Chemical Physics Letters 465 (2008) 1–9
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Chemical Physics Letters
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FRONTIERS ARTICLE On the formation of higher carbon oxides in extreme environments
Ralf I. Kaiser a,*, Alexander M. Mebel b a Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall (Bil 301A), Honolulu, HI 96822, USA b Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA article info abstract
Article history: Due to the importance of higher carbon oxides of the general formula COx (x > 2) in atmospheric chem- Received 15 May 2008 istry, isotopic enrichment processes, low-temperature ices in the interstellar medium and in the outer In final form 22 July 2008 solar system, as well as potential implications to high-energy materials, an overview on higher carbon Available online 29 July 2008 oxides COx (x = 3–6) is presented. This article reviews recent developments on these transient species. Future challenges and directions of this research field are highlighted. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction tion has no activation energy, proceeds with almost unit efficiency, and most likely involves a reaction intermediate. However, neither In recent years, the interest in carbon oxides of higher complex- reaction products (kinetics studies) nor the nature of the interme- 1 + ityP than carbon monoxide (CO; X R ) and carbon dioxide (CO2; diate (kinetics and dynamicsP studies) were determined. On the 1 þ 1 + 3 X g ) of the generic formula COx (x > 2) has been fueled by com- other hand, a CO(X R )+O2(X g ) exit channel was found to have plex reaction mechanisms of carbon oxides with atomic oxygen in an activation energy between 15 and 28 kJ mol 1 in the range of the atmospheres of Mars [1–4] and Venus [5]. Here, carbon dioxide 300–2500 K [14]. Atreya et al. pointed out the necessity to incorpo- represents the major atmospheric constituent [6]. Laboratory stud- rate heterogeneous reactions on aerosols or carbon dioxide ice par- ies suggested that the photodissociation of carbon dioxide by solar ticles in the Martian atmosphere [15–17]. However, these 0 photons (k < 2050 ÅA) generates a carbon monoxide molecule plus processes have not been investigated in the laboratory so far. Also, 3 atomic oxygen. Near the threshold, only ground-state O( P) atoms the explicit structure of the atmospherically relevant CO3 interme- are produced; shorter wavelengths supply also electronically ex- diate has not been unraveled to date [18,19]. cited oxygen atoms O(1D) [1]. Electronically excited oxygen atoms Besides the atmospheric chemistry of Venus, Earth, and Mars are thought to be mainly quenched involving a carbon trioxide and the 18O enrichment processes, solid carbon dioxide presents molecule (CO3) of a hitherto unknown structure. Once ground- an important constituent of interstellar ices as present on sub- state oxygen atoms O(3P) and carbon monoxide have formed, it micrometer sized grain particles in cold interstellar clouds holding is difficult to recycle carbon dioxide, since the reversed reaction temperatures as low as 10 K. Here, ice mixtures comprising water presents a spin-forbidden process. (H2O) (100), carbon monoxide (CO) (7–27), methanol (CH3OH) Besides the quenching of electronically excited oxygen atoms, (<3.4), ammonia (NH3) (<6), methane (CH4) (<2), and carbon diox- the carbon trioxide (CO3) molecule has been implicated as an ide (CO2) (15) were identified unambiguously via infrared spec- important transient species in the 18O isotope enrichment of car- troscopy towards the dense cloud TMC-1 employing the field star bon dioxide in the atmospheres of Earth and Mars [7,8]. Computa- Elias 16 as a black body source; the numbers in parentheses indi- tions and laboratory experiments indicate that the 18O enrichment cate the relative abundances compared to water ice. These clouds in ozone might be transferable to carbon dioxide [9,10], possibly interact with ultraviolet photons (<13.2 eV) and energetic parti- via a CO3 intermediate. Due to the potential role of carbon trioxide cles, predominantly from galactic cosmic rays. Henceforth, pristine in atmospheric chemistry, various kinetics and dynamics studies ice mantles are processed chemically by the cosmic ray-induced have been carried out to access the CO3 surface and to determine ultraviolet radiation present even in the deep interior of dense the temperature-dependent rate constants of the reaction of elec- clouds [20]. This can lead to the formation of new molecules in tronically excited oxygen atoms, O(1D), with carbon dioxide. At the solid state via non-equilibrium (non-thermal) chemistry even room temperature, rate constants of a few 10 10 cm3 s 1 have been at temperatures as low as 10 K. Likewise, carbon dioxide ices have derived [11–13]. This order of magnitude suggests that the reac- been detected in the outer solar system on Triton and Ganymede (Neptune’s and Jupiter’s largest moons, respectively). Here, charged particles from the planetary magnetospheres can poten- * Corresponding author. Fax: +1 808 956 5908. tially release oxygen atoms which may react with carbon dioxide E-mail address: [email protected] (R.I. Kaiser). to form carbon trioxide and higher carbon oxides.
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.07.076 2 R.I. Kaiser, A.M. Mebel / Chemical Physics Letters 465 (2008) 1–9
Finally, it should be recalled that intensive research has been position of carbon trioxide molecules leads to carbon dioxide mol- carried out to develop new high-energy materials (HEMs) to meet ecules and atomic oxygen. Shortly after this study, Weissberger needs for future defense and space science applications like novel et al. carried out photolysis experiments of solid carbon dioxide explosives and rocket propellants [21]. Particular attention has matrices and of mixtures of ozone and carbon dioxide dispersed been devoted to cyclic carbon oxides of the generic formula COx in solid argon. These experiments strongly indicated that an elec- (x = 3–6). These higher-order oxides of the main group IV element tronically excited oxygen atom (O(1D)) can react with carbon diox- carbon are highly energetic and therefore, candidates for high-en- ide to actually synthesize the C2v symmetric carbon trioxide isomer ergy density materials; Shkrob suggested these oxides also as mod- [26]. An early review on carbon trioxide concluded that further el compounds to study high velocities of detonation (VOD) and to studies are necessary to elucidate the structure and the electronic possibly control the energy release from high-energy density configuration of this transient species [27]. material [22]. The subsequent years elucidated the electronic structures of the Due to the importance of higher carbon oxides in atmospheric carbon trioxide molecule(s). Applying Hueckel molecular orbital chemistry, isotopic enrichment processes, low-temperature ices theory, correlation diagrams, and Walsh rules, Gimarc and Chou in the interstellar medium and in the outer solar system, as well predicted that the molecule favors a planar, C2v symmetric struc- as potential implications to high-energy materials, this Frontiers ture [28]. Calculations predicted that this molecule is metastable
Article presents a historical overview on higher carbon oxides COx toward dissociation into carbon monoxide (CO) and electronically 1 (x = 3–6) and reviews recent developments on how to synthesize excited molecular oxygen (O2; a Dg); an alternative decomposition these transient species experimentally and computationally. The pathway could involve linear carbon dioxide (CO2) and electroni- final chapter highlights future challenges and directions of this re- cally excited oxygen atoms (O(1D)). The latter mechanism was also search field. verified in photolysis experiments of ozone in carbon dioxide [29]. Olsen and coworkers [30] applied the extended Hueckel and inter- mediate neglect of differential overlap (INDO) methods to investi- 2. Historical overview gate various molecular geometries of the carbon trioxide molecule
in its pyramidal (C3v) and symmetric trigonal C2v shaped struc- 2.1. Carbon trioxide (CO3) isomers tures. Both methods predicted the C2v shaped structure holding a 1 X A1 electronic ground-state to be favorable in energy. The authors 1 Experimental and theoretical studies of the properties and for- also suggested that the closed shell A1 ground-state correlates 1 0 mation routes of carbon trioxide isomers started more than four with the E wave function in D3h symmetry; consequently, the decades ago. In 1966, Ung and Schiff [23] conducted gas phase adoption of the C2v symmetry of carbon trioxide can be attributed experiments and photolyzed carbon dioxide under bulk conditions. to the Jahn–Teller effect. Subsequent calculations [31,32] corre-
Based on the kinetics, the mass balance of the detected carbon lated with the previous results favoring a C2v symmetric structure monoxide (CO) and molecular oxygen (O2) products, and the reac- of the molecule. The short carbon–oxygen bond distance of the tions of methane (CH4), molecular hydrogen (H2), and dinitrogen- exocyclic unit of only 126.5 pm indicated a carbonyl group; like- oxide (N2O), the authors postulated the presence of a hitherto wise, the extended carbon–oxygen bonds of 147.3 pm as found in elusive reactive species, carbon trioxide (CO3), in the gas phase. the ring suggest the existence of carbon–oxygen single bonds. Fi- However, an explicit detection of this unstable molecule failed; nally, the oxygen–carbon–oxygen angle was derived to be about further, the authors did not recommend any structure of the car- 66°. This geometry was also verified in subsequent matrix studies bon trioxide molecule. by Jacox and Milligan [25]. Finally, high pressure kinetics studies of
In the same year, Moll et al. [24] identified a C2v symmetric car- the gas phase reaction of electronically excited oxygen atoms 1 bon trioxide molecule (CO3) in carbon dioxide and carbon dioxide- (O( D)) have been conducted [33]. The authors inferred the forma- ozone matrices at temperatures as low as 50 K (Fig. 1). Combining tion of transient carbon trioxide molecules with lifetimes in the or- these data with Jacox and Milligan studies [25], both groups as- der of 1–10 ps. The dominant process was suggested to be 1 @ 1 1 3 signed four fundamentals at 2045 cm (C O stretch), 1073 cm quenching of O( D)–O( P), which proceeded via the CO3 intermedi- (O–O stretch), 972 cm 1 (C–O stretch), 593 cm 1 (C–O stretch), ate. However, no structure of the carbon trioxide molecule was and 568 cm 1 (O–C@O) in low-temperature matrices; in argon proposed. In strong contrast to previous assignments, LaBonville matrices, these absorptions were shifted to 2053, 1070, 975, and et al. suggested a Cs-symmetric structure of the carbon trioxide 564 cm 1 [26]; no feature around 593 cm 1 was identified in solid molecule [34]. argon. Absorptions at 1894 cm 1 (argon matrix) and 1880 cm 1 After a decade, the interest in the carbon trioxide system re- (carbon dioxide matrix) were tentatively assigned as a Fermi reso- sumed, predominantly due to the development of the novel elec- nance of the 2045 cm 1 band with an overtone of the 972 cm 1 tronic structure methods [35]. Pople et al. tackled the energies fundamental. These results suggested that photolysis and radio fre- and electron configuration of open and closed-shell carbon triox- quency discharges of ozone and carbon dioxide generate atomic ide molecules utilizing ab initio molecular orbital calculations. At oxygen; at that time, the spin states of the liberated oxygen atom the highest level of theory used (MP2/6-31G*), including the were not assigned. Based on the observed infrared frequencies and effect of electron correlation, the cyclic, C2v symmetric structure 1 isotope substitution pattern, a planar, C2v symmetric structure was was found to be more stable by 230 kJ mol than any open derived in which the carbon atom is bound by a strong, exocyclic shell electron configuration; this result was in agreement with carbonyl bond to one oxygen atom, and by weaker bonds to two a subsequent study by Francisco et al. [36]. The authors also oxygen atoms placed overall in a cyclic geometry. These experi- emphasized that this molecule is only metastable by approxi- ments presented explicit evidence that, a cyclic carbon trioxide mately 17 kJ mol 1 with respect to ground-state oxygen atoms 3 molecule (CO3) is stable in low-temperature matrices. In addition (O( P)) and linear carbon dioxide. Also, the dissociation energy to this identification, the results of photolysis of 18O enriched car- to form carbon dioxide plus electronically excited oxygen atoms 1 1 bon dioxide samples led to the hypothesis that, besides the C2v iso- (O( D)) was found to be very low (75 kJ mol ). Steinfatt postu- mer, a D3h or C3v symmetric carbon trioxide molecule, in which all lated that the decay of the cyclic carbon trioxide molecule is oxygen atoms are equivalent, should also exist as a reactive, also correlated with strong chemiluminescence signals observed not-isolatable intermediate to form the C2v symmetric structure. in the perhydrolysis of phosgene in presence of organic sensitiz- Finally, the authors provided compelling evidence that the decom- ers [37]. R.I. Kaiser, A.M. Mebel / Chemical Physics Letters 465 (2008) 1–9 3
Fig. 1. Optimized geometries of various isomers of the COx molecules (x = 3–8). Bond lengths are given in Å and bond angles in degrees. Symmetries and ground-state electronic terms are also provided.
Ongoing improvements in computational methods pushed the Interest resumed on the carbon trioxide system to tackle the C2v research on the carbon trioxide system to the next level. Van de and D3h symmetric carbon trioxide isomers. Markovits et al. dem- Guchte et al. [38] conducted a systematic ab initio study of D3h onstrated computationally that a carbon trioxide molecule bound and C2v symmetric structures. The authors concluded that the opti- to a Pt(111) surface depicts a similar equilibrium geometry than 2 mized structure in C2v symmetry (at the SCF level) is higher in en- the carbonate ion (CO3 ). In addition to the well-known infrared ergy than the optimum structure in D3h symmetry. However, spectrum of the cyclic carbon trioxide molecule, Abreu et al. [41] Castro and coworkers utilized a more sophisticated approach provided a computed Raman spectrum. Supporting a previous [39]. They performed many-body perturbation theory and cou- computational study [39], the Raman spectrum was found to show pled-cluster calculations on the carbon trioxide molecule in D3h a very sensitive dependence to the treatment of electron correla- 1 and C2v symmetries. The relative energy location of the A1 (C2v) tion. Triggered by its potential relevance to atmospheric chemistry, 1 0 and A1 (D3h) closed-shell states is found to be extremely sensitive further computations probed the electron affinity of both C2v and to electron-correlation corrections. At the highest level of theory, D3h structures. Here, electron affinities of 3.85–4.08 eV were rec- 1 1 the A1 (C2v) state was found to be below the A1 (D3h) state by ommended to yield in both cases the acyclic CO3 anion [42]. The about 18 kJ mol 1. This study showed that actually two metastable latest theoretical study [43] investigated the electronic structure carbon trioxide isomers can exist, and that the cyclic structure is of carbon trioxide and vibronic interactions involving Jahn–Teller more stable compared to the D3h isomer. This conclusion was con- states. The authors concluded that ground-state equilibrium struc- firmed by a consecutive investigation of this system [40]. Froese tures and frequencies are strongly affected by vibronic interactions et al. also studied for the first time the corresponding triplet sur- with low-lying excited states. For example, at the D3h geometry, face which lies at least 251 kJ mol 1 above the lowest closed-shell the vibronic interactions are enhanced by the Jahn–Teller character singlet molecule. of the excited states and the complete theoretical description of 4 R.I. Kaiser, A.M. Mebel / Chemical Physics Letters 465 (2008) 1–9
this molecule might require going beyond harmonic description tified which have a carbonyl group as the C2v symmetric CO3 and within the adiabatic approximation. CO4 molecules. In summary, previous studies demonstrated experimentally the existence of the cyclic, C2v symmetric carbon trioxide molecule in low-temperature matrices and as a transient species in the photo- 3. Present status of the field dissociation of carbon dioxide containing gas mixtures. The second isomer, the D3h symmetric isomer, was proposed theoretically to The overview recapitulated that among the higher carbon oxi- 1 exist, but it is higher in energy than the cyclic molecule by about des COx (x = 3–6), only the C2v symmetric CO3 (X A1) has been de- 18 kJ mol 1. According to calculations, both structures are con- tected experimentally in low-temperature matrices. nected via a transition state located 36 kJ mol 1 above the cyclic Computational studies predicted further that at least a second, molecule [44]. Evidence for an interconversion of both structures D3h symmetric CO3 should exist. Likewise, higher carbon oxides first came when Baulch and Breckenridge, using isotopic labeling, ranging from CO4 to CO6 are expected to be metastable and possi- found that an attacking oxygen atom has an approximately statis- bly detectable experimentally. Low-temperature spectroscopy tical chance of being incorporated into the carbon dioxide mole- combined with a computational study of the structures, energetics, cule [45]. and infrared spectroscopic properties of the higher carbon oxides presents a powerful tool to identify these molecules in icy matri- ces. Here, the electronic structure calculations serve as a ‘guide’ 2.2. Carbon tetraoxide (CO ) isomers 4 to search for the transitions of higher carbon oxides in the chemi- cally relevant, mid infrared regime of 5000–400 cm 1. But how can Studies on the carbon tetraoxide molecule (CO ) have been re- 4 these molecules be ‘made’ in the laboratory? Previous laboratory stricted to theoretical investigations. More than a decade ago, studies, aimed to investigate the interaction of ionizing radiation Averyanov et al. predicted the existence of a metastable carbon tet- with astrophysically relevant low-temperature ices, showed that raoxide molecule [46]. Based on electronic structure calculations, the energy transfer from energetic electrons (keV range) penetrat- the authors inferred a spirane-like, D symmetric structure with 2d ing thin ice layers of a few 100 nm to the surrounding molecules aX1A electronic ground-state (Fig. 1). Applying selection rules 1 can lead to a unimolecular decomposition of the target molecules for chemical reactions, this molecule was postulated to decay forming reactive radicals and atoms [51]. An interaction of solid either to a ground-state, C symmetric carbon trioxide molecule 2v nitrogen molecules embedded in low-temperature (10 K) matrices, (CO ;X1A )(Fig. 1) plus electronically excited atomic oxygen 3 1 for example, with keV electrons generates suprathermal nitrogen (O(1D)) or alternatively to a linear carbon dioxide molecule (CO ; P 2 atoms [52]. These atoms hold access kinetic energies up to a few X1 þ) together with electronically excited molecular oxygen g eV and therefore, are not in thermal equilibrium with the sur- (O ; a1D ); note that the latter route had to proceed via a cyclic car- 2 g rounding matrix. These ‘suprathermal atoms’ can react with mole- bon dioxide intermediate (CO ;X1A ) which had to ring-open to 2 1 cules of the surrounding ice matrix. For instance, the linear the linear carbon dioxide molecule. Subsequent studies by Averya- N (X2P) radical (nitrogen ices) [52] and the C symmetric nov et al. expanded preliminary computational investigations and 3 s OCNO(X2A00) [53] intermediate can be synthesised via the reaction explored the global CO potential energy surface at the SCF, MP2, 4 of a single suprathermal nitrogen atom with one nitrogen and car- MP4, and CISD levels of theory using the 6-31G* basis set bon dioxide molecule, respectively. Likewise, a unimolecular [47,48]. These refined computations located two local minima: decomposition of molecular oxygen [54] and carbon dioxide [53] the previously assigned D symmetric isomer and a newly discov- 2d leads to the formation of two oxygen atoms as well as carbon mon- ered C symmetric isomer. These investigations postulated that, 2v oxide plus atomic oxygen, respectively. Based on these consider- both structures are vibrationally stable, but metastable with re- P ations, we expect that a processing of low-temperature carbon spect to the formation of carbon dioxide (CO ;X1 þ) plus elec- 2 g dioxide matrices by energetic (keV) electrons will lead to the gen- tronically excited molecular oxygen (O ; a1D ). The C 2 g 2v eration of energetic, suprathermal oxygen atoms which can react symmetric structure was found to be lower in energy by about then with the surrounding carbon dioxide matrix to potentially 134 kJ mol 1. The existence of the energetic D symmetric isomer 2d synthesize higher carbon oxides. However, it should be stressed, was confirmed by Song and coworkers exploiting second-order that compared to a photodissociation of molecules in the gas Moller-Plesset (MP2) and a quadratic configuration interaction phase, which may generate atoms in selected electronic states (CI) through single and double excitation (QCISD) calculations (for example the photodissociation of ozone (O ) at 248 nm leads using 6-31G(d) and 6-311G(d,p) basis sets [49]. These computa- 3 to electronically excited oxygen atoms O(1D), the interaction of tions labeled CO (D ) as a potential high-energy oxidizer. 4 2d energetic electrons with molecules in ices can generate both ground-state O(3P) and electronically excited oxygen atoms 1 2.3. Carbon pentaoxide (CO5) and carbon hexaoxide (CO6) isomers O( D). Therefore, it may be difficult to extract the actual reaction mechanisms leading to the synthesis of higher carbon oxides in Computational studies on carbon pentaoxides and carbon hexa- low-temperature carbon dioxide matrices. Nevertheless, recall that oxides are sparse. As a matter of fact, only a single theoretical the first and foremost goal is to actually synthesize new carbon oxi- investigation was conducted [50]. Elliott et al. predicted the exis- des in the laboratory and to identify the nature of the isomer(s) tence of two isomeric pairs. For the CO5 isomers, these are C2 formed spectroscopically via infrared spectroscopy at 10 K. Sec- and C2v symmetric structures where the central carbon atom is ondly, by following the development of the infrared absorption connected to a carbonyl group and an O4 ring (C2) as well as a features in real time during the irradiation exposure on line and bi-cyclic molecule holding a CO3 and a CO4 ring (C2v). The latter in situ, it will be also feasible to extract information on the kinetics 1 molecule was predicted to be less stable by 166 kJ mol compared and reaction mechanisms how the higher carbon oxides form. Fi- to the C2 isomer. In case of the CO6 system, the authors identified a nally, the laboratory experiments can also help to elucidate the C2 and D2d symmetric structure; here, the C2 isomer is more stable stability of the newly formed molecules in the ices. By increasing 1 by 59 kJ mol compared to the D2d symmetric molecule. The most the temperature of the carbon dioxide ice matrix in the post-irra- stable structure is formally derived from a CO4 ring merged with a diation phase and monitoring the decay of the infrared absorptions CO2 ring via the central carbon atom. The D2d symmetric isomer of the higher carbon oxides, it is feasible to extract information on has two CO3 cycles. Surprisingly, no CO5 or CO6 isomers were iden- the stability and decomposition temperature of these molecules. R.I. Kaiser, A.M. Mebel / Chemical Physics Letters 465 (2008) 1–9 5
3.1. Electronic structure calculations zero-point vibrational energy corrections taken from the B3LYP calculations. At the B3LYP level, the influence of the inclusion of Electronic structure calculations and the derived infrared spec- diffuse functions in the basis set appeared to be insignificant. Elliot tra together with their intensities in particular present valuable and Boldyrev [50] have also shown that the differences in geomet- tools to predict the geometries of – if they exist – multiple isomers ric parameters of COx optimized at the B3LYP, CASSCF(8,8), and and the position of the vibrational fundamentals of the newly CCSD(T) levels are rather minor, especially between B3LYP and formed molecules. The optimized geometries of various COx iso- the most accurate CCSD(T) calculations. mers (x = 3–8) are illustrated in Fig. 1 and their symmetries, Two isomers have been found for the CO4 species. The C2v struc- ground electronic state terms, and relative energies with respect ture (O3C@O) includes the carbonyl C@O group merged with the to the most stable structures for each molecule are collected in diamond-shaped CO3 ring via the central carbon atom. The D2d iso- Table 1. mer (O2CO2) is spirane-like and is made from two three-member The most detailed calculations of the potential energy surface CO2 rings located in perpendicular planes. According to the have been performed for the CO3 species [55], for which geometry CCSD(T)/6-311+G(d) calculations, [50] the C2v structure is signifi- 1 optimization and vibrational frequency calculations were carried cantly, by 137.7 kJ mol , more favorable than D2d. Next, the struc- out at the multireference CASSCF level of theory with the active tures of higher COx molecules can be derived from CO4 isomers by space including 16 electrons distributed on 13 orbitals (i.e., the expansion of the CO3 and CO2 rings through the addition of extra complete valence active space excluding 2s orbitals of oxygen oxygen atoms. For instance, the expansion of the CO3 ring in the * atoms) and the 6-311G basis set. Single-point energies of various C2v structure of CO4, while keeping the carbonyl moiety intact, isomers and transition states were refined using the coupled-clus- gives the C2-symmetric isomer of CO5.CO5(C2) has a non-planar ters CCSD(T) and multireference configuration interaction (puckered) CO4 ring attached to the C@O group (O4C@O). Alterna- MRCI(16,13) methods with the 6-311+G(3df) basis set. The calcula- tively, ring-expansion in the D2d CO4 isomer produces the C2v tions have confirmed that the cyclic C2v CO3 isomer is the most sta- structure of CO5 with the merged CO3 and CO2 rings in perpendic- ble, but the acyclic D3h structure is only slightly higher in energy, ular planes (O3CO2). As for CO4, the C2 isomer containing the car- 1 by 13.8 and 0.4 kJ mol at the CCSD(T)/6-311+G(3df) and MRCI/ bonyl group is much more stable than the C2v form; the energy 6-311+G(3df) levels of theory, respectively. The two isomers are difference is as high as 166.1 kJ mol 1. Elliott and Boldyrev [50] 1 separated by a barrier of 25 kJ mol according to supposedly most considered only two forms of CO6,C2 and D2d, both obtained by 1 accurate MRCI calculations. The C2v isomer resides 197.5 kJ mol ring expansions of the least-stable C2v isomer of CO5. In particular, 1 3 below O( D)+CO2 but is nearly isoenergic with O( P)+CO2. How- the C2 structure of CO6 is made by expansion of the diamond- 3 ever, the fragmentation of CO3 to O( P)+CO2 is spin-forbidden shaped CO3 ring to the puckered CO4 cycle, whereas the D2d CO6 and can occur via the minimum energy structure on the singlet- isomer is produced by expansion of the CO2 triangle to the CO3 dia- 1 triplet seam of crossing lyingP 146.4 kJ mol above the C2v CO3 iso- mond. Meanwhile, the expansion of the CO4 ring in the most stable 3 mer. Dissociation to O2ðX g ÞþCO is also spin-forbidden and C2 isomer of CO5, which was not considered earlier, gives a Cs 1 endothermic by 48 kJ mol . Hence, we can conclude that CO3 structure of CO6 combining the carbonyl group with a non-planar should be a rather stable molecule, which can be produced in the CO5 cycle. According to our CCSD(T)/6-311+G(d) calculations, the 1 exothermic O( D)+CO2 reaction. Cs-symmetric O5C@O isomer represents the lowest in energy CO6 For the higher carbon oxides (COx, x > 3), our discussion is based form; it is calculated to be more favorable than the C2 (O4CO2) 1 on B3LYP/6-311G(d) and B3LYP/6-311+G(d) optimized geometries and D2d (O3CO3) isomers by 112.9 and 173.4 kJ mol , respectively. and CCSD(T)/6-311+G(d) relative energies of different isomers with Elliott and Boldyrev [50] calculated only one isomer each for
CO7 and CO8,C2-symmetric O4CO3 and D2-symmetric O4CO4, both made by expansion of the CO2 triangle in the C2-symmetric CO6 Table 1 structure O4CO2 first to CO3 and then to CO4 rings. However, since Calculated symmetry, ground electronic state terms, and relative energies of isomers g the isomers containing the carbonyl fragment are the most stable (Erel) of the COx molecules (x = 3–8) structures of COx with x = 3–6, we searched for a possibility of a 1 Molecule Isomer, symmetry Electronic state Erel (kJ mol ) further ring expansion in the Cs-symmetric CO6,O5C@O, while @ 1 CO3 O2C O, C2v A1 0.0 maintaining the C@O group untouched. The geometry optimiza- OC(O)O, D 1A0 13.8a, 0.4b 3h 1 tion gave new isomers of CO and CO ,OC@OofC symmetry @ 1 7 8 6 s CO4 O3C O, C2v A1 0.0 @ O CO ,D 1A 137.7c and non-symmetric O7C O. At the CCSD(T)/6-311+G(d) level, 2 2 2d 1 1 1 @ CO5 O4C@O, C2 A 0.0 O6C O appeared to be 203.6 kJ mol more stable than O4CO3, 1 c 1 O3CO2,C2v A1 166.1 whereas O7C@O lies 106.2 kJ mol lower in energy than O4CO4. @ 1 0 CO6 O5C O, Cs A 0.0 Summarizing, we can conclude that the O C@O isomers contain- 1 d x 1 O4CO2,C2 A 112.9 1 d ing the carbonyl group are the most stable forms COx and they are O3CO3,D2d A1 173.4 1 1 0 100–200 kJ mol more favorable than the other bi-cyclic O CO CO7 O6C@O, Cs A 0.0 x n n 1 e O4CO3,C2 A 203.6 (n > 1) structures. @ 1 @ @ CO8 O7C O, C1 A 0.0 As seen in Fig. 1, the C O bond length in the Ox 1C O struc- 1 f O4CO4,D2 A 106.2 tures gradually increases from 1.174 and 1.173 Å in CO3 and CO4 a Calculated at the CCSD(T)/6-311+G(3df) level in Ref. [55]. to 1.180, 1.185, and 1.201 Å in CO5,CO6, and CO7, respectively, b Calculated at the MRCI+Q(16,13)/6-311+G(3df) level in Ref. [55]. but then slightly decreases to 1.193 Å in CO8. The single C–O bonds c From CCSD(T)/6-311+G* calculations in Ref. [50]. d are found to be the shortest in the three-member CO2 rings, 1.337– Based on the present CCSD(T)/6-311+G* calculations for the Cs structure and 1.352 Å, and are longer in CO3 and CO4 cycles, 1.369–1.398 and similar calculations for the C2 and D2d isomers in Ref. [50]. e 1.376–1.407 Å, respectively. In the larger rings, the calculated Based on the present CCSD(T)/6-311+G* calculations for the Cs structure and similar calculations for the C2 isomer in Ref. [50]. C–O distances change non-monotonically, from 1.362 Å in CO5 to f Based on the present CCSD(T)/6-311+G* calculations for the C structure and 1 1.340 Å in CO6, but then to 1.313 and 1.415 Å in CO7. On the other similar calculations for the D2 isomer in Ref. [50]. hand, the O–O distances vary in a much wider range, 1.547– g The ab initio calculations produce zero temperature results. Thermal correc- tions to the relative energies of isomers are normally very small and are not 1.668 Å in three-member CO2 cycles, 1.463–1.470 Å in CO3 dia- 1 expected to change relative energies of different isomers by more than 1 kJ mol . monds, 1.400–1.457 Å in CO4 rings, and 1.391–1.491 Å in CO5. Finally, in the CO6 and CO7 rings of CO7 and CO8 we find bonding 6 R.I. Kaiser, A.M. Mebel / Chemical Physics Letters 465 (2008) 1–9
O–O distances as short as 1.217 Å and as long as 1.949 Å, indicating fluxes (1.0 1016 electrons cm 2); this is expected to signifi- that a usual valence-bond consideration can hardly be applied to cantly zenhance the concentration of suprathermal oxygen atoms these exotic structures. formed and could trigger the formation of higher-order carbon
Vibrational frequencies and infrared intensities for various COx oxides. isomers discussed in the subsequent section have been calculated * at the B3LYP/6-311G level of theory (except for CO3, where the 3.2.2. Carbon trioxide CO3(D3h) CASSCF(16,13)/6-311G* method was used) within harmonic An enhanced electron exposure of the carbon dioxide target approximation. It should be emphasized that the harmonic approx- lead to the observation of the novel CO3(D3h) molecule via infrared imation almost always overestimates experimental frequencies spectroscopy. Here, the CO3(D3h) molecule was identified by an [56]. An useful approach widely utilized to correct this deficiency absorption centered at 1165 cm 1 corresponding to the degenerate is given by scaling procedures [57]; since calculated harmonic fre- m1/m2 vibrational modes. This band position matches nicely the cal- quencies are generally larger than the observed fundamental fre- culated position of 1168 cm 1 after being scaled [62]. This correc- quencies, the calculated values are scaled (multiplied) by a tion is necessary to account for the anharmonicity of the scaling factor. The scaling factors are not only intended to account vibrations, which are computed within the harmonic approxima- 12 18 13 16 13 18 16 12 18 for anharmonicity effects that are neglected in the theoretical cal- tion. The observation of the C O3, C O3, C O3, O C O2, 18 12 16 culations, but also for an inadequate description of electron corre- and O C O2 isotopomers of this acyclic isomer confirms the * 12 18 lation, and the use of finite basis sets. For the B3LYP/6-311G level assignments. Here, in the C O2 system, the m1/m2 bands were 1 13 16 of theory, a recommended value for this scaling factor is given as red-shifted to 1152 cm . Considering the C O2 irradiation 13 16 0.9619; based on comparison of the vibrational frequencies from experiment, the m1 and m2 bands of the C O3 molecule were 125 different molecules [58]. Note that the scaled frequencies are found at 1141 cm 1, which agrees nicely with the calculated wave- 1 13 18 essentially computed for the gas phase. In addition matrix effects number of 1139 cm . Also, in the C O2 irradiation experiment, 13 18 can shift these fundamentals by a few wavenumbers; typically, the C O3 molecule was monitored via an absorption at in non-polar matrices such as carbon dioxide, these shifts should 1121 cm 1 compared to the computed and scaled position of 1 12 16 12 18 not be higher than a few wavenumbers [59]. For more accurate 1126 cm . Most importantly, in C O2/ C O2 ice mixture, four 12 16 18 12 16 16 12 18 determination of a scaling factor for a particular subset of mole- acyclic isotopomers were formed: C O3, O C O2, O C O2, 12 18 cules (e.g. COx species) it is often desirable to re-calculate a scaling and C O3. Here, the overlapping m1 and m2 modes resulted in a factor based on bands solely arising from similar molecules. The broad peak centered at 1158 cm 1 that had to be deconvoluted. 12 16 scaling factors that are used here were calibrated for each particu- Also, the substitution of one or two oxygen atoms in C O3 by 18 lar system and were found to be in the range 0.967–0.992, i.e. O reduces the symmetry from D3h to C2v. This eliminates the slightly closer to 1 than the recommended value. degeneracy of the m1 and m2 modes. Summarized, eight isotopomers of the acyclic CO3 isomer were monitored in low-temperature car- 3.2. Low-temperature spectroscopy bon dioxide samples by comparing the experimentally observed peak positions with the computed, scaled data.
3.2.1. Carbon trioxide CO3(C2v) The electron irradiation of the carbon dioxide target leads to 3.2.3. Carbon tetraoxide CO4(C2v) rapidly developing new absorption features of carbon monoxide As the irradiation time increased further, additional absorptions 1 at 2139 cm (m1, CO stretching) [60]. These findings suggest that appeared in the infrared spectra. These could not be assigned to the carbon dioxide molecule undergoes unimolecular decomposi- any CO3 isomer. Likewise, less intense fundamentals, overtones, tion to form carbon monoxide plus oxygen atoms. In addition, and/or combination bands of ozone and carbon monoxide could 12 16 two absorptions of the ozone molecule were monitored at be ruled out. In the C O2 irradiation experiment, the m1 vibration 1 12 16 1 1042 cm (m3, anti symmetric stretch) and the weaker bending of C O4(C2v) was observed at 1941 cm compared to a calcu- 1 1 mode at 704 cm (m2) [61]. A quantitative fit of the temporal evo- lated wavenumber of 1936 cm [63]. The next most intense lution of the ozone profile suggests that two oxygen atoms com- absorption of the carbon tetraoxide molecule would be the m5 bine to form molecular oxygen; the later reacts with a third vibration centered at 1114 cm 1. However, no absorption could oxygen atom to synthesize ozone. Besides the carbon monoxide be identified in this region due to its low intensity. Therefore, a and ozone molecules, four fundamentals of the well-known C2v confirmation of the assignment of the CO4(C2v) molecule requires 1 symmetric cyclic CO3 structure were identified at 2045 cm (m1), an agreement with the predicted isotope shifts in the m1 position. 1 1 1 12 18 12 18 1 1068 cm (m2), 973 cm (m5), and 565 cm (m6). Although the In the C O2 experiment, C O4 was observed at 1908 cm 1 unobserved m3 and m4 modes have larger absorption coefficients which compared well to a scaled value of 1899 cm . In the 13 16 13 16 1 than the detected m6 transition, the m4 absorption overlaps with C O2 experiment, C O4 was observed at 1894 cm which 1 the broad m2 band of the carbon dioxide reactant; the m3 band of agrees with the theoretical value of 1886 cm . Lastly, in the 13 18 13 18 carbon trioxide is too close to the cut-off of the detector of the C O2 irradiation experiment C O4 was observed at infrared spectrometer to be observable. Finally, we detected also 1855 cm 1 compared to the scaled theoretical shift of 1847 cm 1. 1 a transition at 1879 cm , which was assigned tentatively as a Fer- The consistent agreement of the peak positions of the m1 funda- 1 mi resonance of the 2044 cm band with an overtone of the mental vibrations of the CO4(C2v) isotopologues with the theoreti- 973 cm 1 fundamental. These experiments act as control experi- cally predicted shifts confirmed the assignment of the carbon ments and showed explicitly the formation of free oxygen atoms tetraoxide molecule. The assignment of the hitherto elusive D2d in the irradiated carbon dioxide matrices and the capability of symmetric CO4(D2d) isomer still awaits confirmation. How stable the oxygen atoms to react with carbon dioxide to form the ob- is this newly observed CO4(C2v) molecule? After the irradiation, 1 served C2v symmetric CO3(X A1) molecule. It should be stressed the ice was left isothermal at 10 K. There was no observable change 1 that the CO3(X A1) isomer was found to be stable at 10 K, but in the column density of carbon tetraoxide (C2v) indicating that the started to decompose at about 90 K. Finally, at these low irradia- product was stable and that there were no residual reactions tion currents, which exposed the sample to a total of occurring in the ice to form CO4(C2v). The ice was then heated to 1.0 1015 electrons cm 2, no infrared absorptions of hitherto elu- further check the stability of the carbon tetraoxide molecule. In sive higher carbon oxides could be detected. Therefore, successive the infrared spectra carbon tetraoxide absorptions remained until experiments exposed the carbon dioxide sample to higher electron about 120 K. This high-temperature is interesting because CO4(C2v) R.I. Kaiser, A.M. Mebel / Chemical Physics Letters 465 (2008) 1–9 7 shouldP be unstableP with respect to dissociation to 3.3. Formation mechanisms 1 þ 3 þ CO2(X g )+O2(X g ) [48]. However, Averyanov et al. predicted a significant barrier for this spin-forbidden process. A barrier is also Besides the identification of four hitherto unknown carbon oxi- likelyP to be present in the spin-allowed dissociation to des, i.e. CO3(D3h), CO4(C2v), CO5(C2), and CO6(Cs) based on the infra- 1 þ 1 CO2(X g )+O2(a Dg). red features, the studies also followed the temporal evolution of the absorption features in situ. This provided an important chance to quantitate the kinetics of the important reaction pathways of 3.2.4. Carbon pentaoxide CO5(C2) 12 16 higher carbon oxides in the irradiated carbon dioxide samples. During the irradiation of the C O2 target, the m1 vibration of 12 16 1 These reactions were not conducted under gas phase single colli- C O5(C2) was observed at 1912 cm [64], this compares nicely 1 sion conditions, but in a low-temperature carbon dioxide ice to the calculated wavenumber of 1914 cm . Besides the m1 vibra- (10 K). This makes it sometimes challenging to elucidate the reac- tion, the next most intense absorption of the carbon pentaoxide 1 tion pathways based on the kinetic fits of the temporal profiles of molecule would be the m7 vibration centered at 1092 cm . How- the products observed. In particular, the kinetic studies alone could ever, no absorption could be identified in this region due to its not provide any evidence to what extend ground-state O(3P) and/ low intensity. The m7 vibration should be almost four times less in- or electronically excited carbon atoms O(1D) drive the formation tense than the m1 vibration. Therefore, once again, a confirmation of of higher carbon oxides. Nevertheless, based on the temporal pro- our assignment of the CO5(C2) molecule requires an agreement 12 18 files of the reactant molecules and the products, it was evident that with the predicted isotope shifts in the m1 position. In the C O2 12 18 1 the response of the irradiated carbon dioxide target was dictated experiment, C O5 was observed at 1872 cm which agrees well 1 13 16 13 16 by a unimolecular decomposition of carbon dioxide forming car- with the scaled value of 1876 cm . In the C O2 study, C O5 bon monoxide and oxygen atoms (Eq. (1)) via first-order kinetics. was monitored at 1873 cm 1; this data is very close to the scaled 1 13 18 The suprathermal oxygen atoms were inferred to react with the theoretical value of 1865 cm . Lastly, in the C O2 irradiation 13 18 1 carbon dioxide molecules via addition either to the carbon–oxygen exposure, C O4 was observed at 1829 cm compared to the double bond forming the cyclic CO (X1A ;C ) molecule (Eq. (2)), scaled theoretical shift of 1826 cm 1. The consistent agreement 3 1 2v but also by addition to the carbon atom of the carbon dioxide mol- of the peak positions of the fundamental vibrations of the m1 1 0 ecule yielding the acyclic CO3(X A1;D3h) structure. Branching ra- CO5(C2) isotopologues with the theoretically predicted shifts con- tios, i.e. the direct ratios of the rate constants of reactions (2) and firmed the identification of the C2-symmetric carbon pentaoxide (3), were determined to be about 7. Therefore, the cyclic structure molecule. To investigate the stability of the CO5(C2) molecule, a is, as attributed to statistical and sterical factors, formed preferen- similar approach to CO4(C2v) was conducted. Here, the CO5(C2) iso- tially. Note that the authors outlined that the D isomer may also mer was found to be stable at 10 K. Upon warming up the sample, 3h be produced by isomerization of the C structure and vice versa. the infrared spectra suggest that the carbon pentaoxide absorp- 2v However, the kinetic fits were found to be insensitive to both pro- tions remained until about 106 K. This temperature is interesting: cesses meaning that the formation of both CO isomers in the low- according to the calculations conducted by Elliot et al. three exoer- 3 temperature samples via reactions (2) and (3) was more important gic dissociationP pathways exist. These areP the decay to form þ than the isomerization channels. The pathways to synthesize CO (X1 )+O (X1A ), CO(X1R+)+2 O (X3 ), and CO (X1A )+ 2 P g 3 1 2 g 3 1 1 0 1 0 3 CO3(X A1;C2v) and CO3(X A1;D3h) have been also confirmed theo- O2(X ). Consequently, similar to carbon tetraoxide, the experi- g retically via electronic structure calculations [60]; both reactions ments suggest that significant energy barriers are involved to dis- were found to be exoergic (with respect to carbon dioxide and sociate carbon pentaoxide. electronically excited oxygen atoms) by 197.5 and 197.1 kJ mol 1, respectively. 3.2.5. Carbon hexaoxide CO (C ) 6 s 3 1 12 16 CO X1Rþ CO X1Rþ O P= D 1 As the irradiation exposure of the C O2 sample increased, the 2ð g Þ! ð Þþ ð ÞðÞ 12 16 m1 vibration of the Cs-symmetric C O6 isomers was observed at 1 1 þ 3 1 1 1876 cm compared to a scaled, calculated wavenumber of CO2ðX Rg ÞþOð P= DÞ!CO3ðX A1; C2vÞð2Þ 1877 cm 1 [65]. The next most intense absorption of the carbon hexaoxide molecule is the A00 symmetric vibration, which should 1 þ 3 1 1 0 CO2ðX Rg ÞþOð P= DÞ!CO3ðX A1; D3hÞ: ð3Þ be less than half the intensity of the m1 vibration. Therefore, confir- mation of our assignment of the CO6(Cs) molecule requires an The kinetics of the higher-order carbonoxides CO4(C2v), CO5(C2v), agreement with the predicted isotope shifts in the m1 position. In and CO6(Cs) could only be rationalized by successive, stepwise reac- 12 18 12 18 1 the C O2 experiment, C O6 was observed at 1837 cm ; this tions of oxygen atoms with the corresponding lower-order carbon 1 13 16 data is identical to the scaled value of 1837 cm . In the C O2 oxide (Eqs. (4)–(6)). Here, oxygen atoms reacted with carbon triox- 13 16 1 1 experiment, C O6 was observed at 1832 cm which agrees with ide, CO3 (X A1;C2v), to form the C2v symmetric CO4 isomer via a for- 1 13 18 the scaled theoretical value of 1831 cm . Lastly, in the C O2 mal expansion of the cyclic structure by one oxygen atom. Note that 13 18 1 irradiation experiment C O6 was observed at 1791 cm com- in the gas phase, the CO4 molecule would not be stable; however, 1 pared to the scaled theoretical shift of 1790 cm . The coherent the surrounding matrix absorbs the internal energy of the CO4 iso- conformity of the absorptions of the m1 fundamental vibrations of mer via phonon interaction – in a similar manner as for the CO5 and the CO6(Cs) isotopologues with the theoretically predicted shifts CO6 molecules. Stepwise expansions of the ring structure of highlighted the assignment of the Cs-symmetric carbon hexaoxide CO4(C2v) and CO5(C2) by one oxygen atom lead to CO5(C2) and molecule. It should be stressed that neither the C2-symmetric bi- CO6(Cs), respectively. It should be stressed that these processes 1 cyclic structure, O2CO4, nor the D2d symmetric bi-cyclic structure, are exoergic by 165.9, 166.3, and 145.2 kJmol , respectively, as cal- * 3 O3CO3, could be detected. Similar to the CO4(C2v) and CO5(C2) mol- culated at the CCSD(T)/6-311+G level for the reactions of O( P) and, 1 ecules, the CO6(Cs) isomer is stable at 10 K. However, the band re- correspondingly, by 355.8, 356.1, and 335.1 kJmol for the reac- gion containing the carbon hexaoxide molecule decreased in tions of O(1D). We would like to discuss briefly the microscopic intensity at around 60 K. This may be an indication of the instabil- mechanisms of these reactions. As noted above, Eqs. (4)–(6) can ity of carbon hexaoxide above 60 K. Here, we see a trend of be formally interpreted as stepwise expansions of the ring system decreasing stability from about 120 K CO4(C2v) via 106 K CO5(C2v) by oxygen atoms under conservation of the carbonyl group. to 60 K CO6(Cs) as the numbers of oxygen atoms in the ring grow. Electronically excited oxygen atoms can either insert into an 8 R.I. Kaiser, A.M. Mebel / Chemical Physics Letters 465 (2008) 1–9 oxygen–oxygen or carbon–oxygen single bond to expand the cyclic the calculated frequencies, but they remain challenging because structure in one step. Alternatively, electronically excited oxygen of enormous computing requirements. Second, more detailed stud- might add to the non-bonding pair of a ring-oxygen atom forming ies of the CO4,CO5, and CO6 potential energy surfaces are required a weak, exocyclic oxygen–oxygen bond similar to the bonding in order to characterize kinetic stability of these molecules with re- scheme present in oxywater (H2OO) [66]. However, this addition spect to various dissociation channels. To attain this goal, one has to be followed by a ring-opening process leading ultimately to needs to map out not only different COx isomers but also transition a ring expansion by one oxygen atom, but via a two-step mecha- states for isomerization and fragmentation processes. Additionally, nism compared to a one-step insertion pathway. The larger cone since major dissociation channels involve the production of molec- of acceptance of the cyclic moiety compared to the carbonyl group ular and/or atomic oxygen in the ground triplet electronic states likely directs the interaction of the oxygen atom with the ring. Elec- and are spin-forbidden, one also needs to characterize singlet–trip- tronic structure calculations are currently ongoing to verify these let conical intersections. Once the singlet and triplet surfaces and pathways computationally and to expand the knowledge from the their conical intersections are derived from ab initio calculations,
CO3 system to more complex carbon oxides. It should be stressed it will be possible to apply both adiabatic and non-adiabatic statis- that related cyclic molecules containing only oxygen atoms such tical theories, like TST and RRKM, to compute rate constants and as O4,O5, and O6 are unstable. Therefore, the incorporation of a sin- branching ratios for the fragmentation of COx under various gle carbon atom into the oxygen structure leads to a stabilization of conditions. the oxygen ring structure Acknowledgements 3 1 CO3ðC2vÞþOð P= DÞ!CO4ðC2vÞð4Þ This project was supported by the Air Force Office of Scientific 3 1 CO4ðC2vÞþOð P= DÞ!CO5ðC2Þð5Þ Research (W911NF-05-1-0448). The work on the D3h symmetric CO3 isomer was co-sponsored via a Research Assistantship by the 3 1 CO5ðC2ÞþOð P= DÞ!CO6ðCsÞ: ð6Þ National Aeronautics and Space Administration through the NASA Astrobiology Institute (Cooperative Agreement No. NNA04CC08A issued through the Office of Space Science). 4. Future challenges and directions
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[40] R.D.J. Froese, J.D. Goddard, J. Phys. Chem. 97 (1993) 7484. Ralf I. Kaiser received his Ph.D. in Chemistry from the [41] E.P. Abreu, M.A. Castro, M.F. Costa, S. Canuto, J. Mol. Spectr. 202 (2000) 281. University of Münster (Germany) in 1994. He conducted [42] C.D. Cappa, M.J. Elrod, Phys. Chem. Chem. Phys. 3 (2001) 2986. postdoctoral work on the gas phase formation of [43] T. Kowalczyk, A.I. Krylov, J. Phys. Chem. A 111 (2007) 8271. astrochemically relevant molecules at UC Berkeley [44] A.S. Averyanov, Y.G. Khait, Y.V. Puzanov, J. Mol. Struct. 459 (1999) 95. (Department of Chemistry). From 1997–2000 he [45] D.L. Baulch, W.H. Breckenridge, Trans. Faraday Soc. 62 (1966) 2768. [46] A.S. Averyanov, Y. Khait, G. Ross, Opt. Spekt. 78 (1995) 895. received a fellowship from the German Research [47] A.S. Averyanov, Y.G. Khait, Y.V. Puzanov, THEOCHEM 367 (1996) 87. Council (DFG) to perform his Habilitation at the [48] A.S. Averyanov, Y.G. Khait, Y.V. Puzanov, THEOCHEM 459 (1999) 95. Department of Physics (University of Chemnitz, Ger- [49] J. Song, Y.G. Khait, M.R. Hoffmann, J. Phys. Chem. A 103 (1999) 521. many) and Institute of Atomic and Molecular Sciences [50] B.M. Elliott, A.I. Boldyrev, J. Phys. Chem. A 109 (2005) 3722. (Academia Sinica, Taiwan). He joined the Department of [51] C.S. Jamieson, C.J. Bennett, A.M. Mebel, R.I. Kaiser, ApJ 624 (2005) 436. Chemistry at the University of Hawaii at Manoa in 2002 [52] C.S. Jamieson, R.I. Kaiser, Chem. Phys. Lett. 440 (2007) 98. where he is currently Professor of Chemistry. [53] C.S. Jamieson, A.M. Mebel, R.I. Kaiser, Phys. Chem. Chem. Phys. 7 (2005) 4089. [54] C.J. Bennett, R.I. Kaiser, ApJ 635 (2005) 1362. [55] A.M. Mebel, M. Hayashi, V.V. Kislov, S.H. Lin, J. Phys. Chem. A 108 (2004) 7983. [56] (a) A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502; Alexander M. Mebel studied chemistry at the Moscow (b) J.P. Merrick, D. Moran, L. Radom, J. Phys. Chem. A 111 (2007) 11683. Institute of Steel and Alloys and Kurnakov’s Institute of [57] G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093. [58] M.P. Andersson, P. Uvdal, J. Phys. Chem. A 109 (2005) 2937. General and Inorganic Chemistry of Russian Academy of [59] M.E. Jacox, J. Mol. Spectrosc. 113 (1985) 286. Science in Moscow, Russia, where he received his Ph.D. [60] C.J. Bennett, C. Jamieson, A.M. Mebel, R.I. Kaiser, Phys. Chem. Chem. Phys. 6 in physical chemistry. Currently, he is Associate Pro- (2004) 735. fessor at the Department of Chemistry and Biochemistry [61] T. Shimanouchi, J. Phys. Chem. Ref. Data 6 (1972) 993. of Florida International University in Miami, Florida, [62] C.S. Jamieson, A.M. Mebel, R.I. Kaiser, Chem. Phys. Chem. 7 (2006) 2508. USA. His current research interests involve theoretical [63] C.S. Jamieson, A.M. Mebel, R.I. Kaiser, Chem. Phys. Lett. 440 (2007) 105. quantum chemical studies of mechanisms, kinetics, and [64] C.S. Jamieson, A.M. Mebel, R.I. Kaiser, Chem. Phys. Lett. 443 (2007) 49. dynamics of elementary chemical reactions related to [65] C.S. Jamieson, A.M. Mebel, R.I. Kaiser, Chem. Phys. Lett. 450 (2007) 312. combustion, atmospheric, and interstellar chemistry. [66] Y. Ge, K. Olsen, R.I. Kaiser, J.D. Head, Am. Inst. Phys. (2006). [67] V.J. Barone, Chem. Phys. 122 (2005) 014108. [68] (a) J.M. Bowman, Acc. Chem. Res. 19 (1986) 202; (b) R.B. Gerber, M.A. Ratner, Adv. Chem. Phys. 70 (1988) 97; (c) S. Carter, J.M. Bowman, N.C. Handy, Theor. Chem. Acc. 100 (1998) 191.