing stream of Ar gas and flushed to MO2(OH)2(H2O)2 2− + ⇆ ± the adjoining TC column. Volatile oxychlorides were MO4 +2H +2H2O MO2(OH)2(H2O)2 7.50 8.1 8.9 0.1 + ⇆ synthesized by adding air saturated with SOCl as reactive MO2(OH)2(H2O)2 +H 0.93 0.98 1.02 2 + MO(OH)3(H2O)2 agent. The formed oxychloride species migrated downstream in the fused silica chromatography column, to which a longitudinal, negative temperature gradient was applied, and Complex formation of Mo, W, and Sg in HF solutions was fi 308 nally deposited according to their volatility. In contrast to also studied theoretically on the basis of 4c-DFT calculations earlier experiments, no mica plates were inserted, but the fused fl of the following stepwise uorination process silica column itself served as SF track detector. The deposition −− − of Sg was registered after completion of the experiment by MO[or2 MO(OH)]HFMOF+⇄ 4 33 searching for latent SF tracks left by the SF decay of 263Sg. ⇄⇄⇄MO F (H O) MO F (H O)−− MOF Indeed, in several experiments, a number of SF tracks were 22 2 2 23 2 5 − ° (11.1.2) found in the column in the temperature region 150 250 C, which were attributed to the decay of a Sg nuclide. The SF The obtained ΔEC values indicate a very complicated tracks were only found when the quartz wool plug, which was dependence of the complex formation of these elements and inserted as a filter for aerosol particles, was absent. This was trends on pH and HF concentration (Figure 51). Thus, at the attributed to the increased surface and thus a much longer lowest HF concentrations (≲0.1 M HF), a reversal of the retention time. In ancillary experiments, the production cross trends in K should occur in the group, while, at higher HF d sections of transfer reaction products, notably 256Md/256Fm, molarities (≲0.1 M HF), the trend should be continued with were measured. The measured cross sections agreed well with Sg: Mo < W < Sg. At the range of these HF concentrations, previous values measured at Dubna, but they differed by almost separation between the homologs is the best. 386 2 orders of magnitude from a measurement at Berkeley. The The obtained sequences are in agreement with experiments lower cross sections were attributed to the thick target and the for Mo and W.384,385 Future experiments on the AIX collimation technique employed in Dubna. The adsorption separations of group-6 elements from HF solutions should behavior of long-lived 176W was simultaneously studied in the clarify the position of Sg in this group. Sg experiments. The results of these experiments are shown in 11.2. Experimental Results Figure 52. The number of SF tracks has been corrected for 11.2.1. Gas-Phase Chemistry. The first chemical identi- losses due to annealing of tracks throughout the experiment. fication of Sg as volatile oxychloride in TC experiments was These corrections are substantial. At temperatures above 400 reported in a preliminary report by Timokhin et al.251 in 1993 °C, the correction increases the number of observed SF tracks and again in 1996.253 Later, Yakushev et al.252 reported about by a factor of 5. No adsorption enthalpies were deduced from ancillary experiments with Mo and W nuclides and about a the experimental data. Nevertheless, different chemical states further experiment of the same type with Sg. A full paper giving are discussed for Sg and W.250

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column can be expected, compared to about 64 detected events (after applying the annealing correction). Therefore, contribu- tions other than SF of 263Sga have to be considered. A contribution of about nine SF events can be estimated due to SF in 259Rf (or 259Lr after EC in 259Rf), the α-decay daughter of 263Sg. A possible EC decay branch in 263Sg leading to SF in 263Db or in 259Lr after α-decay must also be considered.273,391 Assuming a similar EC-branch as measured for 259Rf (15 ± 3%), a contribution of about 18 SF events must be assumed, which brings the number of expected SF events to roughly 50, not far from the 64 observed events. It is therefore of crucial importance that, under the conditions of the experiment, actinides, as well as the transactinide elements Rf and Db, are much less volatile than the studied element 106 compound. Even though this possibility was dismissed in the work of Zvara 250 et al., it must be noted that Db forms volatile DbOCl3 at temperatures above 250 °C231 (see section 10.2.1), whereas Rf can also be transported in an oxygen containing carrier gas via a transport reaction mechanism230 (see section 9.2.1). The conclusion by Zvara et al.250 that the observed distribution of SF events exclusively reflects the chemical behavior of Sg only can, therefore, not be substantiated. In the upper panel of Figure 52, the registered SF tracks per 5 cm column length (gray histograms) attributed to the SF decay of 263Sga and the corrected number of SF events due to 250,253 263 a Figure 52. Measured and simulated deposition peaks of Sg - the annealing of tracks (white histograms) are displayed. In the and 176W-oxychlorides. The modeled deposition peaks were obtained 288 lower panel, the integrated yield along the length of the column using the microscopic model of Zvara and a Monte Carlo is shown. The influence of the vastly different t values on the simulation technique, with the only adjustable parameter being the 1/2 adsorption enthalpy (for details see text). deposition temperature was discussed, and the authors came to the conclusion that “this factor can hardly account for so large a difference”.250 The distribution of SF tracks can best be As with most of the earlier TC experiments with trans- Δ 0(T) − · −1 described with Ha (Sg-oxychloride) = 110 kJ mol . For actinide elements, the Sg experiments have not unanimously Δ 0(T) − · −1 W, Ha (W-oxychloride) = 102 kJ mol resulted. Both been accepted as the first positive identification of Sg after Δ 0(T) values are very similar. Actually, the Ha value deduced for chemical separation. Serious criticism has been voiced by ̈ 32,259 W-oxychloride is in perfect agreement with the data of Gartner Kratz concerning mostly the magnitude of a SF-branch in et al.392 However, Gartner̈ et al.392 attributed the observed 263Sg but also the assignment of the volatile species. volatile species to WO2Cl2, based on thermodynamic In the reaction 18O+249Cf, at a beam energy of 94 MeV, the 250 ff 264 263 262 considerations, while, in the work of Zvara et al., di erent nuclides Sg, Sg, and Sg could, in principle, be produced chemical species are discussed for Sg- and W-oxychlorides. The in the 3n,4n, and 5n evaporation channels. The nuclide 264Sg +27 387 quantitative analysis performed here shows that the observed decays by SF with t1/2 =37−11 ms. However, according to differences in the deposition temperature are mainly due to the HIVAP calculations, the 3n evaporation channel is expected to ff large di erences in t1/2 of the two nuclides and that indeed the have a 1 order of magnitude lower production cross section Δ 0(T) Ha values of both compounds are very similar. In TC than the 4n channel at 94 MeV. The 4n deexcitation channel experiments by Yakushev et al.252 with short- and longer-lived to 263Sg, for which two isomeric states are known: 263Sga 263 b Mo and W isotopes, it was observed that, depending on t1/2, and Sg with t1/2 = 0.9 and 0.3 s, respectively. The nuclide ff ff 263 b two di erent chemical species of W with di erent volatility Sg is populated by the decay of were formed. The less volatile compound was deposited at αα ° 271Ds→→ 267 Hs263 Sg b about 250 C, whereas the more volatile one was deposited at about 160 °C. It was concluded that first the less volatile 263 a (refs 70 and 388) while Sg is predominantly populated in MO2Cl2 (M = Mo, W) was formed, which was then slowly 263 a ± the direct synthesis reaction. For Sg a SF branch of 13 8% converted to the more volatile MOCl4 (M = Mo, W). Thus, 387 was measured, in contrast to an earlier indirect determi- short-lived Sg was deposited in the column as SgO2Cl2, whereas 389 nation of 70% by Druin et al. based on cross section longer-lived W was observed as WOCl4.Again,these arguments. The 5n deexcitation channel leads to 262Sg, a experiments were only analyzed qualitatively. Due to the α ff nuclide decaying by SF and by -particle emission with a t1/2 = possibility of di erent chemical states of Sg and W, the TC +5 250 15−3 ms. The calculated cross section at 94 MeV is again 1 experiments of Zvara et al. allow no conclusions about order of magnitude smaller than that for 263Sg. Considering the chemical similarities of Sg to either W or Mo. Furthermore, the 262,264 short t1/2 and low production cross sections of Sg, the uncertainties about the origin of the observed SF tracks, which most likely candidate responsible for the observed SF tracks might be partly due to lighter transactinide elements, render must be 0.9-s 263Sga. Using the efficiencies given by Zvara et conclusions about the chemical speciation and properties of Sg al.,250 a SF branch of 13%,387 and a production cross section of oxychlorides questionable. 0.3 nb as measured independently by Ghiorso et al.68 and The announcement of the synthesis of longer-lived 265,266Sg Gregorich et al.,390 detection of about 21 SF events in the in the reaction 248Cm(22Ne, 4−5n) by Lazarev et al.393 laid the

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· −1 104 foundation for further chemical experiments with Sg in the kJ mol was deduced, whereas, for MoO2Cl2, 232,234,239,279,394 Δ 0(T) ± · −1 liquid- as well as in the gas-phase. These Ha (MoO2Cl2)=90 3kJmol resulted, in good experiments were all successful; however, due to an erroneous agreement with theoretical predictions.382 However, it should assignment of mass numbers and decay properties in the be noted that, due to the very limited number of observed physics discovery experiment,393 which served as a basis for the events, it was not possible to, for example, provide 95% or 99% search for Sg isotope decays in all subsequent Sg chemistry error intervals, and there remains an about 15% chance that experiments, it was believed that also in the chemistry SgO2Cl2 is more volatile than MoO2Cl2 compared to an 85% experiments two different isotopes of Sg, namely 265Sg and chance that it is less volatile, as reflected by the deduced 266 232−234,239 −Δ 0(T) 104 Sg, were observed. Much later, after the discovery Ha value. The observed yield curves for MoO2Cl2, 270 266 109 168 265 of Hs (the α-decay mother of Sg) , it became evident WO2Cl2, and SgO2Cl2 are displayed in Figure 53. Thus, that all decay chains observed in the Sg chemistry experiments were due to 265Sg only.395,396 There is now conclusive evidence for two isomeric nuclear states in 265Sg.397 The nuclide 265Sga ≈ 261 a decays with t1/2 9 s preferentially to Rf , which further α 257 decays by -particle emission and t1/2 =68sto No, whereas 265 b ≈ 261 b Sg with t1/2 14 s decays preferentially to Rf , which fi ≈ 397,398 undergoes spontaneous ssion with t1/2 3s. Both states are formed in the direct synthesis reaction 248Cm(22Ne, 5n)265Sga,b but also in the α-decay of 269Hs.395 Following the considerations discussed in section 11.1, the obvious choice was to study Sg as the volatile SgO2Cl2 compound. For online gaschromatographic studies of Sg, the OLGA(III) system in conjunction with the ROMA detection system was employed (see section 7.2.1). Preparatory experi- ments with short-lived Mo-, W-, and U-isotopes were described 392 by Gartner̈ et al. Experimental results with Sg were Figure 53. Relative yield of 104 MoO Cl , 168WO Cl , and 265SgO Cl 232 2 2 2 2 2 2 communicated in a short account by Schadel̈ et al. Later, as a function of isothermal temperature in the chromatography further experiments involving the measurement of a break- column.234 through curve were published in more detail by Türler et al.233,234 Typically, the reaction products recoiling from the both relativistic theory and experiment have shown that the target were transported to the OLGA(III) setup with the aid of volatility of group-6 dioxidichlorides should decrease with   a C-aerosol gas-jet. As chlorinating agents, Cl2 saturated with increasing Z due to from the theoretical point of view an SOCl2 and traces of O2 were introduced to the reaction oven, increase in the molecular dipole moments. where the formation of oxychlorides occurred. Volatile species Hübener et al.239 investigated the behavior of group-6 oxide − traveled through the colder, isothermal section of the column hydroxides including Sg in the system O2 H2O(g)/SiO2(s).Itis and were reattached to KCl aerosol particles while exiting into well-known that in an O2/H2O atmosphere the solid MO3 (M the recluster chamber. This second aerosol gas-jet transported = Mo, W) are in equilibrium with gaseous MO2(OH)2. Under the separated activities to the ROMA detection system. The the assumption that, in one-atom-at-the-time experiments, the α registered spectra were dominated by lines originating from gaseous MO2(OH)2 undergoes a dissociative adsorption isotopes of Po and Bi. Presumably, these nuclides were process, the process can be described as follows: produced in multinucleon transfer reactions from Pb impurities MO(OH)⇄+ MO HO (M = Mo,W,Sg) in the Cm target. These elements were not or only partly 2 2(g) 3(ads) 2 (g) retained in the chromatographic column. Except for 211Pom and (11.2.1) 212Pom, all very short-lived Po activities were due to longer-lived The gas chromatographic investigation of Sg oxide − Bi and/or Pb precursors. Therefore, the mother daughter hydroxides in quartz-glass columns with He/O2/H2O as carrier recoil technique had to be implemented in ROMA. In two runs gas must be highly characteristic, since neither actinides nor the at isothermal temperatures between 300 and 400 °C, ten decay lighter transactinides should form volatile species that would chains attributed to 265Sg were registered, which were obscure the unequivocal identification of Sg. Preparatory summarized in one data point of 350 ± 50 °C. One of the experiments involving TC and high-temperature online IC decay chains consisted of a complete chain were carried out, which demonstrated that Mo and W oxide hydroxides are not transported by simple reversible adsorption 265 αααa261 257 253 Sg→→→ Rf No Fm of MO2(OH)2 (M = Mo, W) but can be best described by a microscopic description of the dissociative adsorption process. which unambiguously demonstrated that Sg was chemically The relative yields of 104Mo and 168W oxide hydroxides as a isolated. At 250 °C, an additional three decay chains were function of isothermal temperature are shown in Figure 54. observed. Due to the complicated detection technique, not only In the actual experiment with Sg, the synthesis reaction 22Ne the chromatographic transport through the column but also the + 248Cm was employed. Reaction products recoiling from the decay and detection of Sg nuclei was modeled with a Monte target were stopped in He seeded with MoO3 aerosol particles Carlo procedure. This way, the most probable number of and transported to the HITGAS237 setup. At the entrance to 265 initially produced Sg nuclei and the chemical yield could be the chromatography column, moist O2 was added to the gas-jet. determined, which allowed extracting a probability density The temperature of the quartz chromatography column was −Δ 0(T) +2 · −1 distribution of Ha (SgO2Cl2)=98−5 kJ mol (68% 1325 K in the reaction and 1300 K in the isothermal zone. 168 −Δ 0(T) ± fi error interval). For WO2Cl2, Ha (WO2Cl2)=96 1 Loosely packed quartz wool in the reaction zone served as lter

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with 265Sg, produced in the reaction 248Cm(22Ne, 5n), in 3900 collection and separation cycles with ARCA, three decay chains attributed to the decay chain αα 261Rfa,b →→ 257 No 253 Fm were observed.232,278 On average, the measurement of a sample started 38 s after the end of collection. As the experiment was designed to retain the group-4 element Rf, the decay chains are due to chemically isolated 265Sga,b. Thus, decay of the Sg nuclides occurred after the Rf/Sg separation, which, on average, occurred within 5 s. Since the operation of ARCA still involved also manual labor, the experiment was a tour de force, which also demonstrated that the method had reached its limits. fi fi Figure 54. Relative yields in isothermal gas chromatography of 104Mo Nevertheless, the experiment signi ed the rst chemical ○ 168 ● separation of Sg in aqueous solution and demonstrated that, ( ) and W( ) oxide hydroxides in quartz columns using humid − − 2− oxygen as reactive carrier gas component. Sg was observed at an presumably, either SgO3F , SgO2F3 , or SgO2F4 complexes isothermal temperature of 1300 K. The solid lines are the result of a were formed. Also, a neutral complex such as SgO2F2 cannot be Monte Carlo model based on a microscopic description of the excluded. However, Sg did not behave similarly to the 236 Δ 0 2+ dissociative adsorption process with H diss.ads(MoO2(OH)2)= pseudohomolog U, which forms UO2 under the present − · −1 Δ 0 − · −1 54 kJ mol and H diss.ads(WO2(OH)2)= 56 kJ mol . The conditions (which is natural, in view of 5f contributions to the dashed line represents a hypothetical yield curve assuming that group- bonding of uranyl). Due to the low fluoride concentration used, 6 oxide hydroxides are transported by simple reversible adsorption 2− “ ” Δ 0 − · −1 239 the anionic SgO4 ( seaborgate in analogy to molybdate, with Ha = 220 kJ mol . Copyright 2011 Oldenbourg MoO 2−, or tungstate, WO 2−) could not be excluded.26 Wissenschafts-verlag GmbH. 4 4 A new series of Sg experiments279 was performed to study 2− the possible formation of the seaborgate anion, SgO4 . It was for aerosol particles. Retention times of about 8 to 9 s were attempted to elute Sg from a cation-exchange column with pure determined from measurements with short-lived Mo and W 0.1 M HNO3 (without the addition of HF). If Sg had been eluted from the cation-exchange column, this would have nuclides at isothermal temperatures above 1270 K. By 2− demonstrated the formation of the seaborgate anion SgO4 ,in condensing the separated volatile species directly on metal 2− analogy to WO4 . In 4575 experiments, only one candidate foils mounted on the circumference of the rotating wheel of the α−α ROMA detection system, the time-consuming reclustering step correlation was observed, with an expected background of 0.49 correlations. If Sg would have been eluted with the same could be avoided, however, at the expense of a reduced +3.7 fi ffi α chemical yield as W, then 4.7−2.5 (68% con dence interval) detection e ciency for -particles. Due to the thickness of the 261 a,b 257 fi π α−α correlations from Rf and No would have been metal foils, nal samples could be assayed only in a 2 279 geometry. The search for genetically linked decay chains detected. Therefore, the non-W like behavior of Sg was attributed to a weaker tendency to hydrolyze,26 in agreement revealed two candidate events, which must be attributed to the 306 sequence with the theoretical predictions. Thus, in the presence of fluoride ions,278 which have a strong α sf − 265Sgb →→ Rf b261 tendency to replace water or OH ligands, the formation of neutral or anionic fluoride complexes seems to be favored, fl 2− (ref 395). In the original publication, these events were whereas, in the absence of uoride ions, the elution of SgO4 erroneously attributed to the decay of 266Sg. The probability anions is rather unlikely.279 that both of these events were entirely random was only 2%.239 Since Sg appeared to be volatile under the conditions of the 12. BOHRIUM (Z = 107) experiment, it showed the typical behavior of a group-6 element and was transported presumably as Sg oxide hydroxide. Under 12.1. Theoretical Predictions the given conditions, this coincides also with the behavior of The ground state of Bh is 6d57s2, so that it is a homolog of Tc the pseudohomolog U(VI). U is also known to form a volatile and Re in group 7. MCDF calculations179 have given the first IP oxide hydroxide. In order to answer the question about the of 7.7 eV (corrected value obtained by an extrapolation sequence of volatility of oxide hydroxides within group 6, procedure, while the calculated one is 6.82 eV), which is slightly further experiments have to be conducted at lower isothermal lower than IP(Re) of 7.83 eV. The first ionized electron both in temperatures. Bh and Re is the (n−1)d one, which is more bound in Re than 11.2.2. Liquid-Phase Chemistry. As described in section in Bh. Multiple IPs(M → MZ+) for group-7 elements179 reveal 2− − 11.1, group-6 elements form MO4 ,MO3F ,MO2F2, the same decreasing trend in the group with increasing Z as that 2− − − MO2F4 ,MO2F3 ,andMOF5 depending on the HF found for group-4, group-5, and group-6 elements (see Figure 278 − concentration. In test experiments, optimum conditions for 28). IR values obtained via a linear correlation with Rmax[(n an isolation of group-6 elements were established. In 0.1 M 1)p] AO in the group are also shown in Figure 28,179 following × −4 HNO3/5 10 M HF, Mo and W are rapidly eluted from a the established trends for group-4 through group-6 elements. cation-exchange column while di- and trivalent actinides, as well The estimated IR(Bh7+) of 0.58 Å is also typically larger than 2+ 7+ as group-4 elements or UO2 , are retained on the column. The IR(Re ) of 0.53 Å. This is also in line with the CR of these group-6 elements form anions of the type MO 2−,MOF−,or elements.319,320 − 4 3 MO2F3 (M = Mo, W). Also, the formation of a neutral Among gas-phase compounds of Bh in the highest oxidation compound such as MO2F2 cannot be excluded. In experiments state, oxyhalides should be most stable. As 4c-DFT calculations

1282 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review − have shown, there is a trend of decreasing metal ligand bond transport reaction of ReO3: strength of the group-4 to group-6 halides with increasing 1 1 group number. In addition, the metal−ligand bond strength HReO4(g )⇄++ ReO 3(ads)4O 2(g)2HO 2 (g) (12.2.2) decreases also from the 5d to the 6d compounds within the 315 mobile adsorption of HReO4: same group. Thus, SgCl6 was shown to be unstable. Consequently, BhCl7 should not exist. This is also connected HReO4(g) ⇄ HReO4(ads) (12.2.3) with a decrease in the relative stability of the maximum 169 m oxidation state along the transactinide series (see Figure 5 in While formation of volatile Re O3 (t1/2 = 16 s) was observed Pershina et al.315). Oxyhalides of Bh should be rather stable, in online isothermal experiments at temperatures above 950 K, fi 169 m like those of the lighter homologs. The 4c-DFT calculations of the signi cantly more volatile H Re O4 could not be synthesized online.413 This is probably the reason why two the spectroscopic properties of MO3Cl (M = Tc, Re, and 291 early attempts to chemically identify Bh in the form of volatile Bh) showed that the electronic structure of BhO3Cl is very 254,414 similar to that of TcO Cl and ReO Cl. The Bh molecule should oxides or oxide hydroxides failed. It is interesting to note 3 3 that in both attempts the utilized production reactions led to be stable with D of 22.3 eV, only slightly less stable than e isotopes that were unknown at the time. ReO Cl, with D of 24.3 eV. R (BhO Cl) values are also 3 e e 3 Two developments were instrumental in leading to the first typically larger than R (ReO Cl). μ and α of MO Cl should e 3 3 successful chemical identification of Bh. First, the nuclides increase in group 7, which is connected with an increase in the 266Bh and 267Bh were synthesized in the reaction 249Bk(22Ne, molecular size. Increasing dipole moments and electric dipole 4−5n) and their decay properties and production cross sections polarizabilities in the group suggest a decreasing volatility in the determined.415 Especially interesting for chemical investigations sequence TcO3Cl > ReO3Cl > BhO3Cl. 267 +14 is the relatively long-lived Bh with t1/2 =17−6 s. Second, due Using calculated molecular properties, the volatility of group- to the not well suited properties of the oxide-hydroxide system 7 trioxychlorides as adsorption on a surface of a chromatog- to rapidly isolate group-7 elements, chlorides and oxychlorides raphy column was predicted via a physisorption model for long- 291 were investigated as potential candidate compounds for an range interactions. Since the surface of the quartz column is online gas chemical isolation of Bh.399 This approach had fi obviously modi ed with in the gas-phase experiments, already been successful in studies of volatile Db and Sg the adsorption model takes into account the following types of oxychlorides. The first TC experiments with Tc and Re using a − ff the molecule surface interactions: the dipole-e ective surface mixture of He(g)/O2(g)/HCl(g) revealed only one single (Cl) charge, the polarizability-surface (Cl) charge, and the deposition zone for each element despite the large number of dispersion one (see section 8.1.1 and eq 8.1.3). All those known chloride and oxychloride compounds of group-7 fi contributions were evaluated, and they all were shown to elements. The formed compound was identi ed as MO3Cl increase in the group with increasing Z. As their sum, the (M = Tc, Re) as the most likely one and was also formed in Δ 0(T) − 399 following adsorption energies Ha (BhO3Cl) = 78.5 online isothermal gaschromatography experiments. Interest- · −1 Δ 0(T) − · −1 kJ mol and Ha (TcO3Cl) = 48.2 kJ mol were then ingly, TcO3Cl was so volatile that it could no longer be determined with respect to the experimentally measured reclustered in the OLGA(III) setup with CsCl aerosol particles, Δ 0(T) − · −1 399 fi Ha (ReO3Cl) = 61 kJ mol . Thus, the sequence in while these worked ne for ReO3Cl. Aerosol particles with a volatility was predicted as TcO3Cl > ReO3Cl > BhO3Cl. This reducing surface such as FeCl2 increased the yield of Tc trend is caused by an increasing μ in this group. significantly. This property allowed the distinction between a The aqueous chemistry of Bh has not yet been studied “Tc-like” and a “Re-like” behavior in future experiments with theoretically. Predictions of other chemical properties of Bh Bh. fi fi based on earlier DF calculations are given in refs 33 and 34. The rst successful chemical isolation and identi cation of Bh was accomplished in an experiment of four weeks duration 12.2. Experimental Results at the PSI Philips cyclotron employing the OLGA(III) setup 12.2.1. Gas-Phase Chemistry of Bohrium. Similar to and the ROMA detection system.235 A mixed 249Bk/159Tb group-6 elements, group-7 elements form a number of target was irradiated with 22Ne ions, producing simultaneously mononuclear compounds, of which some are appreciably 17-s 267Bh and 5.3-m 176Re. Nuclear reaction products recoiling volatile and can thus be utilized for gas chromatographic from the target were attached to carbon aerosol clusters and investigations. Oxides and oxide hydroxides of Tc and Re are transported with the He carrier gas through a capillary to the typically formed in an O /H O containing gas phase. They OLGA(III) setup. As reactive gas, a mixture of HCl and O2 was 2 2 fi were extensively studied, mostly using the method of added. After chemical separation, the nal products were − TC.244,400 410 The technique has also been applied to develop attached to CsCl aerosol particles and transported to the α Tc and Re generator systems for nuclear medical applica- detection system, where -particle and SF decays were tions.411,412 Schadel̈ et al.224 and Eichler et al.413 studied the registered in an event by event mode. The yield of Re and Bh was determined at isothermal temperatures of 180 °C, 150 oxide and the oxide hydroxide chemistry of trace amounts of Re ° ° in an O /H O-containing system with respect to its suitability C, and 75 C. Altogether nearly 180,000 samples were 2 2 collected and measured. A total of six genetically correlated for a first gas chemical identification of Bh. In TC experiments, decay chains of 267Bh were observed, four at 180 °C, two at 150 the formation of ReO and HReO was observed. However, 3 4 °C, and zero at 75 °C. One of the decay chains at 180 °C was three transport processes probably took place simultaneously, complete and consisted of the sequence depending also on the pretreatment of the quartz columns: 267ααα 263 259 255 mobile adsorption of ReO3: Bh→→→ Db Lr Md Due to a non-negligible background created by the presence of ReO⇄ ReO 3(g) 3(ads) (12.2.1) Po and Bi nuclides, 1.3 of the 4 decay chains at 180 °C and 0.1

1283 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review of the 2 decay chains at 150 °C had to be subtracted. While, in for group-7 oxychlorides by V. Pershina et al.291 The results of 169 m test experiments, 16-s Re O3Cl was still observed with 80% these calculations showed that he electronic structure of ° yield at an isothermal temperature of 75 C, no decay chain of BhO3Cl is very similar to that of TcO3Cl or ReO3Cl. Increasing 267 17-s BhO3Cl was observed at this temperature, indicating a dipole moments and electric dipole polarizabilities in the group 267 less volatile BhO3Cl compared to ReO3Cl. The fact that Bh suggest a decreasing volatility in the sequence TcO3Cl > fi “ Δ 0(T) was identi ed after chemical separation already excludes a Tc- ReO3Cl > BhO3Cl. Also, the calculated Ha (BhO3Cl) = like” behavior of Bh, since CsCl was used as the recluster −78.5 kJ·mol−1 is in perfect agreement with the experimental 108 aerosol material. The relative yields of compounds TcO3Cl, result. Despite the success of the experiment, it should be noted 169 m 267 ffi α−α Re O3Cl, and (most likely) BhO3Cl as a function of that the overall e ciency to detect a correlated pair was isothermal temperature are shown in Figure 55. The deduced only about 4%. Significant improvements of the overall efficiency were mandatory to continue chemical investigations of even heavier elements.

13. HASSIUM (Z = 108) 13.1. Theoretical Predictions The ground state of Hs is 6d67s2, so that it is a homolog of Ru and Os in group 8. MCDF calculations179 have given the first IP of 7.6 eV (corrected value obtained by an extrapolation procedure, while the calculated one is 6.69 eV), which is lower than the IP(Os) of 8.44 eV. The first ionized electron in Hs is the 6d one, while in Os it is the 6s.313 The 6d(Hs) electron is slightly more bound than the 6s(Os),122 so that a larger IP of Hs than that of Os is expected. The opposite trend obtained in 179 108 MCDF calculations should therefore be revisited on the Figure 55. Relative yields of the compounds TcO3Cl (blue circles), Z+ 169 m 267 basis of more accurate calculations. All multiple IPs(M → M ) Re O3Cl (green circles), and (most likely) BhO3Cl (red squares) as a function of isothermal temperature. The error bars indicate a 68% are also given in ref 179 and those to the 8+ oxidation state are confidence interval. The solid lines indicate the results of simulations also shown in Figure 28, revealing the same decreasing trend 288 → Zmax+ with the microscopic model of Zvara with the adsorption enthalpies with Z in the group in IPs(M M ), as for group-4 given in the text. The dashed lines represent the calculated relative through group-7 elements. The IR values obtained via a linear fi Δ 0(T) − yield concerning the 68% con dence interval of Ha (BhO3Cl) correlation with Rmax of the (n 1)p AOs are also shown in from −66 to −81 kJ·mol−1.235 Figure 28,179 following the trend of increasing IR for group-4 through group-7 elements. The IR(Hs8+) of 0.45 Å is also enthalpies of adsorption on the column surface were typically larger than the IR(Os8+) of 0.39 Å. This is in −Δ 0(T) ± · −1 −Δ 0(T) 319,320 Ha (TcO3Cl) = 51 3kJmol , Ha (ReO3Cl) = agreement with the CR of these elements. ± · −1 −Δ 0(T) +6 · −1 61 3kJmol , and Ha (BhO3Cl) = 75−9 kJ mol (68% As with other group-8 elements, Hs should form volatile confidence interval). Therefore, the sequence in volatility is tetroxides, whose volatility was studied experimentally (see TcO3Cl > ReO3Cl > BhO3Cl. The probability that BhO3Cl is below). The group-8 elements Ru and Os are the only elements equally or more volatile than ReO3Cl was estimated to be less which can form an 8+ oxidation state (with the exception of Xe, 416 than 10%. This sequence in volatility agrees well with which is known to form tetrahedral XeO4 ), the highest predictions from fully relativistic density-functional calculations oxidation state known for a .417 The perfect

α Figure 56. Relativistic (rel.) and nonrelativistic (nonrel.) bond lengths, Re, ionization potentials, IP, and polarizabilities, ,ofMO4 (M = Ru, Os, and Hs). Reproduced with permission from ref 293. Copyright 2008 by The American Physical Society.

1284 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review tetrahedral symmetry of the neutral OsO4 makes this molecule change of the reaction is indicative of the following trend in the ° ≫ nonpolar and highly volatile (Tm = 40.25 C). This outstanding complex formation in group 8: Os > Hs Ru. The predicted property distinguishes Os from any other transition metal oxide lower reactivity of HsO4 with NaOH as compared to that of ff 420 and allows, thus, a clear di erentiation of the chemistry of Hs OsO4 has so far not clearly been revealed experimentally. from that of the actinide and lighter transactinide elements. To 13.2. Experimental Results predict their properties and volatility, very accurate 4c-DFT calculations with maximum large basis sets were performed.293 13.2.1. Gas-Phase Chemistry. The experimental chemical investigation of the transactinide element hassium (Hs, Z = The results have shown that group-8 MO4 molecules should all 108), an expected member of group 8 and homolog of Os and be very similar and stable, with the same trend reversal in De, Ru, presented a number of challenges. For obvious reasons RuO4 < OsO4 > HsO4, as for the earlier 6d elements with respect to the 5d ones. SO effects on the 6d AO are responsible described above, from the very beginning, all attempts concentrated only on the isolation of Hs as a very volatile for such a trend reversal. Re(HsO4) should also be larger than Re of RuO4 and OsO4, as in compounds of group-4 through HsO4, probably very similar in volatility to OsO4. group-7 6d elements with respect to the 5d ones. The The first synthesis of Hs was reported by Münzenberg et 421 fi 265 calculations have also revealed an inversion of the trend in α al. in 1984, which identi ed the nuclide Hs with t1/2 = 1.5 and IPs values beyond Os (Figure 56). This trend reversal is ms, far too short for any currently available chemical separator 269 ≈ explained by the behavior of the valence (n−1)d AO system. The more neutron-rich nuclide Hs with t1/2 10 s contributing predominantly to bonding. suitable for chemical investigations was observed much later in fl the α-particle decay chain originating from 277Cn.80 However, For the MO4 (M = Ru, Os, and Hs) molecules, the in uence − of relativistic effects on properties important for gas-phase the production cross section of only about 1 pb (10 36 cm2) for experimental investigations was studied in detail.292,293 Figure the reaction 208Pb(70Zn, 1n)277Cn was discouragingly small. A α 56 shows the relativistic and nonrelativistic IP, , and Re values somewhat larger production cross section of about 7 pb was of these molecules. One can see that relativistic effects decrease calculated for the direct production of 269Hs in the reaction ff 248 26 422 Re, increase IPs (with the strongest e ect on HsO4), and Cm( Mg, 5n). This production cross section is 1 order of decrease α. However, they do not change trends in these magnitude smaller than that for the synthesis of 267Bh, the properties in the group, since those for the relativistic and nuclide that was used for chemical investigations of the next nonrelativistic (n−1)d AOs are the same. lighter transactinide element. Therefore, new techniques had to There are also ab initio DF418 and infinite-order regular be introduced for irradiation, separation, and detection in order approximation with modified metric method (IORAmm/ to accomplish the required sensitivity of chemically investigat- 419 HF) theoretical studies of the electronic structures of MO4 ing the element Hs. (M = Os and Hs). These works, however, revealed some Three early attempts to chemically identify Hs as volatile fi de ciency of the calculations that resulted in predicting a wrong HsO4 demonstrated the high chemical selectivity of the chosen trend in properties from Os to Hs, as compared to a more approach; however, the experiments lacked the overall accurate calculation.293 (See a critical analysis in ref 292.) efficiency and the long-term stability to reach the required Δ 0(T) Using a model of dispersion interaction (eq 8.1.1), Ha sensitivity. In experiments conducted at Dubna, Zhuikov et values of group-8 tetroxides on a silicon nitride surface of the al.255,256 employed the reaction 40Ar + 235U to produce short- detectors of the chromatography column were predicted.293 lived α-decaying isotopes of element 110 and their Hs daughter The inversion of the trend in α and IPs beyond Os (Figure 56) nuclides. The technique employed fission track detectors, −Δ 0(T) resulted in a trend reversal in Ha : RuO4 > OsO4 < HsO4. assuming that the produced Hs nuclides would decay mainly by This prediction turned out to be in agreement with the SF. No SF decays were registered, resulting in a production −Δ 0(T) 197 experimentally observed trend in Ha : OsO4 < HsO4. cross section limit of 10 pb for nuclides with t1/2 > 150 ms. In a −Δ 0(T) · −1 255,256 249 22 267 Also, the calculated Ha (HsO4) = 45.1 kJ mol proved to second attempt, the reaction Cf( Ne, 4n) Hs was be in excellent agreement with the measured −ΔH0(T) (HsO ) used to search also for short-lived α-particle emitting isotopes − a 4 =46± 2kJ·mol 1.197 Relativistic effects were shown to have no of Hs. The decontamination from actinides (separation factor fl Δ 0(T) fl 6 3 in uence on the trend in Ha , as they have no in uence on >10 ) as well as that from Po (>10 ) was excellent. the trends in the molecular properties (Figure 56), since Nevertheless, no α-particles in the energy range above 8.5 relativistic and nonrelativistic (n−1)d AOs change in the same MeV and no SF events were registered. An upper limit way with increasing Z in group 8.293 production cross section of 100 pb for α-decaying nuclides with 295 ≤ ≤ fi In a work of Pershina, thermodynamic equations to 50 ms t1/2 12 h and of 50 pb for spontaneously ssioning predict Ta of a heaviest element with respect to Ta of a homolog nuclides was established. A similar experiment was reported by in a comparative study are given. As an example, the adsorption Dougan et al.423 by applying the so-called On-line Separation of group-8 MO4 (M = Os, Hs) was considered. Accordingly, and Condensation AppaRatus (OSCAR), which was installed at Ta(HsO4) with respect to Ta(OsO4) was predicted. In the same the LBNL 88-Inch Cyclotron. The OSCAR setup was used to work, various measures of volatility were critically compared. search for α-decaying 272Hs, the expected EC decay daughter of 272 ≈ Volatile tetroxides of Hs, like other group-8 elements, should Mt (estimated t1/2(EC) 25 m), produced in the react with a moisturized NaOH surface, forming the 254Es(22Ne, 4n) reaction. However, no α-decays between hassate (VIII), Na2[HsO4(OH)2], by analogy with 8.7−11.0 MeV were observed and an upper limit for the Na2[OsO4(OH)2], according to the reaction: production cross section of 1 nb was derived. The first successful Hs chemistry experiment was conducted 2NaOH+→ HsO Na [HsO (OH) ] (13.1.1) 4242 in the spring of 2001 in the framework of an international fi The reactivity of RuO4, OsO4, and HsO4 with NaOH was collaboration at the GSI, Darmstadt. In order to signi cantly studied on the basis of 4c-DFT calculations of the components increase the level of sensitivity, all features of earlier of the reaction of eq 13.1.1.309 The calculated free energy experiments were either improved or replaced by new

1285 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review techniques. The stationary target was replaced by the rotating graphic column, was used to evaluate the adsorption enthalpy 196 window and target system ARTESIA (see section 7.1.1), of HsO4 and OsO4 on the silicon nitride detector surface. The fi −Δ 0(T) ± · −1 which could accept signi cantly higher beam intensities from modeled distributions with Ha (HsO4)=46 2kJmol 248 26 269 fi −Δ 0(T) ± the accelerator. The reaction Cm( Mg, 5n) Hs was chosen (68% con dence interval) and Ha (OsO4)=39 1 as the most promising one, also due to the availability of kJ·mol−1 are shown as solid lines in Figure 57. The obtained ffi 248 −Δ 0(T) su cient quantities of the Cm target material for the Ha value for HsO4 and the trend from OsO4 to HsO4 are preparation of three segments of the target wheel and its still in excellent agreement with the 4c-DFT theory.293 manageable radioactive decay properties. With the rotating target wheel, synthesis of about 3 atoms of 269Hs per day could be expected. The transport of reaction products with the aid of an aerosol gas-jet was abandoned. Instead it was observed that Os tetroxides were formed directly in the recoil chamber in test 206 reactions when adding O2 to the He stopping gas. The addition of an oven heated to 600 °C at the exit of the recoil chamber helped to complete the oxidation reaction and increased the yield. Volatile tetroxide species could then be transported essentially without loss through Teflon capillaries to the chemistry and detection setup. Instead of IC, TC was applied where the column was replaced with a narrow channel formed by silicon particle detectors;201 see also Figure 21 (section 7.2.2). The efficiency of detecting an α-particle or a SF event was about 80%. In addition, the deposition temperature Figure 57. Relative yields of HsO4 and OsO4 for each of the 12 269 detector pairs. Measured values are represented by bars: HsO4, dark of each detected single Hs atom also provided chemical 172 information. The required gain in sensitivity of 1 order of blue; OsO4, light blue. The dashed line indicates the temperature profile (right-hand scale). The maxima of the deposition distributions magnitude compared to the OLGA setup used in experiments − ± ° − ± ° were evaluated as 44 6 C for HsO4 and 82 7 C for OsO4. with Bh was thus accomplished. In an experiment conducted at Solid lines represent results of a simulation of the adsorption process the GSI, valid data was collected during 64.2 h. During this −Δ 0(T) · −1 −Δ 0(T) with Ha (HsO4) = 46.0 kJ mol and Ha (OsO4) = 39.0 × 18 26 248 − time, 1.0 10 Mg beam particles passed through the Cm kJ·mol 1. target. Only α-lines originating from 211At, 219,220Rn, and their fi 211 decay products were identi ed. While At and its decay 13.2.2. Liquid-Phase Chemistry. In an independent 211 fi daughter Po were deposited mainly in the rst two detectors, experiment, von Zweidorf et al.420 used the CALLISTO 219,220 Rn and their decay products accumulated in the last three (continuously working arrangement for cluster-less transport detectors, where the temperature was sufficiently low to partly of in situ produced volatile oxides) setup to demonstrate that adsorb Rn. During the experiment, seven correlated decay the volatile Hs compound formed in situ with oxygen chains were observed in detectors 2 through 4 and were containing carrier gases reacts readily with a thin layer of 269 assigned to the decay of either Hs or the yet unknown hydroxide in the presence of water. This behavior is well-known 270 197 Hs. This assignment was based on an erroneous for OsO4, which behaves as an acid anhydride, forming with assignment of mass numbers and decay properties of Sg aqueous NaOH sodium osmate(VIII) of the stoichiometry 393 isotopes in the physics discovery experiment. The excellent Na2[OsO4(OH)2] (see section 13.1). In an experiment similar separation from unwanted activities resulted in a background to the one of Düllmann et al.,197 the reaction 248Cm(26Mg, α − 269 269 count-rate of -particles with energies between 8.0 9.5 MeV of 5n) Hs was employed to form volatile HsO4 by stopping 0.6 per hour and detector, leading to very low probabilities the fusion reaction products in a mixture of He/O and passing − − 2 between 2 × 10 3 and 7 × 10 5 for any of the chains being of the gas stream through a hot quartz wool filter. The addition of random origin. One of the observed decay chains was complete water vapors significantly improved the deposition yield of consisting of four consecutive α-particles attributed to the tetroxides on freshly prepared NaOH surfaces. Therefore, the decay sequence He/O2 gas stream (1 L/min He, 0.1 L/min O2) containing ααα α HsO4 or OsO4 was mixed with 0.1 L/min He saturated with 269Hs→→→ 265 Sg 261 Rf 257 No → 253 Fm water at 30 °C. The detection system consisted of four detection arrays, each containing four silicon PIN-diodes of 10 The efficiency and selectivity of the system were so superior, mm × 8 mm active area facing a stainless steel plate, coated that it was also used later on for the search and discovery of the with NaOH at a distance of about 1 mm. Hs-isotopes 270Hs and 271Hs.109,261 With the correct identi- The gas stream was passing through three of these arrays, fication of 270Hs and its α-decay daughter 266Sg, it became whereas the fourth one was in so-called “service mode”. Every obvious that all 7 observed chains in the first Hs experiment 60 min, one of the stainless steel plates had to be replaced with were due to 269Hs.109 The thermochromatogram of these 7 a freshly coated one, since the reactive surfaces were loosing events along with the distribution of Os is shown in Figure 54. deposition efficiency with time, probably due to the The maximum of the Hs distribution was found at a neutralization of the alkaline surface with CO2, which was − ± ° 172 fl temperature of 44 6 C. The distribution of OsO4 probably formed under the in uence of the heavy ion beam by (t1/2 = 19.2 s) measured before and after the experiment a reaction of the carbon beam dump with the oxygen of the jet showed a maximum in detector 6 at a deposition temperature gas. The flow of the gas stream was controlled by four of −82 ± 7 °C. As in experiments with lighter transactinide computer controlled valves. The working principle of the elements, the Monte Carlo model of Zvara,288 that describes deposition and detection system is illustrated in Figure 58. A the microscopic migration of a molecule in a gas chromato- disadvantage of the one-sided detection system is the reduced

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269 Figure 59. Deposition pattern of OsO4 (blue) and HsO4 (red) on a NaOH surface from a moist gas stream. The Os α-radioactivity was mostly due to the decay of 19.2-s 172Os and 22.4-s 173Os.424

14. MEITNERIUM (Z = 109), DARMSTADTIUM (Z = 110), ROENTGENIUM (Z = 111) 14.1. Theoretical Predictions Mt, Ds, and Rg have 6d77s2,6d87s2, and 6d97s2 ground states and belong to groups 9, 10, and 11 of the Periodic Table, respectively. The ground states for Ds and Rg are different from those of their lighter homologs Pt (5d96s) and Au (5d106s), which is explained by the relativistic stabilization of the 7s AO in the heavier elements. The M+ configurations Ds+(6d77s2) and Rg+(6d87s2) are also different from those of their homologs Pt+(5d9) and Au+(5d10), which is also a relativistic effect. Belonging to groups 9, 10, and 11, Mt, Ds, and Rg are expected to exhibit a behavior similar to that of their lighter homologs in these groups, the noble metals Ir, Pt, and Au. Strong relativistic effects should influence the properties of Mt, Ds, and Rg and Figure 58. Comparison of two different states of the deposition and their compounds to a larger extent than those of the homologs detection system of CALLISTO. In the upper part of the figure, of the sixth period. The chemistry of these elements predicted detection array 4 is in “service mode”; in the lower part of the figure, on the basis of atomic DF calculations is described else- detection array 1 is in “service mode”, allowing replacement of the 33,34 424 where. steel plate of array 1 with a freshly prepared NaOH surface. These early DF calculations33,34 have given an IP of 8.7 eV for Mt, which is obviously too low (it is lower than the IP of Ir of 8.967 eV313), while it should probably be larger, because the 6d(Bh) AO is more stabilized than the 6s(Ir) AO.122 For Ds, detection geometry compared to a two-sided geometry in a 33,34 the DF calculated IP of 9.6 eV is larger than that of Pt fi cryo TC detector, which signi cantly lowers the probability to (8.959 eV313), because the 6d(Ds) AO is more stabilized than detect complete nuclear decay chains. In total, five nuclear the 6s(Pt) AO. In any case, more accurate calculations are decay chains attributed to the decay of Hs isotopes were needed for these two heaviest elements. For Rg, the best DCB 425 registered.420 The distribution of the five Hs events in relation FSCC calculated IP is 10.6 eV, which is also larger than the IP(Au) of 9.2255 eV313 because the 6d(Rg) is more stabilized to the lighter homolog Os on the 3 times 4 detectors, that is, 12 than the 6s(Au) AO. According to the higher IPs of the positions, is depicted in Figure 59. The gas stream always heaviest elements in groups 9 through 11, they should be even entered the detection setup before detector position 1 and left more inert and noble than their homologs of the sixth period. ff after passing detector 12. For Mt, the Kα1 transition energies for di erent ionization The authors420 concluded that, for the first time, an acid− states were predicted using the DHF theory, taking into account QED and nuclear-size effects. The results were base reaction was performed with the tetroxide of hassium, compared with recent experiments in the α-decay of 272Rg.128 leading to the formation of a hassate(VIII). Evidence for a The main oxidation states for Mt and Ds are 3+ and 2+, lower reactivity of HsO4 with respect to moisturized NaOH as respectively, though some other oxidation states are also 426 33,34 compared to OsO , as tentatively suggested by the maximum of foreseen. In Rg, the most stable state should be 3+, while 4 the 1+ state should be very unstable. Due to the relativistic the Hs distribution on detector 3 and predicted by the 4c-DFT destabilization, that is, the 6d AOs that start to be chemically 309 fi theory, was not judged as signi cant due to the few detected active at the end of the 6d series, the 5+ state of Rg is also events. expected. The −1 state is also expected due to the EA of Rg of

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− − 1.56 eV, which is, however, much lower compared to Au (2.31 stabilizes the molecules in the following order: RgF6 > RgF4 425 − eV). > RgF2 . This order is consistent with the relative involvement The AR of Rg (1.2 Å) is smaller than the AR of Au (1.35 Å) of the 6d electrons in bonding for each type of molecule. due to the stronger relativistic contraction of the 7s AO,33,34 as The aqueous-phase chemistry of Rg received some was also mentioned earlier. From group 9 onward, there is a theoretical attention on the basis of DFT theory. It was decrease in the difference in the single and triple bond CR shown that Rg(I) is the softest metal ion.438 No chemistry between the 6d and 5d compounds, reaching negative values in experiments have been conducted with elements Mt, Ds, and groups 11 and 12 (Figure 29).319,320 The relativistic bond Rg so far. contraction is caused mainly by the relativistic stabilization of the ns AO. 15. COPERNICIUM (Z = 112) On the MO level of theory, Mt and Ds have received little attention so far. It was suggested that volatile hexafluorides and 15.1. Theoretical Predictions fl octa uorides might be produced and used for chemical Heavy group 12 elements all have a closed shell d10s2 ground separation experiments. DS-DV calculations for DsF6 indicate state and should therefore be rather inert. With increasing that DsF6 should be very similar to PtF6, with very close values relativistic stabilization and contraction of the ns AO in group 427 of IPs. Relativistic effects were shown to be as large as ligand- 12, elements become even more inert. Thus, bulk Hg is known field splitting.427 ff − to be a liquid; however, it is very di erent from the condensed Bond lengths in MtH3, MtC and DsH2, and DsH3 were noble gases. In the case of Cn, relativistic effects are expected to 319,320 calculated using the ADF ZORA method. Using the same be further amplified. Pitzer439 in 1975 found that the very high approach, electronic structures of DsC and DsCO were excitation energy of 8.6 eV from the s2 closed shell into the sp 428 calculated, suggesting that these compounds are chemically valence state of Cn will not be compensated by the energy gain similar to the corresponding 5d homologs. of the chemical bond formation. Thus, Cn should reveal a In contrast, Rg received much attention from theory. A noble-gas character. special interest in the chemistry of Rg is explained by the The atomic properties of Cn are very well studied (for older expectation of unusual properties of its compounds due to the predictions, see refs 33 and 34). The DCB FSCC calculations maximum of relativistic contraction and stabilization of the 7s have given the first IP of Cn of 11.97 eV and the second one of 122 AO in this group. The electronic structure of the simplest 22.49 eV.440 It is the highest in group 12 (Figure 60) and in the molecule RgH, a sort of a test system such as AuH, was studied seventh row of the Periodic Table, evidencing a very high 429−434 fi at various levels of theory, REPP, DHF, and 4c-DFT. inertness of Cn. However, the rst ionized electron is the 6d5/2, Because the obtained Re(RgH) between 1.503 Å and 1.546 Å in difference to Hg (6s) (Figure 11). The DCB FSCC 432 (the DHF CCSD(T) value is 1.523 Å ) is so close to calculations of EA found no bound anion for Cn.440 Excitation 435 Re(AuH) of 1.5236 Å, a very high accuracy is needed to energies, IPs, and oscillator strengths for neutral and up to 5+ predict the correct trend. At the best present level of accuracy, ionized states of Cn and Zn, Cd, and Hg were calculated using the bond lengths of these molecules are about the same. the MCDF method.441 The calculated MCDF IPs441 are, A comparison of relativistic (DF or ARPP) with non- however, less accurate than the DCB FSCC ones.440 IPs of relativistic (HF or NRPP) calculations shows bonding to be internal conversion electrons, that is, of K-shell (1s) and L-shell considerably increased by relativistic effects doubling the (2s), of Cn are predicted to an accuracy of a few tens of dissociation energy, though the SO splitting diminishes it by electronvolts using DHF theory taking into account QED and 429 127 0.7 eV. The trend to an increase in De from AgH to AuH nuclear-size effects. This data can be used for experimental should be reversed from AuH to RgH, as was also shown by the studies of Cn involving K conversion electron spectroscopy. BDF calculations.436 The PP and BDF calculations disagree, The main oxidation state of Cn is expected to be 0 due to its however, for the trend in Re(MH) in group 11 from Au to Rg. closed shell structure and strongest relativistic effects. However, The trend to an increase in force constant, ke, was found to be the 2+ state could also be foreseen. As in the case of Rg, the 6d continued, with RgH having the largest value of all known electrons start to be chemically active in Cn. As a consequence, diatomic molecules.429 The μ was shown to be relativistically an increase in the stability of the higher oxidation states, 4+, is decreased from AgH to AuH and to RgH, indicating that RgH expected also for this element. Due to the maximum relativistic is more covalent and element Rg(I) is more electronegative contraction of the 7s AO in group 12 and in the seventh period than Au(I).429,436 (Figure 12), the AR of Cn should be the smallest in group 12 Results of 4c-BDF436 and 4c-DFT calculations433 for AuX (1.71 Å)442 (see also Figure 60). This also results in the and RgX (X = F, Cl, Br, O, Au, Rg), indicate that relativistic shortest bond lengths of Cn compounds in group 12, with the effects follow a similar pattern to that for RgH, except for RgF predominant contribution of the 7s AOs (see also CR in Figure 319,320 and RgO, where SO splitting increases De. The singlet state was 29 ). found to be the ground state for Rg2, in comparison with the The static dipole polarizability of Cn of 27.64 au was triplet state.433 The dissociation energy was found to change in calculated most accurately at the DC CCSD(T) level of 442 the following order: Au2 > RgAu > Rg2. theory. Because of the relativistic 7s AO contraction, it is the To study the stability of higher oxidation states, energies of smallest in group 12 (Figure 60) and the smallest in the seventh − → − − → − α the MF6 MF4 +F2 and MF4 MF2 +F2 (M = Cu, Ag, period. Due to the smallest , Cn should be very volatile over Au, and Rg) decomposition reactions were calculated at the PP, inert surfaces, which guarantees its transport from the target MP2, and CCSD levels of theory.437 Relativistic effects were chamber to the chemistry setup through Teflon capillaries in shown to stabilize higher oxidation states in the high- chemical experiments. Deposition of Cn on ice and quartz at coordination compounds of Rg due to the destabilization of higher temperatures than those for Hg was also predicted.442 − fl ff the 6d AOs and their larger involvement in bonding. RgF6 was The in uence of relativistic e ects on the atomic properties shown to be the most stable in this group. SO coupling of group-12 elements, as the most interesting case, was

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Figure 60. Relativistic (solid lines) and nonrelativistic (dashed lines) ionization potentials, IP, polarizabilities, α, and atomic radii, AR, of group-12 elements. Reprinted with permission from ref 294. Copyright 2005 Elsevier. investigated.294 Figure 60 shows relativistic and nonrelativistic occupied 7s(Cn) AO and singly occupied 6s(Au) AO, as well as α values of IPs, and AR, which change in an opposite way from between the 6d5/2(Cn) AO and 5d5/2(Au) AO. Thus, CnAu Hg to Cn. One can see that, exceptionally due to relativistic should be chemically bound, having a σ2σ*12Σ+ ground state effects, Cn should have the largest IP and smallest α and AR configuration with two electrons in the bonding and one in the and, therefore, be chemically rather inert, much more than the antibonding MOs (Figure 61). lighter homologs in the group. Taking into account the strong relativistic effects on the AOs of Cn, the following questions were of high interest, especially for gas-phase chemical experiments: Is Cn metallic in the solid state, or is it more like a solid ? How volatile and reactive toward Au is the Cn atom in comparison with Hg and Rn? Since bonding in the solid state in the first approximation is described by bonding in a homonuclear dimer, De(M2) values Δ 0(298) were calculated to estimate HS of the Cn metal. Moreover, Hg2 and Cn2 have been of special interest in chemical theory in testing the accuracy of quantum-chemical methods in treating closed-shell interactions. Accordingly, the electronic structures of these dimers were calculated using a variety of methods, such as 4c-BDF, ECP CCSD(T), QP-PP CCSD(T),443 and 4c-DFT.294,433 The calculations have shown that even though bonding in both Hg2 and Cn2 is preferentially of the van der Waals type, a partial overlap occurs. Both the DFT and PP calculations agree on an increase in De of about 0.04 eV from Hg2 to Cn2 with the corresponding bond shortening. Thus, due to the relativistic 7s AO contraction, Cn2 Figure 61. Bond formation (principal MOs) of the CnAu and FlAu molecules. Reproduced with permission from ref 25. Copyright 2011 should be more stable than Hg2. This is in agreement with the LDA DFT solid-state calculations for solid Cn.444 A cohesive Oldenbourg Wissenschaftsverlag GmbH. energy of 1.13 eV was obtained for Cn at the SR-level of theory, which is larger than that of Hg (0.64 eV) and is an order of Overall, Cn should be about 0.1−0.2 eV more weakly bound magnitude larger than those of the solid noble gases. It was also with a transition metal atom M than Hg, due to the stronger concluded that Cn is not a metal, but rather a semiconductor relativistic contraction of the 7s(AO) in comparison with the with a band gap of at least 0.2 eV. In this sense, Cn resembles 6s(Hg) AO, while the bonds should be longer, because of the the group-12 metals more closely than it does the noble gases. more extended 6d(Cn) than the 5d(Hg). Among the group-11 Predictions of the interaction of Cn with Au were also very and group-12 metals, bonding with Ag was found to be the important to compare the volatility of Cn with that of Hg as weakest while that with Pt the strongest. adsorption on (Au) surfaces of detectors in gas- The influence of relativistic effects on properties of MAu (M phase chromatography experiments. With this aim in view, = Hg and Cn) was studied as well.294 Relativity was shown to electronic structure calculations were performed for HgM and increase De(HgAu) by 0.13 eV but to decrease it by about the CnM, where M = Ag, Au, Pt, Pd, and Cu using the 4c-DFT same amount (0.12 eV) in CnAu due to the contraction of the method.445,446 It was demonstrated that Cn forms a chemical 7s(Cn) AO. This makes trends in nonrelativistic vs relativistic nr bond with Au primarily due to the overlap between the doubly De values opposite from HgAu to CnAu, so that De (CnAu) >

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nr rel rel ff ff + De (HgAu), while De (CnAu) < De (HgAu). Re is decreased this di erence are greater relativistic e ects in CnH than in by relativity in both systems, and the trends are the same for RgH. both the nonrelativistic and relativistic Re. Due to the larger involvement of the 6d AOs in bonding, Further on, 4c-DFT calculations were performed for Hg and higher oxidation states should be observed for high- Cn interacting with Aun clusters simulating Au (111) and (100) coordination compounds of Cn, as was proven by the PP 290a,446,447 455 fi surfaces. Convergence of Eb with cluster size was CCSD(T) calculations. No de nite conclusion, however, reached for n = 95 (top position), n = 94 (bridge), n = 120 about the existence of CnF4 can be drawn from its → · −1 (hollow1), and n = 107 (hollow2) on the Au(111) surface. The decomposition MF4 MF2 +F2 energy between 100 kJ mol − and 200 kJ·mol−1. Nonrelativistically, CnF would be definitely obtained M Aun binding energies, Eb, evidence that an 4 − Au(111) surface is a good approximation of the real one, unstable. It was also found that the addition of F ions to HgF2 459,460 which is usually unknown in the experiments. The bridge and to HgF4 is energetically favorable. By analogy, it is adsorption position was found to be preferential for Hg, while a assumed that, in combination with an appropriate polar solvent, − − 455 hollow2 one was preferential for Cn (Figure 62). CnF5 and/or CnF3 may be formed. The small energy of → fi the decomposition reaction MF2 M+F2 con rms the prediction that Cn should be more inert than Hg, though the difference to Hg is not that large. A comparison with nonrelativistic results shows that this is a pure relativistic effect. 15.2. Experimental Results 15.2.1. Gas-Phase Chemistry. With the synthesis of partly long-lived isotopes of Cn through element 118 in 48Ca induced − reactions on actinide targets,2,89,461 463 the focus of chemists shifted to the chemical exploration of these superheavy elements. Of special interest are chemical investigations of Cn with its 6d107s2 closed shell and strong relativistic stabilization of the 7s AO that should result in a high inertness, higher than that of Hg. Experimental investigations of Cn hold the promise to study this element as the first transactinide in the elemental state. Suitable isotopes of Cn with sufficiently long half-lives are 283 285 283 Cn (t1/2 = 3.8 s) and Cn (t1/2 = 29 s). The nuclide Cn Figure 62. 4c-DFT calculated binding energies of Pb, Hg, Cn, and Fl can be synthesized either directly in the reaction 238U(48Ca, 3n) −Δ 0(T) 287 with the Aun clusters in comparison with experimental Ha of Pb, or indirectly as a decay product of Fl in the reaction 194,448,449 Hg, and Cn on Au. Reproduced with permission from ref α 290a. Copyright 2009 American Institute of Physics. 242Pu() 48 Ca, 3n 287 Fl→ 283 Cn with the latter reaction having the higher production cross 285 The Eb(Cn-Aun) for the hollow2 position was calculated as section. The heavier, but longer-lived Cn can only be 290a fi −Δ 0(T) 0.46 eV. This is signi cantly lower than Ha (Hg) of synthesized indirectly in the 449 1.0 eV on Au. The reason for that, as in the case of the α dimers of these elements, is the relativistic stabilization of the 244Pu() 48 Ca, 3n 289 Fl→ 285 Cn 7s(Cn) AO. Thus, a lower volatility as adsorption on a gold surface was expected for Cn in comparison with Hg. reaction. From the viewpoint of identification, the decay chain 283 Works on RECP and 2c-DFT (SO corrected) calculations of Cn is preferred since the α-decay (Eα = 9.54 MeV) is 279 for Hg and Cn interacting with small Au clusters (n =1−4 and followed shortly in time by SF of Ds (t1/2 = 0.18 s). This 10) arrived at the same conclusion, namely that Eb(Cn-Aun)is decay chain constitutes quite a unique signature and allows the − 450−453 fi 283 about 0.2 eV smaller than Eb(Hg Aun). safe identi cation of Cn after chemical isolation. In this 285 The influence of relativistic effects on the adsorption process respect, the decay of Cn (Eα = 9.16 MeV), which is followed 281 of Hg and Cn on metal surfaces was investigated on the by SF of Ds (t1/2 = 13 s), is less favorable. For the ensuing − 294 ff fi example of small M Aun clusters. Relativistic e ects were discussions, it is important to note that, in the rst attempts to fi − 283 shown to de ne a decreasing trend in Eb(M Aun) from Hg to chemically identify Cn, it was believed that Cn decays by SF − ≈ 461,462 Cn, even though they increase Eb(M Aun) in these systems, with t1/2 3m. especially at the hollow position due to the involvement of the A first attempt to chemically identify Cn in the elemental ff − 464 6d(Cn) AOs in bonding. This makes the di erence in Eb(M state was made by Yakushev et al. in Dubna. The isotope ff 283 Aun) between Hg and Cn very small. Relativistic e ects were Cn was produced by bombarding a U target of natural 48 shown to decrease Re, the distance of the adatom to the surface, isotopic composition with Ca ions. In test experiments, short- in all the cases. lived Hg isotopes could be isolated in the elemental form from The relativistic contraction of the 7s AO is expected to other reaction products, transported in He quantitatively manifest itself also in properties of other Cn compounds, e.g., through a 30 m long Teflon capillary, and adsorbed + 434,454−458 in shortening Re in CnH and CnH . Another quantitatively on Au, Pt, or Pd coated silicon detectors at interesting point is that, in contrast to the group-11 hydrides, room temperature. If Cn behaved chemically like Hg and all the trend in the dissociation energies from Cd to Hg is efficiencies measured for Hg were also valid for Cn, detection + + + +4.3 continued with Cn, i.e. De(CdH ) De(RgH). The reasons for section value for the production of Cn measured in ref 461.

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However, no SF events were observed. Therefore, no is reduced to 5% while for 283Cn it is still 68%. A total of five unambiguous answer as to the chemical and physical properties decay chains was detected in two experiments. They started of Cn was obtained.464 In a next experiment, the question with the α-decay of 283Cn followed by SF-decays within less whether Cn remained in the gas phase and passed over the Au than one second.192,194 The positions of the five atoms inside of and Pd surfaces was addressed.190 Therefore, a special the detector array are depicted in Figure 63. They represent the ionization chamber to measure SF fragments of nuclei remaining in the gas was added at the exit of the Au or Pd coated silicon detector array. Again, zero SF events were registered on the Au and Pd coated silicon detectors, confirming the result of the first experiment. However, eight high energy events were detected in the ionization chamber ascribed to SF decays because they were accompanied by neutrons registered in the surrounding neutron counters, while only one background count was expected.190 The majority of the events were attributed to the decay of an isotope of Cn, since there are no other known volatile nuclides decaying by SF. From this experiment it appeared that the interaction of Cn with an Au or Pd surface is much weaker than that for Hg.190 These results were seemingly confirmed in a TC experiment using the COLD detector where one side of the channel was replaced by an Au covered surface, allowing only measurements in a 2π geometry, i.e. no coincident detection of both SF fragments. The temperature in the gradient started at +35 °C and reached down to −185 °C, the adsorption temperature of Rn. Using the same 48Ca + 238U synthesis reaction, reaction products were stopped in He carrier gas and transported through a 25 m long capillary within about 25 s to the detector array. Seven events were detected that were attributed to fission fragments from the SF decay of 283Cn. The position of most of these events was identical to the deposition peak of Rn. However, the fragment energies were lower than expected, which was attributed to a thin layer of ice that has formed on the detectors at such low temperatures. It was concluded that all three chemical studies on Cn yielded a consistent picture, namely that Cn is not interacting with Au and is more similar to Figure 63. Deposition of the five detected atoms (indicated by arrows) Rn.465 However, since later physics studies could not confirm assigned to 283Cn in 48Ca + 242Pu experiments. The dotted lines the SF-decay of 3-min 283Cn but rather showed that this isotope indicate the temperature gradient inside the detector array (right axis α ≈ 279 in °C). Three different regimes in terms of temperature range inside decays via -emission with t1/2 4sto Ds, the transport fl time for nuclei produced at the accelerator to the detector array the detector array and gas ow rates were applied (see text). The solid in all chemistry experiments performed so far was too long for red lines depict results of a Monte Carlo model prediction (left axis in fi rel. units), including the given experimental parameters and assuming identi cation of a 4-s isotope. Therefore, without safe −Δ Au · −1 27,194 fi the deposited atoms to have always Ha (Cn) = 52 kJ mol . identi cation of the isolated nuclide, all conclusions concerning The vertical dashed lines at detectors 17, 19, and 21, respectively, the chemical properties of Cn were questionable and called for indicate the start of ice layer formation toward lower temperatures significantly improved experiments. At present it remains corroborated by reduced resolutions in the α-spectra. unclear what the source of the signals was that were measured in these early experiments. outcome of three different experiments with varying values of A new attempt to investigate the chemical properties of Cn the temperature range inside the detector array and velocity of took advantage of the higher production cross section of the the carrier gas, respectively. Also shown are the deposition patterns of α-decaying isotopes of Hg and Rn formed in nuclear 242 48 287α 283 Pu() Ca, 3n Fl→ Cn reactions in the same experiment with a minor admixture of Nd to the 242Pu target (for Hg) or formed in transfer reactions with synthesis reaction. A prerequisite of this approach is, however, the target (for Rn). In the first experiment, the temperature fi − − ° that rst an Fl isotope is formed that has a too short t1/2 for range inside the detector array was 24 to 185 C. Under this chemical study, followed by an isotope of Cn with a sufficiently condition, the first decay chain of 283Cn was observed in the fi 287 long t1/2, which is ful lled in this case as t1/2( Fl) = 0.48 s. second detector, at a position where also major Hg deposition Again the COLD setup was used for this study, now with a 4π was found. To search for a possible difference in chemical detection geometry where one side of the channel was behavior between Hg and Cn, the temperature at the beginning equipped with Au-covered silicon detectors. Moreover, the of the detector array was increased to the maximum value at setup was operated in a closed loop mode to reduce the water which a semiconductor detector is still operational (+35 °C). vapor content of the carrier gas. This was decisive to reduce Indeed, in the second experiment, one decay chain of 283Cn was formation of ice layers at low temperatures. Still it was observed at −5 °C, which is at the edge of Hg deposition. impossible to exclude ice formation at temperatures below Therefore, a third experiment was conducted with increased gas about −100 °C. With a transport time of 2.2 s, the yield of 287Fl flow rate (increase from about 1 L/min to 2 L/min). Under

1291 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review this experimental condition, two atoms of 283Cn were detected α. The extremely small α of element 113, caused by the − − ° on the Au covered detector array at 29 and 39 C, contraction of the 7p1/2 AO, is the main reason for the very low −Δ 0(T) respectively. One further atom was found on the ice covered Ha on inert surfaces. part at −124 °C. All three atoms were observed where only The main oxidation state of element 113 is expected to be little if any Hg deposition occurred. The adsorption enthalpy of 1+, though the 3+ state should also appear.33,34 Cn on Au surfaces was evaluated from the deposition There are quite a number of molecular calculations for −Δ Au fl ff temperature of the four atoms adsorbed on Au as Ha (Cn) element 113. The in uence of relativistic e ects on the 7p +4 · −1 fi =52−3 kJ mol (68% con dence interval). For Hg, compounds was studied on the example of the hydrides MH −Δ Au · −1 − Ha (Hg) > 65 kJ mol was evaluated, in agreement with (M = 113 118) using various methods: DFC, RECP, 2c- and 449 −Δ Au ± · −1 166,433,434,456,471−479 literature data, where Ha (Hg) = 98 3kJmol was 4c-DFT. In 113H, the 6d and 7s AOs of determined. For the noble gas Rn on an ice surface, element 113 participate little in bonding and all the effects are −Δ ice ± · −1 fi Ha (Rn) = 19 2kJmol was measured, in excellent de ned by the 7p1/2 shell. A large relativistic contraction of the −Δ ice ± · −1 agreement with Ha (Rn) = 20 2kJmol from literature 7p1/2 AO results in a large contraction of the 113-H bond. The 466 283 ff Δ − data. In later experiments, one decay of Cn and one decay SO e ects on the bond lengths, Re(SO), are about 0.2 Å. of 285Cn were observed, the latter one in experiments where the Such a large bond contraction is not found in the other 7p MH. online thermochromatographic setup was attached to the exit In moist oxygen atmosphere, element 113 should react with of a gas-filled separator.467,468 The adsorption temperatures of OH, forming 113OH by analogy with TlOH. The compound is −Δ Au the additional events were in full agreement with Ha (Cn) predicted to be stable, with De of 2.42 eV in comparison with +4 · −1 −Δ Au 480 =52−3 kJ mol . The theoretically predicted Ha (Cn) of De(TlOH) of 3.68 eV, according to 4c-DFT calculations. 0.46 eV290a on Au turned out to be in good agreement with the The lower binding energy is due to the relativistic stabilization −Δ Au 0.04 194 experimental Ha (Cn) = 0.54−0.03 eV. This value, of the 7p1/2 AO. If element 113 adsorbs on a Au surface in the fl −Δ 0(T) re ecting the fact that Cn was observed in a range of form of 113OH, Ha should be smaller than that of adsorption temperatures that are significantly higher than those TlOH.480 − of Rn, is indicative of an Eb(Cn Aun) that is larger than that for Element 113 should also form 113F, as other group-13 pure van der Waals interaction. Thus, in agreement with theory, elements. Results of PP, DCB, RECP, and 4c-DFT 433,473,474 μ chemical bond formation obviously takes place in the case of calculations reveal increasing Re and e from TlF to Cn with Au, similar to, but weaker than for Hg with Au. Thus, 113F, in contrast to decreasing values from TlH to 113H. ff μ Cn is a d-metal, and not an inert gas, and its place in group 12 These di erent trends in Re and e for the MF compounds as of the Periodic Table is legitimate. compared to MH are explained by a more ionic nature of the MF molecules. DF calculations481 have shown that in the 16. ELEMENT 113 (113)(117) molecule there is a reversal of the dipole direction and a change of the sign of μ in comparison with other group- 16.1. Theoretical Predictions 17 homologs. The reason for that is the energetically higher- In elements 113 through 118, filling of the 7p shell takes place. lying 7p3/2 of the element 117 shell that donates into the low- The 7p AOs experience a very strong influence of relativistic lying 7p1/2 shell of the element 113 atom, while, in lighter effects: a large SO splitting and a strong contraction and homologs, the single electron of the group-13 atom usually 481 stabilization of the 7p1/2 AO, as well as an expansion and completes the valence p shell of the group-17 atom. destabilization of the 7p3/2 AO, that all increase along the 7p Like in Cn, the relativistic destabilization of the 6d AOs is series. The 7s2 stabilization is so large that it practically expected to influence properties of high-coordination com- becomes an inert pair in these elements. Early predictions pounds of element 113, as was confirmed by PP and RECP 33,34 473,482 indicated that these elements should be very volatile. calculations for 113X3 (X = H, F, Cl, Br, and I). As a Extrapolations from lighter homologs in the chemical groups consequence of the involvement of the 6d AOs, a T-shaped have, indeed, shown that elements 113 through 117 should rather than trigonal planar geometric configuration was Δ 0(298) have smaller HS or formation enthalpies of gaseous atoms, predicted for these molecules, showing that the valence shell Δ * 299 H 298(E(g)), than their lighter homologs (Figure 27). electron pair repulsion (VSEPR) theory is not applicable to the In element 113, the SO splitting is 3.1 eV, and the relativistic heaviest elements. Vest et al.483 have shown that the 37 stabilization of the 7p1/2 AO is 2.2. eV. The DCB FSCC decomposition energy of 113H3 into 113H and H2 becomes fi 2 calculations con rmed the 7s 7p1/2 ground state of element 113 more favorable in going down group 13. The reason for that is 469 ff and have given the first IP of 7.306 eV. An EA of 0.68(5) eV enhanced relativistic e ects on the 7p1/2 AO. was calculated in the same work, which is larger than EA(Tl) A stable high-coordination compound of element 113, − due to the relativistic stabilization of the 7p1/2 AO. The 113F6 , with the metal in the 5+ oxidation state is also 474 polarizability of element 113 was calculated as 29.85 au via the foreseen. 113F5 will probably be unstable, since the energy of 470 → DC FSCC approach. A trend reversal is observed in group the 113F5 113F3 +F2 decomposition reaction is less than α − · −1 → 13 in IPs (an increase), (a decrease), and AR (a decrease) 100 kJ mol . The calculated energies of the reaction MX3 from In on. This is connected with a trend reversal in the MX + X2 (from M = B through element 113) suggest a energies (an increase) and Rmax (a decrease) of the np1/2 AOs, decrease in the stability of the 3+ oxidation state in this group. fi Δ 0(298) Δ 0(T) that de ne those atomic properties, at In. Using the calculated In order to predict HS of the 7p metals and Ha of Δ 0(T) fl atomic properties, Ha of element 113 on Te on and the 7p elements on a Au surface that might be measured in polyethylen were estimated via an adsorption model (eq 8.1.1) future gas-phase experiments, 4c-DFT calculations of M2 and as 14.0 and 15.8 kJ·mol−1, respectively, which guarantees its MAu (M = 113 through 118) were performed.484,485,490 Some transport through Teflon capillaries in chemical experiments. In other calculations were also performed for these species: ab group 13, ΔH0(T) values were shown to exhibit a trend reversal initio DF for (113) ,486 4c-BDF, and 2c-SO ZORA; DC/MP2- a 2 − beyond In, due to the trend reversal in the atomic IP, AR, and DFT for the element 113−117 dimers165,166,471,472,487 489 and

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RECP ones for Fl- and 118 dimers.456 The obtained 4c-DFT 17. FLEROVIUM (Z = 114) D (M ) and D (MAu) are shown in Figure 64.484,490 e 2 e 17.1. Theoretical Predictions 2 2 Element 114, Fl, has a quasi-closed shell 7s 7p1/2 ground state caused by the large SO splitting of 4.7 eV122 and relativistic stabilization of the 7p1/2 AO. A high excitation energy into the fi 2 → 2 valence state con guration, 7p1/2 7p , of 6.2 eV calculated by Pitzer439 was a reason for him to believe that Fl, like Cn, would be an inert-gas like element. The most accurate DCB IHFSCC value of the IP(Fl) is 8.626 eV, which is, indeed, indicative of a very high inertness of Fl, though less than that of Cn.493 Other IPs up to the M3+ state for Fl and Pb were also calculated.493 The MCDF IPs and IR of the neutral to the 4+ ionized state of Fl and other group-14 homologs are also given,494 though being less accurate than the DCB values.493 The first IP of Fl and energies of several excited states were also calculated with the use of the relativistic complete active space MC CI method.142,143 The IPs of group- 14 elements show a decreasing trend from C to Sn and an Figure 64. Calculated atomization energies of MAu and M2 (M are increasing trend from Sn to Fl, so that the IP of the latter is elements Hg/Cn through Rn/118). Filled and open squares are D (MAu) and D (M ) of the 6p elements, respectively, while filled and even higher than the IP of Si. The reason is the relativistic e e 2 stabilization of the np AO with Z. Due to the quasi-closed open rhomboids are De(MAu) and De(M2) of the 7p elements, 1/2 484,490 2 respectively. . Reprinted with permission from ref 490. Copyright 7p1/2 shell, Fl will have a zero EA, as was shown by DCB 495 2010 American Institute of Physics. FSCC calculations. The main oxidation state should be 2+.33,34 IPs of internal conversion electrons, that is, of K-shell (1s) and L-shell (2s), of Fl are predicted to an accuracy of a few 10 According to these results, the (113)2 dimer should be eV using DHF theory, taking into account QED and nuclear- ff 127 weakly bound because the 7p1/2 electron yields a weak bond size e ects. This data can be used for experimental studies of having 2/3π bonding and 1/3σ antibonding character (or 2/3π Fl involving K conversion electron spectroscopy. σ 486 A polarizability of Fl of 30.6 au (as well as of Pb of 46.96 au) antibonding and 1/3 bonding). 442 Δ 0(298) Δ * was calculated using the DC CCSD(T) method. Polar- HS = H 298(E(g)) values were estimated then for the izabilities of group-14 elements including Fl were also 7p metals via a correlation with De(M2) in the respective chemical groups.484 They are given in Table 5 along with calculated using the DK and DC methods, though with a slightly smaller basis set496 (without h-functions taken into ΔH* (E ) predicted via a linear extrapolation in the groups 298 (g) account in ref 442). The DC CCSD(T) value of 31.49 au is (Figure 27),299 in good agreement with each other. One can see Δ * very close to that of ref 442. A Gaunt contribution was that H 298(E(g)) changes almost linearly with Z in these estimated in this work as 0.38 au, being rather significant. Δ 0(298) groups. For element 113, HS is lower than that of Tl due Finally, the recommended value of 31.0 au is that of ref 442, to the bonding preferentially made by the 7p1/2 AO. corrected for the Gaunt term. To study the interaction of element 113 with Au, 4c-DFT As in group 13, α shows a reversal of the increasing trend in calculations were performed for TlAu and 113Au.480 In 113Au, group 14 at Sn, so that α(Fl) is smaller than that of Ge. This is bonding should be weaker than in TlAu due to the relativistic due to the relativistic contraction of the np1/2 AO, which is 490 stabilization of the 7p1/2 AO. One can therefore expect that documented by a correlation between polarizabilities and the 122 element 113 will adsorb on Au at much lower temperatures mean radius of the np1/2 AO in group 14. −Δ 0(T) ± · −1 The AR of Fl was estimated as 1.75 Å, while the van der than Tl. An Ha (113) = 159 5kJmol on Au was − 442 −Δ 0(T) ± · 1 Waals radius, RvdW, of 2.08 Å, which both are relativistically estimated with respect to Ha (Tl) = 240 5kJmol ff contracted. The trend in AR and RvdW is also reversed at Sn due using the di erence in De(MAu), where M = Tl and element 480 to the same reason, the contraction of the np1/2 AO with 113. Adsorption of an element 113 atom and Tl on Au(111) α −Δ 0(T) − increasing Z. Using the and RvdW values, Ha (Fl) = 10.4 and Au(100) surfaces was also modeled by M Aun (n = 20) · −1 fl 491 kJ mol on Te on was predicted via a model of clusters and 2c-DFT calculations. The results show that the 442 α −Δ 0(T) ff − physisorption. As do and radii, Ha shows also a di erence in binding energy, Eb(M Aun), between Tl and −Δ 0(T) − − reversal of the increasing trend at Sn, so that Ha (Fl) is element 113 stays within ±15 kJ·mol 1 of 82 kJ·mol 1 obtained −Δ 0(T) smaller than that of Ge. The very low value of Ha in ref 480. Thus, the cluster calculations performed on a larger indicates that Fl should be easily delivered to chemical scale confirmed the estimate of Pershina et al.,480 so that experiments through Teflon capillaries. −Δ 0(T) ± · −1 Ha (113) can be given as 159 15 kJ mol . Due to the very strong stabilization and contraction of the 2 16.2. Experimental Results 7p1/2 (Fl) shell and, therefore, expected van der Waals nature of the M−M bonding, the homonuclear dimer Fl was of fi 2 16.2.1. Gas-Phase Chemistry. The rst experiments were particular interest for theory. Also, the knowledge of the Fl−Fl conducted by Dmitriev et al. at FLNR Dubna, studying the binding energy was important to estimate its sublimation adsorption of element 113 on Au surfaces. Only preliminary enthalpy. The ECP and 2c- and 4c-DFT calcula- 492 166,471,484,485 results have been communicated. tions agree on the fact that Fl2 is more strongly

1293 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review

−Δ 0(T) 448 bound than a typical van der Waals system. At the 4c-DFT level experimental Ha (Pb) = 2.43 eV on Au, so that − of theory, it is slightly more strongly bound than Cn2, but much Eb(Fl Aun)(n = 94) was given as 0.71 eV (Figure 62). The −Δ 0(T) · −1 more weakly bound than Pb2. A Mulliken population analysis obtained Ha (Fl) = 68.5 kJ mol is indicative of indicates that both the 7p1/2 and 7p3/2 AOs of Fl take part in formation of a chemical bond with Au. A comparison with the bond formation.484,485 The participation of the more group-12 Hg (where, however, Hg dissolves into Au) and Cn −Δ 0(T) ≪ extended 7p3/2(Fl) AO in bonding in comparison with the shows that the trend in Ha should be Cn < Fl < Hg Pb 290a − 6p3/2(Pb) AO explains an increase in Re from Pb2 to Fl2.SO (Figure 62). Calculations for the C Aun and Fl-Aun systems ff 450−453 e ects were shown to decrease De, but increase Re in both using other relativistic DFT methods came to the same systems.471 conclusion: Fl should form a rather strong chemical bond with Δ 0(298) HS of Fl was estimated using a correlation with the Au, stronger than that with Cn. A comparison of results of calculated De(Fl2) in group 14 (see Table 5). The obtained various calculations is given in Table 6. Many other, mostly simple, compounds of Fl were treated Table 5. Standard enthalpies of monotaomic gaseous theoretically using various approaches. In the 7p MH, from Fl ΔH* · −1 elements, 298(E(g)) (in kJ mol ), of the 7p Elements on, both the relativistically contracted 7p1/2 and expanded 7p3/2 AOs take part in the bond formation, so that Re(FlH) is longer method E113 E114 E115 E116 E117 than Re(PbH). De(MH) (M = 113 through 117) is reduced by extrapolation299 138.1 70.3 146.4 92.1 83.7 large SO effects, with the lowest value at FlH. Trends in the correlation484 144.7 70.4 152 ± 12 101.3 91.7 stability of hydrides were predicted as follows: RnH ≪ HgH < PbH and 118H ≪ FlH < CnH. RECP and DC CCSD(T) value is in very good agreement with that obtained via a linear calculations for PbH+ and FlH+ have also given a 50% weaker extrapolation in the group (Figure 27).299 E of Fl was coh bond and a shorter Re in the latter due to the contraction of the predicted from the SR and SO-GGA-DFT solid-state 434,456 − 7p1/2 AO. CAS-SCF/SOCI RECP calculations for FlH2 calculations.497 The obtained SO-PW91 value of 48.2 kJ·mol 1 1 demonstrated breakdown of the conventional singlet (X A1) is in reasonable agreement with other estimates.299,484 SO 3 ff 498 ff and triplet ( B1) states due to large SO-e ects. SO-e ects are effects were shown to lower E and lead to structural phase coh shown to destabilize FlH2 by almost 2.6 eV. transitions for solid Fl (the hcp structure in contrast to the fcc Electronic structures of FlX (X = F, Cl, Br, I, O) and FlO2 for Pb). In a nonrelativistic world, all group-14 elements would were calculated using 2c-RECP CCSD(T), 2c-DFT SO ZORA, adopt a diamond structure. 166,471 and 4c-BDF methods. In contrast to PbO2 (De = 5.60 To estimate the interaction strength of Fl with noble metals, eV), FlO2 (De = 1.64 eV) was predicted to be thermodynami- particularly with Au, 4c-DFT calculations were performed for cally unstable with respect to the decomposition into the metal MM′, where M are group-14 elements and M′ are group-10 atom and O2. According to results of these calculations, Fl and 11 metals.485,490 The results have shown that Fl interacts should not react with O2 under typical experimental conditions, most strongly with Pt, while least strongly with Ag. De(FlAu) is as was discussed in ref 442. fi 481,499 signi cantly (1.42 eV) smaller than De(PbAu) (Figure 64) due Ab initio DF and PP calculations for the decomposition to the very large 7p AO stabilization. However, the 7p AO → → 1/2 3/2 reactions MX4 MX2 +X2 and MX2 M+X2 (M = Si, Ge, also takes part in the bond formation, which results in an Sn, Pb, and Fl; X = H, F, and Cl) also predicted a decrease in − increase in Re from the Pb to Fl dimers. The Fl Au bond the stability of the 4+ oxidation state in group 14. The − 290a,490 should, however, be stronger than the Cn Au one. This instability was shown to be a relativistic effect. The neutral state  is due to the fact that in FlAu even though both FlAu and was found to be more stable for Fl than for Pb. As a σ* CnAu are open shell systems with one antibonding consequence, Fl is expected to be less reactive than Pb, but electronelectron density is donated from the 7p (Fl) AO, 2‑ 1/2 about as reactive as Hg. The possibility of the existence of FlF6 lying higher in energy, to the 6s(Au) AO, while, in CnAu, some was also suggested in ref 499. excitation energy is needed to transfer electron density from the 17.2. Experimental Results closed 7s2(Cn) to the open 6s(Au) shell (Figure 61). Large-scale 4c-DFT calculations were also performed for M 17.2.1. Gas-Phase Chemistry. The heaviest element = Pb and Fl interacting with large Aun (n > 90) clusters investigated experimentally to date is Fl, which would be simulating a Au(111) surface.290a Both Pb and Fl were found to placed as eka-Pb into group 14 of the Periodic Table. Suitable 287 +0.16 89 prefer the bridge adsorption position. The calculated Eb value nuclides for chemical experiments are Fl (t1/2 = 0.48−0.09 s) 288 +0.17 289 +0.8 500 for Pb (2.40 eV) is in very good agreement with the or Fl (t1/2 = 0.69−0.11s) and Fl (t1/2 = 2.1−0.4s), which

Table 6. Cn-Aun and Fl-Aun Binding Energies (in eV) Simulating Interactions of Cn and Fl with Au(100) and Au(111) Surfaces (Bold Values Are for the Preferential Positions)

method n position surface Cn Fl ref 4c-DFT 1 top 0.51 0.73 290a 2c-DFT 1 top 0.47 0.72 450−452 SO DFT 3 top, bridge 0.47 0.77 450−452 2c-DFT 26 bridge Au(100) 0.33 0.55 452 2c-DFT 37 hollow-2 Au(111) 0.49 453 4c-DFT 95 top Au(111) 0.30 0.47 290a 4c-DFT 94 bridge Au(111) 0.42 0.71 290a 4c-DFT 107 hollow-2 Au(111) 0.46 0.59 290a −Δ Au ∞ 0.04 0.6 ≥ Ha (exp) unknown unknown 0.54−0.03 0.35−0.1,or Cn 192, 195, 260

1294 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review can be synthesized in the reaction 48Ca + 242Pu or 244Pu, The vertical dash-dotted line indicates the temperature at which respectively.89 In the experiment where three decay chains of the dew point in the gas was reached. Left of the dash-dotted 283Cn were registered in the COLD setup (see Figure 63),194 line, the surface of the COLD detector was either Au (top also a three member decay chain was registered in detector pair detectors) or SiO2 (bottom detectors); right of the dash dotted 19 held at −88 °C that was attributed to an atom of 287Fl line, the surfaces were covered by a thin layer of ice. In panel 1 185 reaching the detector despite its short t1/2 of only 0.5 s. The the deposition of the nuclide Hg (t1/2 = 49 s) (gray bars), transport efficiency was estimated to be only 5%.195 produced from an admixture of Nd of natural isotopic An additional experiment conducted by the PSI group in composition to the target material, is shown. Single atoms of 2007 at FLNR employing a 244Pu target in combination with Hg show the expected diffusion controlled deposition pattern the COLD setup revealed two further decay chains that were from irreversible adsorption on the Au surface of the top attributed to 288Fl.195 The transport efficiencies of 288Fl and detectors. Due to the high flow rates and since only one side of 289 288 Fl can be estimated using the currently best t1/2 of Fl and the detector channel was Au covered, the distribution of Hg 289 219 Fl to be 11% and 48%, respectively, due to their longer t1/2 extends far into the COLD detector. The nuclide Rn (white compared to that of 287Fl. Indeed, two events attributed to the bars) being produced in transfer reactions was deposited on the decay of 288Fl were detected. One α-decay event of 9.95 MeV ice surface only at very low temperatures close to the exit of occurred on the bottom detector of detector pair 18, followed COLD, as expected for the noble gas Rn. The deposition 185 219 0.109 s later by one SF fragment in the neighboring top patterns of both Hg and Rn could be satisfactorily − ° 288 described by a microscopic model of the adsorption chromato- detector 19 held at 90 C. The deposition of Fl occurred 288 most likely on the Au covered detector 19 top, where the SF graphic process based on a Monte Carlo approach (solid −Δ 0(T) ≥ · −1 fragment was observed, whereas the α-particle was emitted lines) with Ha (Hg) 50 kJ mol on a Au surface and −Δ 0(T) · −1 across the gap and registered in the detector 18 bottom. The Ha (Rn) = 19 kJ mol on an ice surface, respectively, in α good agreement with literature data. In panel 2, the location of second decay chain started with an -particle of 9.81 MeV in 283 the top detector of pair 3 and ended 0.104 s later with a single three decays of Cn in COLD (see Figure 63) is depicted that fission fragment observed in bottom detector 6 at −4 °C. The was observed in the same experiment as the decay chain attributed to 287Fl (panel 3). In panels 3 and 4, the locations of observation that the decay of a daughter atom is displaced from 287 288 that of the mother atom was explained by the recoil of the deposition of one atom of Fl and two atoms of Fl, daughter atom out of the detector during the α-particle respectively, are shown. From these three events, a most −Δ 0(T) +20 · −1 195 probable adsorption enthalpy of Ha (Fl) = 34−3 kJ mol emission, followed by a transport with the carrier gas. A 195 background of α-particle events in the region where decays of (68% c.i.) on a Au surface was deduced. The corresponding 289Fl and its daughter 285Cn were expected prevented the calculated model distributions are truncated at the dash-dotted fi line indicating the dew point. positive identi cation of the rather long decay chain from −Δ 0(T) +20 · −1 289Fl.195 An adsorption enthalpy of Ha (Fl) = 34−3 kJ mol on a Au surface is surprisingly low, since Fl is expected to be more The location of the three detected events attributed to reactive than Cn and should thus deposit at higher temper- deposition of Fl atoms in relation to the elements Cn, Hg, and atures than Cn and not a lower temperatures, as observed. Rn is shown in Figure 65, panels 1 through 4. The dashed line 290a −Δ 0(T) Theoretical calculations predicting Ha (Cn) = 45 (right-hand axis) always indicates the temperature gradient − kJ·mol 1 on Au, in good agreement with experiment, predicted established during the experiments. The left-hand axis indicates − −ΔH0(T)(Fl) = 68 kJ·mol 1 on Au, corresponding to a relative yields per detector pair in percent of a given element. a somewhat less volatile Fl compared to Cn. The DFT solid- state calculations give a higher Ecoh of the Cn solid in comparison with the Hg and Fl ones,444,497 (though a direct comparison of the calculations performed in different approximations is not straightforward). Because in group 12 Δ 0(298) −Δ 0(T) there is no correlation between HS and Ha on Au Δ 0(298) Δ 0(298) HS (Hg) < HS (Cn), while, on a Au surface, Δ 0(T) Δ 0(T) Ha (Hg) > H a (Cn). On the contrary, in group 14 Δ 0(298) −Δ 0(T) there is a correlation between HS of metals and Ha Δ 0(298) Δ 0(298) −Δ 0(T) on Au, so that HS (Pb) > HS (Fl) and Ha (Pb) −Δ 0(T) > Ha (Fl) on Au. This case shows clearly that there is no Δ 0(298) −Δ 0(T) general correlation between HS on metals and Ha on Au. In groups 15 through 17, this is another obvious case, as will be shown later. The observation that Fl exhibits an unexpected high volatility, together with the fact that a background in the detector array made the positive identification of 289Fl events impossible, caused some skepticism.501 In order to remove the background of undesired reaction products, the chemical techniques developed so far were fi Figure 65. Deposition patterns of the elements Hg, Rn, Cn, and Fl in coupled to a physical preseparator (see section 7.1.2). A rst the COLD detector as observed in experiments by Eichler et al.195 chemical experiment with Fl using a combination of IC and TC Reproduced with permission from ref 195. Copyright 2010 Old- on Au surfaces was conducted behind the TASCA separator in enbourg Wissenschaftsverlag GmbH. For a detailed discussion see text. the fall of 2009. Two events attributed to the decay of Fl were

1295 dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312 Chemical Reviews Review observed.260 Both α-particle decays of Fl isotopes occurred in 10.8 kJ·mol−1 on Teflon should allow for its transport through the isothermal section at room temperature. Teflon capillaries to the chemistry setup. The obtained almost −Δ 0(T) A detailed comparison with the experiment of the PSI-FLNR equal values of Ha of element 118 and Rn on various collaboration195 cannot be made, especially since no analysis of surfaces are indicative, however, that experimental distinction the experiment behind TASCA has been published so far. between these elements by using these surfaces will be Despite this, it is quite remarkable that two independent impossible. A possible material could be activated charcoal; experiments were able to separate and detect Fl in a chemistry however, further studies are needed to test this assumption. experiment, demonstrating the power of gas chemical To estimate the strength of the M−M bonding in the solid investigations. state of elements 115 through 118, 4c-DFT calculations were performed for their homonuclear dimers.484 Results shown in − 18. ELEMENT 115, LIVERMORIUM (Z = 116), ELEMENT Figure 64 indicate that the M M bonding is weaker in (115)2 117, AND ELEMENT 118 through (117)2 compared to Bi2 through At2, respectively, due to the availability of practically only 7p3/2 AOs for bonding. 18.1. Theoretical Predictions Δ * This means that H 298(E(g)) of elements 115 through 117 fi should be the smallest in the groups. Their estimates made In elements 115 through 118, lling of the 7p3/2 shell takes place that will determine their chemical properties. For element using the calculated De(M2) values are in good agreement with 115, DCB FSCC calculations give an IP of 5.579 eV.502 For Lv those obtained via linear extrapolations in groups 15 through and element 117, older DF calculations give IPs of 6.6 and 7.7 17 (Figure 27), meaning that these elements should be rather 33,34 eV, respectively, while the DCF FSCC calculations are in volatile. Bonding of (118)2 is, however, stronger than that of 503,504 progress. For element 118, the DC FSCC value of the IP Rn2, since it is of van der Waals type and caused by the largest is 8.914 eV.505 Since 7p AOs are relativistically more polarizability of the 118 atom in group 18. 3/2 Δ 0(T) destabilized than the np3/2 AOs of the lighter homologs, IPs of To predict Ha of elements 115 through 118 on a Au elements 115 through 118 are smaller than those of the lighter surface, 4c-DFT calculations were performed for the MAu 490 homologs in groups 15 through 118. This means that the 7p3/2 dimers and their 6p homologs. Results are shown in Figure elements should be chemically more reactive than the 6p3/2 64. One can see that, in groups 15 through 17, De(MAu) values ones. are about the same for the 7p and 6p elements. This is in Due to the relativistic destabilization of the np3/2 AOs, EAs of contrast to the trends in De(M2) in these groups, where De(Bi2) 502 ≫ ≫ the elements 115 (with a DCB FSCC value of 0.383 eV ), Lv De[(115)2], De(Po2) De(Lv2), and De(At2)>De[(117)2]. (with a DCB FSCC value of 0.905 eV504), and 117 (with a DC The relatively strong M−Au bonding of elements 115 through CCSD(T) value of 1.589 eV504) will also be smaller than those 117 with Au is explained by the relativistic destabilization of the fi of the 6p elements. For element 118, DCB FSCC+QED 7p3/2 AOs tting energetically better to the 6s(Au) AO, thus 131   σ calculations have given a positive EA of 0.056 eV. The making together with the 7p1/2 AO a full -bond in MAu, in ff reason for that is the relativistic stabilization of the 8s AO. di erence to M2, where only the 7p3/2 AOs are involved in 484 IPs of internal conversion electrons, that is, of K-shell (1s) bonding. In group 18, a reversal of the trend takes place, so and L-shell (2s), of Lv and element 118 are predicted to an that De(118Au) > De(RnAu), in agreement with the trend in accuracy of a few 10 eV using DHF theory, taking into account De(M2). This is due to the relativistically more destabilized 127 QED and nuclear-size effects. This data can be used for 7p3/2(118) AO than the 6p3/2(Rn) AO, thus better overlapping experimental studies of Fl involving K conversion electron with the valence AOs of Au. Hence, for elements 115 through spectroscopy. 118, temperatures as high as for their 6p homologs will be For elements 115 through 118, lower oxidation states should needed to detect their equilibrium adsorption position on Au. be more stable in comparison with those of the lighter The calculations have also revealed that the M−Au bond homologs due to the inaccessibility of the relativistically strength does not decrease linearly with Z in groups 15, 16, and −Δ 0(T) stabilized 7p1/2 AO for bonding and the SO destabilized 17, which means that Ha on Au will not correlate with Δ 0(298) 7p3/2 electrons. Thus, for element 115, the 1+ state should be HS in these groups. the most important one. The 3+ state should also be possible, Formation enthalpies of MX2 and MX4 (X = F, Cl, Br, I, 2− 2− − 3− while 5+ should not. For Lv, a decrease in the stability of the 4+ SO4 ,CO3 ,NO3 , and PO4 ) for Po and Lv were estimated oxidation state is expected, and the 2+ state should be on the basis of the MCDF atomic calculations.508 They important. For element 117, the 1+ and 3+ oxidation states confirmed the instability of the 4+ oxidation state of Lv. should be the most important ones, while the 5+ and 7+ states The influence of SO effects on the molecular structure of are less important. The 1− state of element 117, having one MX2 (X = F, Cl, Br, I, At, and element 117) of Lv and its lighter electron hole on the 7p3/2 AO, should, therefore, be less homologs was studied with the use of the 2c-HF and DFT ECP important (its EA is the smallest in the group). For element methods.509 The results have shown that while the molecules 118, the 2+ and 4+ states are possible, while the 6+ one will be are bent at a scalar relativistic level, SO coupling is so strong 2 less important, since the 7p1/2 pair is very much stabilized. that only 7p3/2 AOs of Lv are involved in bonding, which favors The AR of the 7p3/2 elements should be larger than those of linear molecular geometries for MX2 with heavy terminal their 6p3/2 homologs due to the spatially more expanded 7p3/2 halogen atoms. AOs. Accordingly, the polarizability of these elements should The influence of relativistic effects on the electronic structure also be larger. For element 118, the polarizability of 167.4 au, a of 117H was investigated on the basis of the HF and DC DCB FSCC result, is also the largest in the 18th group.505 CCSD(T) calculations.434 Relativistic effects, including SO fi Using the calculated atomic properties of element 118, its ones, were shown to signi cantly decrease De(117H) and Δ 0(T) Ha values on noble metals (Au and Ag) and increase Re(117H). (quartz, ice, Teflon, and graphite) were predicted using a Electronic structures of IF, AtF, and 117F were considered at 505 −Δ 0(T) 482 physisorption model. A very low value of Ha (118) of the DC and RECP levels of theory. De(117F) was shown to

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fi 518 Figure 66. Dissociation energies, De of group-1 and group-2 M2 ( lled rhomboids are 4c-DFT calculations; open squares are experimental Δ 0(298) fi values), as well as HS ( lled triangles are experimental values; open ones are estimates). Reprinted with permission from refs 518 and 519. Copyright 2012 Elsevier and 2012 American Institute of Physics, respectively.

fl 515,516 517 be the largest among the group-17 uorides. For 117F3, the attempts have failed. The currently best options based RECP calculations have shown that the D3h geometry is not the on the parameters reaction asymmetry and Q value appear to proper one, in difference from the sixth period compound of be the reactions 50Ti + 249Bk and 50Ti + 249Cf to synthesize At, thus again indicating that the VSEPR theory is not elements 119 and 120, which are currently attempted at applicable to the heaviest elements.482,510 117Cl is predicted to TASCA at GSI. However, the t1/2 values of the synthesized be bound by a single π bond and have a relativistically (SO) 511 nuclei are expected to be of the order of milliseconds or even increased Re. microseconds. Provided suitable long-lived isotopes of these Due to the relativistic destabilization of the 7p3/2 AO in elements are found, the volatility of their atoms might be element 118, it is predicted to be rather reactive. It should form studied in the long term using some advanced chromatography the 118-Cl bond.512 The RECP calculations for the reactions M (e.g., vacuum) techniques that can cope with extremely short +F → MF and MF +F → MF , where M = Xe, Rn, and 2 2 2 2 4 lifetimes of their isotopes. To foresee the outcome of such element 118, confirmed the increasing stability of the fluorides experiments, as well as to study the influence of relativistic in the group as a result of the increasing polarizability of the ff central atom.482,513 Also, the following trends in the stability of e ects on the 8s AOs of these heaviest elements and their fl compounds, some theoretical studies have been under- the uorides were established: RnF2 < HgF2 < PbF2, while fl ff taken.506,507,518,519 For earlier predictions of properties of CnF2 < FlF2 < 118F2. The in uence of relativistic e ects on the electronic structure of 118H+ was investigated on the basis of these elements based on the atomic calculations, see refs 33 and HF and DC CCSD(T) calculations.434 Relativity (including SO 34. Properties that are of interest for gas chromatographic ff fi + Δ 0(298) Δ 0(T) e ects) was shown to signi cantly decrease De(118H ) and studies, i.e., HS and Ha of elements 119 and 120 on + increase Re(118H ). noble metals were predicted on the basis of 4c-DFT fl The in uence of the SO interaction on the geometry of calculations for intermetallic M2 and MAu (M are group-1 518,519 group-18 MF4 was investigated by the RECP-SOCI/CCSD and group-2 elements) compounds. 478,513,514 calculations. It was shown that in 118F4,aTd Figure 66 demonstrates the obtained D (M ) and their fi e 2 con guration becomes more stable than the D4h one known trends in groups 1 and 2. One can see that, in these groups, for the lighter homologs. The reason for this unusual geometry there is a reversal of the trends in De(M2) at Cs and Ba, is the availability of only the stereochemically active four 7p3/2 respectively, though in an opposite way. The reason for the electrons for bonding. This is another example of the different behavior is a different type of M−M bonding in these inapplicability of the VSEPR theory for the heaviest elements. fl groups: a covalent one in group 1, while a van der Waals one in An important observation was made that the uorides of group 2, even though both are defined by the behavior of the ns element 118 will most probably be ionic rather than covalent, AOs. as in the case of Xe. This prediction might be useful for future Thus, (119) , having a σ 2 ground state, should be bound the gas-phase chromatography experiments. 2 g strongest by covalent forces among the homologs and have a 19. ELEMENT 119 AND ELEMENT 120 short bond length (about that of Rb2) caused by the contraction of the 8s AO. On the contrary, (120)2 with a 19.1. Theoretical Predictions σ2 σ*2 g u ground state should be the most weakly bound, only by At the time being, the chemistry of elements heavier than Z = van der Waals forces, among the homologs (the number of 118 rests on a purely theoretical basis. The synthesis of bonding and antibonding electrons is the same), and the bond elements beyond Z = 118 appears to be difficult, and several should be the longest.

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−Δ 0(T) fi Figure 67. 4c-DFT dissociation energies, De, of group-1 and group-2 MAu, as well as adsorption enthalpies Ha ( lled symbols are 520 semiempirical calculations, while open ones were obtained via correlations with De(MAu), where M = Au, Pt, and Ag). Reprinted with permission from refs 518 and 519. Copyright 2012 Elsevier and 2012 American Institute of Physics, respectively.

Δ 0(298) · −1 Δ 0(298) HS (119) = 94 kJ mol and HS (120) = 150 elements due to the relativistic ns AO contraction. The 119F kJ·mol−1 values were obtained via a correlation with the was found to be less ionic than lighter alkaline fluoride Δ 0(298) calculated De(M2) in groups 1 and 2, respectively. The HS homologs, in contrast to expectations based on periodic trends. values show the same reversal in the groups as the De(M2) values do. According to these results, element 119 metal should 20. ELEMENTS BEYOND Z = 120 be as strongly bound as K, while element 120 metal should be 20.1. Theoretical Predictions the most weakly bound in group 2, though it is more strongly The chemistry of these elements will be defined by many open bound than element 119 (Figure 66). shells and their mixing.33,34 Due to very strong relativistic Binding energies of MAu of group-1 and group-2 elements effects, things will be much more different than anything known are shown in Figure 67. ΔH0(T) values, estimated using a a before. However, without relativistic effects, it would also be correlation with the calculated De(MAu) in these groups, are very different due to the very large orbital effects. also depicted there. According to the data, elements 119 and Very few molecular calculations exist in this superheavy 120 should form stable compounds with gold. The De(MAu) domain. Properties of elements heavier than 120 predicted on values reveal a reversal of the increasing trend at Cs and Ba in the basis of atomic calculations were discussed.33,34,185,524,525 groups 1 and 2, respectively, so that both 119Au and 120Au More recent considerations of their chemistry can be found in should be the most weakly bound among the considered fi refs 526 and 527. homologs in these groups. The trend is de ned by the behavior A list of possible molecules of elements in the range Z = of the ns AOs, whose relativistic stabilization in the groups 121−164 was suggested,527 though their verification should be starts to dominate over the orbital expansion beyond Cs and left to future theoretical studies. Interesting examples are those Ba, respectively. where the elements are in unusual valence states or −Δ 0(T) · −1 −Δ 0(T) The Ha (119) = 106 kJ mol and Ha (120) = 172 coordination, such as, for example, 144F (an analog of · −1 8 kJ mol were determined via a correlation with De(MAu) in PuF ) or 148O (an analog of UO ). −Δ 0(T) 8 6 6 these groups. Using correlations with Ha (M) on other Quasi-relativistic multiple-scattering calculations on 125F6 −Δ 0(T) 1 noble metals, Ha of these elements on Ag and Pt were have found that bonding is defined by the 5g electron, with the −Δ 0(T) 1 528 also predicted (Figure 67). The very moderate Ha values situation being analogous to NpF6 with the 5f electron. of elements 119 and 120, the lowest in groups 1 and 2, There are noncorrelated DF calculations for fluorides of − − especially on Ag (63 kJ·mol 1 and 50 kJ·mol 1, respectively), element 126.529,530 would allow adsorption chromatographic measurements of Accurate predictions of properties of specific compounds will these elements. be quite a challenging task in this area. This may need inclusion Δ 0(298) −Δ 0(T) ff The obtained HS and Ha values show that there of QED e ects to reach the required accuracy. is no correlation between these quantities in group 1, as they change in the opposite way with Z. In group 2, there is a 21. CONCLUSIONS AND OUTLOOK Δ 0(298) −Δ 0(T) correlation between HS and Ha . Despite the fact that only single atoms of transactinide elements “ ” Thermodynamic properties of metals of elements 113 can be studied one-atom-at-the-time and the short t1/2 are of through 120 were also predicted521 using atomic calculations the order of seconds, the experimental body of data is already and mathematical models. substantial and impressive. It was shown that the heaviest Hydrides and fluorides of elements 119 and 120 were elements are basically homologs of their lighter congeners in considered within the PP and ab initio DF approxima- the chemical groups, though their properties may be rather tions.434,522,523 It was shown that bond distances decrease different due to very large relativistic effects on their electron from the seventh to the eighth period for group-1 and group-2 shells. Relativistic effects were found to be predominant over

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Table 7. Trends in Volatility of the Heaviest Element Compounds and Their Lighter Homologs in the Chemical Groups

group species theor pred ref exp obsd ref

4 MCl4, MBr4 Hf < Rf 38 Hf < Rf 332, 333 ≈ 5ML5 (L = Cl, Br) Nb < Ta < Db 331 NbCl5 DbCl5 376

DbCl5 > DbOCl3 331 DbCl5 > DbOCl3 229 → ¯ MBr5 MBr6 Nb > Ta > Db 364 Nb > Ta > Db 226 Db > Nb > Ta 377

6MO2Cl2 Mo > W > Sg 382 Mo > W > Sg 232, 234

7MO3Cl Tc > Re > Bh 291 Tc > Re > Bh 235

8MO4 Ru < Os > Hs 293 Os > Hs 197 12 M Hg < Cn 290a, 445, 447, 450 Hg < Cn 192, 194 14 M Pb ≪ Fl < Cn 290a, 452, 453, 485 Fl ≥ Cn 195 Fl ≤ Cn 260

Table 8. Trends in Hydrolysis and Complex Formation of the Heaviest Element Compounds and Their Lighter Homologs in the Chemical Groups

group extracted complex theor pred ref exp obsd ref 4 hydrolysis of M4+ Zr > Hf > Rf 307 Zr > Hf > Rf 360 z−x ≤ MFx(H2O) 8−x (x 4) Zr > Hf > Rf 307 Zr > Hf > Rf 272, 354 2− ≥ ≥ MF6 Rf Zr > Hf 307 Rf Zr > Hf 272, 350, 355 Zr > Hf ≫ Rf 2− MCl6 Zr > Hf > Rf 307 Rf > Zr > Hf 357

MCl4 Rf > Hf > Zr 307 Zr > Rf > Hf 271 Zr > Hf ≈ Rf 361 4− ≫ ≫ M(SO4)4 Zr > Hf Rf 310 Zr > Hf Rf 362, 363 5 hydrolysis of M5+ Nb > Ta > Db 303 Nb > Ta 360 − − ≥ ≥ MOCl4 , MCl6 Nb Db > Ta 304 Nb Db >Ta 277 − − MF6 , MBr6 Nb > Db > Ta 305 Nb > Db > Ta 277 6 hydrolysis of M6+ Mo > W > Sg 306 Mo > W > Sg 279

hydrolysis of MO2(OH)2 Mo > Sg > W 306 Mo > W 279

MO2F2(H2O)2 Mo > Sg > W 308 Mo > W 385, 531, 532 − MOF5 Mo < W < Sg 308 Mo < W 385 2− ≫ ≥ 8MO4(OH)2 Os > Hs Ru 309 Os Hs 420 the orbital ones in the electronic structures of the elements of routine application to the heaviest systems lies still in the the seventh period and heavier. They are responsible for trends future. in the chemical groups (a continuation, or a reversal) with Using the methods described in this review, reliable increasing Z from the elements of the sixth period. Thus, for predictions of properties of the heaviest element and their elements of the seventh period and heavier, the use of compounds became available. Theoretical calculations permit- relativistic methods is mandatory. Straightforward extrapola- ted establishment of important trends in spectroscopic tions of properties from lighter congeners may, therefore, result properties, chemical bonding, stabilities of oxidation states, in erroneous predictions. ligand-field effects, complexing ability, and others in the groups Spectacular developments in relativistic quantum theory, of the Periodic Table including the heaviest elements, as well as computational algorithms, and computer techniques allowed assessment of the role and magnitude of relativistic effects. for accurate calculations of properties of the heaviest elements Detailed studies were offered for elements Rf through 120, as and their compounds. Nowadays, most accurate atomic DC(B) well as for some species of even heavier elements. A high correlated calculations including QED effects are available for accuracy of total energy calculations allowed for predictions of the heaviest elements, reaching an accuracy of a few stability of species, of their geometry and energies of chemical millielectronvolts for electronic transitions and ionization reactions in the gas and aqueous phases, as well as of adsorption potentials. These calculations allow predictions of electronic on surfaces of metals. However, fully relativistic descriptions of configurations of the heaviest elements up to Z = 122. For adsorption processes on complicated or inert surfaces are still heavier elements, as well as for the midst of the 6d-element problematic. Therefore, some models were used in practical series, MCDF calculations are still the source of useful applications. Also, physicochemical models were helpful in information. predicting some other properties that are difficult to handle in a Most of the molecular calculations were performed with the straightforward way, such as, for example, extraction from use of relativistic DFT and RECP methods that turned out to aqueous solutions or ion exchange separations. Such studies be complementary both conceptionally and quantitatively. were performed for elements Rf through Hs, Cn, Fl, and Their combination is presently the best way to study properties element 113. Some estimates of adsorption enthalpies of even of complex systems of the heaviest elements. DC ab initio heavier elements, up to Z = 120, on noble metals are also molecular methods are in the phase of development, and their available.

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21.1. Summary of Volatility Studies of the Heaviest separation will also need new technological developments to Elements and Their Compounds cope with the very low production rates and short t1/2. Predicted trends in volatility of the heaviest elements and their In this area, theoretical chemistry will have a number of compounds compared to the experimental observations are exciting tasks to predict the experimental behavior in chemical summarized in Table 7. One can see that almost all the separation experiments. Even though some basic properties of predictions for group 4 through group 8, as well as for group 12 these elements have been theoretically outlined, more detailed were confirmed by the experiments. In addition, the calculated studies should follow, taking into account experimental details. Δ 0(T) Some further methodical developments in the relativistic absolute values of Ha were in very good agreement with the experimental ones, as discussed above. Thus, due to their quantum theory, such as, for example, fully relativistic ab initio highest covalency, the pure halides of Rf and Db were expected molecular, cluster, and solid state codes, also with inclusion of ff to be more volatile than their next lighter homologs in their QED e ects on a SCF basis, will be needed to achieve a respective groups. This was clearly observed experimentally for required accuracy of the predicted quantities for those very high Rf in group 4 (see Figure 37). Open questions remain in the Z numbers. These future calculations will also need powerful interpretation of the volatility of group-5 pure halides, which supercomputers. might need further experimental or/and theoretical consid- In the long run, new accelerators delivering higher beam 250 erations. In groups 6 and 7, the trend to a decrease in volatility intensities and even more exotic target materials, such as Cm, 251,252 254 is clear as defined by decreasing dipole moments of the Cf, and Es, will allow production of nuclides closer to 534 oxyhalides. In group 8, hassium tetroxide should be less volatile the line of β-stability in superheavy element factories. than the Os homolog because of its larger polarizability. Cn is Possible electron-capture branches in the members of the α- more volatile than Hg due to its high inertness. It should also particle decay chains may lead to the formation of longer-lived be more volatile than Fl. While predictions of the adsorption nuclides of Mt, Ds, Rg, and Cn, that could be stored and properties of Cn on a Au surface were in line with experimental studied in traps. observations, predictions for Fl are awaiting further exper- The theoretical and experimental investigation of trans- imental verifications. actinide elements continues to be a fascinating branch of 21.2. Summary of Aqueous Chemistry Studies of the chemistry that, by its nature, is pure fundamental research. Heaviest Elements AUTHOR INFORMATION A summary of the predicted trends in hydrolysis, complex formation, and extraction of the heaviest element complexes Corresponding Author and their homologs as compared to the experimental results is *Electronic address: [email protected]. given in Table 8. As one can see, most of the predictions were Notes confirmed by experiments for the heaviest elements and their homologs, while some of them are still awaiting verification, as The authors declare no competing financial interest. in the case of Sg in HF solutions. Biographies The calculations have shown that the theory of hydrolysis301 based on the relation between the cation size and charge did not explain all the experimental behavior, such as, for example, the difference between Nb and Ta, or Mo and W. Only by performing relativistic calculations for real chemical equilibrium in solutions can complex formation and hydrolysis constants, as well as distribution coefficients between aqueous and organic phases (or sorption coefficients), and their order in the chemical groups be correctly predicted. Being often conducted in a close link to the experiment, those theoretical works helped design chemical experiments and interpret their outcome. In turn, experimental results put theoretical predictions (and hence models used for making these predictions) to the test, and in this way, they help to improve the models. The synergism between the theoretical fi and experimental research in this eld led to better under- ̈ standing of the chemistry of these exotic species. Andreas Turler was born in Winterthur, Switzerland. He received his Diploma and Ph.D. in chemistry from University of Bern, Switzerland, 21.3. Future Developments having carried out research on nucleon transfer reactions under the In the near future, first results concerning the chemistry of supervision of Prof. Hans-Rudolph von Gunten. He then joined the element 113 can be expected. Also, element 115 may come group of Prof. Darleane C. Hoffman at Lawrence Berkeley National within reach with present day technologies, provided a constant Laboratory, Berkeley, United States, as postdoctoral fellow. In 1992, he supply of 249Bk target material. The knowledge of lighter moved back to Switzerland and in the following years worked as staff transactinides will grow, and new classes of compounds, such as scientist at Paul Scherrer Institute, Villigen, with Prof. Heinz Gaggeler.̈ volatile carbonyls, open new perspectives,533 especially to first In 1994 he was awarded the “Fritz-Strassmann-Preis” of the chemical studies of elements such as Mt. For the not yet Gesellschaft Deutscher Chemiker. The habilitation in 2000 at studied elements, such as Mt, Ds, Rg, and element 115, University of Bern was followed with an appointment as full professor isotopes with t1/2 suitable for chemical studies have already and director of the Institute of Radiochemistry at Technical University been identified. For elements 116 through 118, new isotopes of Munich, Germany. In 2009 he returned to Switzerland as head of suitable for chemical studies must first be discovered. Their the Laboratory of Radiochemistry and Environmental Chemistry at

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Paul Scherrer Institute and University of Bern. His scientific interests CI configuration interaction are nuclear and radiochemistry in general, with one focus on the CIX cation exchange physics and chemistry of transactinide elements. COLD cryo online detector COMPACT cryo online multidetector for physics and chemistry of transactinides CR covalent radius CTS cryo thermochromatographic separator DCB Dirac−Coulomb−Breit De atomization energy DF Dirac−Fock DFT density functional theory DGFRS Dubna gas-filled recoil separator Δ 0(T) Ha enthalpy of adsorption at standard conditions at Ta or T50% Δ Hevap enthalpy of evaporation Δ 0(298) HS sublimation enthalpy Δ * H 298(E(g)) standard enthalpy of monatomic gaseous ele- ments Valeria G. Pershina was born in Cheliabinsk, Russia. She received her DHF Dirac−Hartree−Fock Diploma from Mendeleev University of Chemistry and Technology DKH Douglas−Kroll−Hess (Moscow) and her Ph.D. degree (with Professors G. Ionova and V. DS Dirac−Slater Spitsyn) from Institute of Physical Chemistry, USSR Academy of DS-DV Dirac−Slater discrete variational method Sciences (Moscow). In the following years she worked as a researcher, E(x) energy of the charge transfer transition group leader, and deputy head of the quantum chemistry lab at the EA electron affinity same Institute of Physical Chemistry. Since 1990, she has been at the Ecoh cohesive energy University of Kassel (with Prof. B. Fricke), and from 1996 on, at the ECP effective core potential GSI, Darmstadt, as a senior researcher. In 1994 she habilitated at the ECR electron cyclotron resonance Institute of Physical Chemistry, Russian Academy of Sciences, EH effective Hamiltonian Moscow, and received a degree of a professor of the Russian Academy fb femtobarn (10−39 cm2) of Sciences. Her scientific interests are in general relativistic quantum FC frontal chromatography chemistry, , and physical chemistry, with special- FLNR Flerov laboratory of nuclear reactions ization in the chemistry of the heavy and heaviest elements. FS Fock-space GANIL grand accelérateuŕ national d’ions lourds ACKNOWLEDGMENTS GARIS gas-filled recoil isotope separator fi GGA generalized gradient approximation One of the authors gratefully acknowledges the nancial GSI Helmholtzzentrum für Schwerionenforschung support of this work through the Swiss National Science GmbH, Darmstadt Foundation Grant No. 200020_126639, Paul Scherrer Institute, IC isothermal chromatography and University of Bern. HEVI heavy element volatility instrument HITGAS high-temperature online gas chromatography ABBREVIATIONS AND QUANTITIES apparatus 4c four component vector function IH intermediate Hamiltonian A mass number IP ionization potential ADF Amsterdam DFT IR ionic radius AIDA automated ion-exchange separation apparatus HSCC Hilbert space coupled cluster coupled with the detection system for α- IUPAC International Union of Pure and Applied spectroscopy Chemistry AIMP ab initio model potential IUPAP International Union of Pure and Applied Physics AL average level IVO in situ volatilization AO atomic orbital JAEA Japan atomic energy agency ̈ ̈ AR atomic radius JYFL Jyvaskylan Yliopiston Fysiikan Laitos ARCA automated rapid chemistry apparatus Kads adsorption constant ARTESIA a rotating target wheel for experiments with Kdes desorption constant superheavy-element isotopes at GSI using LBNL Lawrence Berkeley National Laboratory actinides as target material LDA local density approximation fi LSC liquid scintillation counting BGS Berkeley gas- lled separator −27 2 CALLISTO continuously working arrangement for cluster- mb millibarn (10 cm ) less transport of in situ produced volatile oxides MBPT many-body perturbation theory fi − CASMCSCF complete active space multiconfiguration self- MCDF multicon guration Dirac Fock fi fi consistent field MCSCF multicon guration self-consistent eld CC coupled cluster MCT multicolumn technique CCSD(T) coupled cluster single double (and perturbative MeV mega electronvolt triple) excitations MG merry-go-round

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