Deuterium Atom Pickup in Srq Solvated by Ammonia

7 May 1999

Chemical Physics Letters 304Ž. 1999 350±356

Cluster size specific chemistry: deuterium atom pickup in Srq solvated by ammonia

David C. Sperry, James I. Lee, James M. Farrar ) Department of Chemistry, UniÕersity of Rochester, Rochester, NY 14627-0216, USA Received 14 January 1999; in final form 25 February 1999

Abstract

We report on recent experimental evidence for deuterium atom pickup in metal ion±polar solvent clusters. Mass spectral q analysis shows the existence of species that correspond to the empirical formula SrŽ. ND3 nx D , where x is as large as 4 for clusters in the size range ns10±15. Photodissociation spectra of several of these clusters indicate that their structures are q very different from the purely ammoniated analogs, SrŽ. ND3 n . Based on deuterium scavenging data and absorption q spectra, we propose the formation of SrD to account for one of the excess deuteriums, and the existence of solvated ND4 to account for additional deuterium atoms in the cluster. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction mine structure and reactivity. Johnson and collabora- torswx 4,5 reported on reactions of neutral electron y Clusters are a unique form of matter, intermediate scavengers withŽ. H2 On , arguing that an electron to isolated gas-phase molecules and condensed me- transfer mechanism explains the formation of sol- dia. These small aggregates of molecules and atoms vated charge transfer products. are free to exhibit properties distinct from either the Spectroscopic techniques have been used exten- gas or condensed phases. Much attention has been sively to understand the structures of solutions as focused on determining the structure and electronic modeled by cluster systemswx 6±12 . Metal ions sol- properties of small and medium-sized clusters. vated by ammonia are interesting, as metal±am- Castleman's group has used a unique form of mixed monia solutions are particularly well-studied exam- cluster formation to provide experimental evidence ples of solvated electron systems at low metal con- for the structure of protonated water clusters and centrations. In this Letter we present results of stud- protonated water±methanol clusterswx 1±3 . By `cap- ies on monovalent strontium ion solvated by ammo- ping' the non-hydrogen-bonded surface hydrogen nia. The data exhibit the formation of a new chemi- atoms of the cluster with triethylamine added to the cal species that incorporates multiple deuterium seed mixture, the resulting mass spectra were inter- atoms into the purely ammoniated cluster within a preted as providing evidence for a clathrate structure. specific range of cluster sizes. In this study, we have Scavenger techniques have also been used to deter- employed our usual technique of mass spectrometry coupled with photodissociation spectroscopy, but have also added reactivity studies to gain insight into ) Corresponding author. E-mail: [email protected] the nature of these unique species.

2. Experimental laser fluence. . In previous work from our laboratory, we have placed emphasis on the formation of small We have reported on the experimental apparatus clusters, in order to build up an understanding of employed for these studies previouslywx 7 . Therefore, their structures from the singly-solvated species up only a brief description of the experimental proce- through clusters with six solvent moleculeswx 6,7 . In dure and details specific to this system will be the present case, we examine the behavior of larger reported here. clusters, and to assist the discussion, we show a mass q A distribution of parent clusters is produced when spectrum for SrŽ. NH3 n , with n ranging from 1 to r a supersonic expansion of neat NH 33ND is ex- 22 in Fig. 1. Under low-resolution conditions, the panded through a pulsed valve into a cloud of Srq mass spectrum is striking because it shows a very ions generated by laser vaporization of a rotating clear intensity distribution with three distinct en- strontium disk. The clusters thus formed drift into a velopes: the first group of peaks corresponds to the vacuum chamber that defines the extraction region of local maximum that peaks at ns6; a second distri- a Wiley±McLaren time-of-flight mass spectrometer. bution of clusters, with a local maximum at ns13 The chamber pressure in the source region with the appears; and a third group of clusters with a maxi- reagent gas on is typically 1=10y5 Torr. A port in mum occurring at ns20 is also readily apparent in the source chamber allows the optional introduction this mass spectrum. This pattern in the mass spec- of a steady stream of gasŽ. neat NO or He that can trum is suggestive of a shell structure in the clusters, interact with the clusters after their formation but with shell closings occurring at 6, 13, and 20 solvent before extraction. The relative gas concentration can molecules. The data we present in this Letter, how- be monitored via the chamber pressure. The clusters ever, show that the clusters in the second cluster are then accelerated down the flight tube to a space group have dramatically different structure, elec- focus point. At an appropriate delay, collimated light tronic properties, and chemical reactivity compared from an optical parametric oscillator transversely intersects the mass separated beam. Spectra of all clusters were probed with a Nd:YAGŽ Spectra- Physics GCR-190. pumped singly-resonant optical parametric oscillatorŽ. Spectra-Physics MOPO-730 . The laser beam diameter is 5 mm, and the energy is attenuated to 1 mJrpulse throughout all experiments. The products of the photodissociation process are mass separated by a reflectron mass spectrometer. The ion signal is collected by microchannel plates, digitized by a transient recorder, and stored on a computer. We obtain a dissociation spectrum by integrating each of the peaks arising from the dissociation process and normalizing to beam flux at a series of wavelengths. The total dissociation spectrum, produced by summing all daughter peaks, is equated with the total absorption cross-section.

3. Results

The cluster size distribution of solvated metal ion Fig. 1. Low-resolution mass spectrum showing the intrinsic inten- clusters produced with a laser vaporization source q sity distribution of SrŽ. ND3 n formed in our source. The inset is can be altered by varying the operating conditions of a scan with conditions optimized for resolution of ns6 showing the pulsed sourceŽ i.e., backing pressure, timing, the natural Sr isotope distribution. 352 D.C. Sperry et al.rChemical Physics Letters 304() 1999 350±356 to those in the first cluster group. Thus, we believe that the interpretation of the overall intensity distribution is more subtle, and an accurate description must account for the structural differences between the cluster groups. In the inset of Fig. 1, one can see the mass spectral lines for smaller clusters show the characteristic triplet structure associated with the three most abundant isotopes of strontium: 86 Sr, 87Sr, and 88 Sr. While the resolution in Fig. 1 is less than optimal, there is evidence of peak broadening in the larger cluster groups, indicating the presence of unre- solved structure in the mass spectrum. In order to understand the cluster mass distribution more fully, we optimized the focusing conditions for the heavier clusters and carried out these experiments with ND3 , to reduce the likelihood for mass coincidences. A close look at the mass spectrum for the solvent number range ns12±14 is shown in Fig. 2. It is immediately clear from the mass spectrum that additional masses begin to ap- q Fig. 3. Absorption spectra of SrŽ. ND312 Dx . pear in this group of heavier clusters. Furthermore, deuterium substitution shows that the clusters are lating from the known constant of proportionality incorporating deuterium atoms in excess of the num- between mass and time in the smaller clusters, out to q ber that a cluster of the stoichiometry SrŽ. ND3 n the second and third groups of mass lines. The graph would have. In Fig. 2, we assign the masses, extrapo- shows the mass assignments corresponding to q s SrŽ. ND3 nx D , with x 0±4, indicating clearly the propensity for the larger clusters to incorporate up to four additional deuterium atoms. Numerous repeti-

tions of the mass spectrum, both with NH33 and ND , confirm the assignments of the multiplicity of mass spectral lines as arising from the incorporation of excess hydrogen atoms in `conventional' clusters. These mass assignments lead to the firm conclusion that we are observing a qualitatively new cluster species given symbolically by the formula Srq-

Ž.ND3 nx D . For n in the range from 10 to 15, i.e., the second cluster group, x is as large as 4. The intensities of many of these cluster ions are sufficiently large that we can measure photodissociation spectra by our usual techniques. As we have reported previously, the dissociation spectra for q SrŽ. NH3 n show a monotonic red shift with increasing cluster sizewx 7 . In more recent work, we have found the absorption maximum appears to approach an asymptotic value of ;6200 cmy1 for large F Fig. 2. Mass spectrum of clusters in the second cluster group. clusters in the group where n 9, which will be Labels over peaks indicate mass assignment according to the reported in a future paper. Qualitatively, this red q designation: SrŽ. ND3 nx D . shift is consistent with the increasing volume for D.C. Sperry et al.rChemical Physics Letters 304() 1999 350±356 353 delocalization of the single metal-centered valence first deuterium atom is added to the cluster in a electron on Srq as the solvation number increases. A location distinct from the second and third. The representative set of absorption spectra for clusters in position of the extra deuterium atom within the the second cluster group is shown in Fig. 3. The cluster could dramatically affect the reactivity of the spectra have several interesting features. Each of the cluster. extra-deuterium containing clusters has a strong, We have used a deuterium scavenging technique, broad absorption band around 11 000 cmy1, and a which allows NO gas to react with the entire cluster much weaker absorption band at 18 000 cmy1. Both distribution before the extraction pulse for mass bands shift 3000 cmy1 higher in energy as x varies analysis. The resultant mass spectrum shows the from 1 to 3. Clusters containing at least two addi- cluster distribution after reaction with a given con- tional deuterium atomsŽ. i.e., xG2 have an absorp- centration of NO. Nitric oxide is an odd-electron tion band peaking at 6200 cmy1. The clusters con- molecule and can therefore effectively scavenge rad- taining only one extra deuterium, xs1, do not ical species selectively compared to a closed-shell exhibit this low-energy absorption feature. These atom such as helium. Fig. 4a shows a representative same trends in absorption profile have been observed set of mass spectra with varying concentrations of for other cluster species of similar size. It appears NO. In the lowermost mass profile, peaks from s q s s that the x 2, 3 species have a chromophore that is SrŽ. ND3 nx D , x 0±4, are clearly visible for n absent in the xs1 species. Thus we expect that the 12±14. This spectrum was taken without adding

q Fig. 4. Mass spectra of SrŽ. ND3 nx D after addition of varying amounts of reactant gas to the source chamber:Ž. a nitric oxide and Ž. b helium. Source chamber pressures chamber are indicated next to each trace. 354 D.C. Sperry et al.rChemical Physics Letters 304() 1999 350±356

P nitric oxide to the source chamber; it represents the involve intra-cluster reactions eliminating ND2 or inherent cluster intensity distribution formed in our other stable or metastable species from a larger source. The chamber pressure under these conditions cluster. In fact, intra-cluster reactions have been is 1=10y5 Torr. The mass spectrum directly above shown to break covalent bonds in the solvent in q wxq was taken after adding enough NO to the source many cases, such as MgŽ. H2 On 13±15 , Ca - = y5 wx q wx q chamber to raise its pressure to 7 10 Torr. The Ž.H2 On 12,14 , MgŽ. CH3 OHn 16 , and M - s s wx peaks corresponding to x 2±4 have clearly lost Ž.ŽCH3 OHn M Mg, Ca, Sr, Ba . 17 . Given the intensity relative to the peaks corresponding to xs1 presence of extra deuteriums in the cluster, we have for the ns13 cluster group. Because of lower par- considered the energetics of deuterium atom transfer ent cluster intensity the quenching is not as obvious and charge transfer in the cluster. Possible configura- in the ns12, 14 cluster groups; it is, however, still tions include the deuterium bound in the solvent as a q q present. The next two traces show mass spectra at neutral SrŽ. ND3 n ND43 or cationic Sr Ž. NDn ND4 higher NO concentrations. It is evident that as the species, or the deuterium bound to the metal in either qq concentration of scavenger gas is increased, nitric a neutral SrDŽ. ND3 n ND33 or charged SrDŽ. NDn - oxide preferentially reacts with species containing ND3 species. Thermodynamic Born cycle calcula- two or more excess deuterium atoms. At higher tions indicate that the latter metal deuteride ion scavenger concentrations the entire mass spectrum is species is the lowest-energy configuration. Data for a quenched. The overall reduction of signal intensity is Born cycle calculation are not complete, but reason- a result of the interference of the background of able estimates of solvation enthalpies and bond ener- nitric oxide with the efficiency of the extraction gies can be made from data available in the litera- process in the instrument. This effect can also be ture. observed when using an unreactive gas in the source Alkaline-earth metal hydride ions, despite being chamber such as helium. Fig. 4b shows a similar stable species, have received relatively little atten- experiment using He instead of NO. The lowermost tion. Only a few reports of the ground state proper- trace shows the intrinsic mass distribution of the ties of SrHq existwx 18±22 . The hydrogen is bound ns12, 13, 14 clusters present with no reactant gas. by ;2 eVwx 19,21 , while the electron affinityŽ 5.58 The two spectra above this trace show the same eVwx 18. is very close to that of the metal ion, Srq . clusters after the addition of He into the source Experimental determinations of the electronic prop- chamber. The intrinsic mass distribution in the beam erties exist only for MgHq wx 23 , while some theoreti- is persistent, independent of He concentration. The cal predictions on the general electronic structure overall intensity is depleted as a result of the ineffi- were made by Schilling et al.wx 19 . cient extraction as in the NO case. Since no electronic information is available for The results of the scavenging experiment coupled SrHq, we turn to the isolectronic RbH molecule. with the electronic spectroscopy clearly show that Properties of low-lying electronic states of the alkali q G wx the SrŽ. ND3 nx D Žx 2 . are chemically distinct hydrides were compiled by Stwalley et al. 24 . The from the xs1 species. The heavier species are more A 1 Sq state of RbH lies ;18 000 cmy1 above the reactive with a deuterium scavenging molecule than X 1 Sq ground state. This transition energy is in the those containing only one extra deuterium. Further- region of the two highest energy bands in the spec- q more, the second and third additional deuterium trum of SrŽ. ND312 Dx . We believe it is reasonable atoms in the cluster appear to create a chromophore to attribute the transitions at 11 000 and 18 000 cmy1 that is not present in the species containing a single to a solvated SrDq chromophore. These bands shift excess deuterium atom. to higher energy with the addition of deuterium atoms probably as a result of a solvent reorganiza- tion which removes ligands from the local solvation 4. Discussion environment of the metal deuteride to accommodate the extra deuteriums. Extra deuterium atoms could conceivably be in- Based on Born thermodynamic cycle calculations, corporated into clusters by a mechanism that would a second additional deuterium atom incorporated into D.C. Sperry et al.rChemical Physics Letters 304() 1999 350±356 355 the cluster is energetically more stable as a neutral could then be explained by the existence of an

ND4 radical species than a charge transfer species. additional identical ND4 absorber. This is somewhat surprising considering the I.P. of Kassab and collaborator have proposed a bubble wx NH4 was determined by high-resolution zero-kinetic- structure for the solvated ND4 radical 34,35 . This energy photoelectron spectroscopy to be 4.65 eVwx 25 model accounts for the repulsive character of the compared to 5.58 eV for SrH. This fact indicates that hypervalent electron. The reactivity data shown in the positive charge would prefer to be associated Fig. 4 are consistent with this picture. We believe with ND4 . This is most likely not the case, however, that the Rydberg electron in the radical forces the q since the preferable solvation enthalpy of SrD ND4 to exist on the surface of the cluster. Thus, the would outweigh the 1 eV gain from the charge weakly bound deuterium atom is highly reactive with transfer. The same energetic arguments apply to the a hydrogen scavenger like nitric oxide. The first third additional deuterium atom ; it can exist as a deuterium atom incorporated into the cluster is more neutral ND4 radical as well. tightly bound in the strontium deuteride ion and is The hypervalent ammonium radical has received therefore not as reactive as the second and third. much attention. Many experimental and theoretical probes of the structure and energetics exist in the literature. The absorption spectrum was first identi- 5. Conclusions fied by Herzbergwx 26 and named after SchulerÈ who was among those who first observed it. The SchulerÈ We have shown the existence of unique chemical Ž; y1 . band of the isolated ND4 radical 17 200 cm species in metal-centered polar solvent clusters cor- has been assigned to the transition between the 3p responding to the empirical formula SrqŽ. ND D , 22 wx 3 nx F21 state and 3s A ground state 27 . The lifetime where x is a large as 4 for clusters in the size range - wx of the free radical was estimated to be 150 ns 28 ns10±15. Photodissociation spectra obtained for while the fully deuterated analog has a much longer several clusters show three distinct features: two ) m wx y lifetime Ž.10 s 27±29 . However, ammoniation bands at ;11 000 and ;18 000 cm 1 present in all y of the NH4 radical has been shown to increase the 1 wx clusters, and a low-energy band at 6200 cm , pre- metastable lifetime by several fold 30,31 . In our sent only in clusters with at least two additional apparatus, clusters exist for durations approaching 1 deuterium atoms. Deuterium scavenging studies show ms before detection. We speculate that solvation of that clusters containing at least two excess deuterium the radical enhances the metastability, i.e. greatly atoms Ž.xG2 are more reactive with nitric oxide increases its lifetime. than clusters containing a single excess deuterium q While the isolated ND4 species has been studied atom. We propose the formation of SrD to account § spectroscopically, no data exist on the 3p 3s like for one of the excess deuteriums and attribute the transition for a solvated species. The I.P. of two high-energy absorption features Ž;11 000 and Ž. wx y NH43 NHn was determined by Fuke et al. 32 to ;18 000 cm1 . to this chromophore. We argue the decrease approximately linearly with ny1r3 and ex- existence of solvated ND4 to account for additional trapolates to the bulk photoelectric threshold limit. deuterium atoms in clusters where xG2 and assign This is similar to results seen in solvated alkali Ž y1 . wx wx the low-energy feature in our spectra 6200 cm to metals 33 . As has been suggested previously 26 , the 3p§3s like transition of the solvated ammo- the ammonium radical should bear electronic similar- nium radical. ities to the isolectronic sodium atom. The 3p§3s splitting in the solvated radical may track similarly to other solvated one-electron systems. We propose § References that the solvated ND4 radical has a 3p 3s like transition at ;6000 cmy1, which coincides with wx1 S. Wei, Z. Shi, A.W. Castleman Jr., J. Chem. Phys. 94 the lowest-energy feature in the spectrum of Ž.1991 3268. q s SrŽ. ND312 Dx , x 2,3. The approximate doubling wx2 X. Zhang, A.W. Castleman Jr., J. Chem. Phys. 101Ž. 1994 in intensity of this band between xs2 and xs3 1157. 356 D.C. Sperry et al.rChemical Physics Letters 304() 1999 350±356

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