J . Phys. Chem. 1993,97,11371-11372
11371
Neutralization-Reionization Mass Spectrometry Experiments Confirm the Predicted Existence of Cyclic Si20 and SizN Cluster Molecules Muhammad Iraqi, Norman Goldberg, and Helmut Schwarz' Institut fur Organische Chemie der Technischen Universitat Berlin, Strasse des 17. Juni 135, D- 10623 Berlin, F.R.G. Received: August 16, 1993; In Final Form: September 23, 1993a
Collision experiments are reported for Si2X+ (X = 0,N) generated by dissociative ionization of (SiH3)20 and (SiH3)3N in the gas phase. It is argued that both the ionic and the neutral Si2X clusters have a triangular geometry as predicted for Si20 by a recent, thorough a b initio MO study (Boldyrev, A. I.; Simons, J. J . Phys. Chem. 1993, 97, 5875). While the linear connectivity SiSiX cannot rigorously be ruled out, the symmetrical linear structure SiXSi is certainly not generated in the present experiment.
Silicon oxides not only play a role in some important aspects of interstellarchemistry,' but simplemolecules like the Si20 cluster may be viewed as a prototype for the interaction between an oxygen atom and silicon sites in clusters or surfacesa2 However, in contrast to the isoelectronic C20 molecule, which has a linear CCO singlet structure,3 only preliminary experimental4 and theoretical resultss were available on Si20 prior to Boldyrev and Simons' recent, thorough ab initio MO study6 on Si,O clusters (n = 2,3). According to the argon matrix experimentsof Weltner and co-workers,4the ESR spectrum has been tentatively assigned to a linear SiSiO structure with a triplet 32-state. The stateof-the-art calculation of Boldyrev and Simons? however, clearly demonstrates that the global minimum on the Si20 potential energy surface corresponds to the triangular Si20 cluster 1 (Ch, 1A1). At the QCISD(T)//6-31 1+G(2df) level of theory, 1 is predicted to be 17.5 kcal/mol more stable than its geometrically closely-relatedtriplet electromer (C,, 3B1). Next in energy comes the linear SiOSi molecule 2 (D-h, 3Zg-), which is 20.7 kcal/mol less stable than 1. The SiSiO cluster 3 (C,,, %-) corresponds to the least stable structure (27.0 kcal/mol, relative to 1).
/"\
Si-0
-Si
Si-Si-0
Si -Si (1)
(2)
(3)
In addition,it is predicted6 that the lowest-energy decomposition path of Si20 involves loss of SiO. While this dissociation needs 50.1 kcal/mol at its thermochemical limit, evaporation of an oxygen atom from Si20 to generate Si2 is much more energy demanding (162.4 kcal/mol). In this Letter we will report the results of gas-phaseexperiments which lend strong support to the theoretical prediction that triangular Si20 1 is indeed a viable molecule. In addition, we demonstrate that Si2N*, a cluster which has not yet been structurally characterized and which is of potential interest in material sciences, has an analogous atom connectivity (4 rather than 5 or 6). Abstract published in Aduance ACS Abstracts. October 15, 1993.
0022-3654/93/2097-11371%04.00/0
I
Recovery Signal Si20+'
I-
I
SiO+'
mlz
-
Figure 1. Neutralization-reionization (NR) mass spectrum of Si20'+ (xenon, 80% T; oxygen, 80% T).
By using the technique of neutralization-reionization mass spectrometry (NRMS),' we were recently able to generate* numerous elusive silicon-containing molecules of interstellar interest, which due to facile intermolecular processes could not be studied in the condensed phasesSg All collision experiments were performed by using a modified large-scale four-sector tandem mass spectrometer of BEBE configuration (B stands for magnetic and E for electric sector). The details of the facility and its operation have been described elsewhere.10 The Si20*+ions, which served as a precursor for the neutralization experiment,were generated by dissociativeelectron impact ionization of disilyl ether, (SiH&O,l using the following mass spectrometric conditions: electron energy, 70 eV; repeller voltage, ca. 30 V; ion source temperature, 200 "C; acceleration voltage, 8 kV. The Si2N+ ions were generated from (SiH3)3N11 using the same ion source conditions. For the N R experiment the 8-keV beams of ions of interest, Le., Si20*+or Si2N+, were mass-selected by means of B( 1)/E( 1) at a resolution m/Am = 2500. Neutralization was brought about in the first cell of a tandem collision cell by colliding the beam with xenon (80% transmission, T). The deflector electrode, mounted between the two collision cells, was charged to 1000 V, thus preventing any ions from entering the second collision cell in which collisional reionization of the beam of neutrals takes place (oxygen, 80%T). All product ions formed upon collisional ionization were recorded by scanning B(2), and 10-20 spectra were accumulated and online processed using the AMD-Intectra data system. The minimal lifetime t of the neutrals Si20 and S&N*(identical with the transit time from the first to the second collision cell) is in the present experiment of the order oft > 10 ps. In Figures 1 and 2 the NR spectra of Si20*+and Si2N+ are given, and the interpretation of these spectra is quite straightforward. 0 1993 American Chemical Society
11372 The Journal of Physical Chemistry, Vol. 97, No. 44, 1993
Letters silicon bond,14 thus ruling out theconnectivity Si-XSi (structures 2 and 5). While an absolutely unambiguous distinction between the triangular structures (1,4) and the linear systems SiSiX (3, 6 ) cannot be made, recourse to the ab initio resultsS very much suggests that in our experiment we are actually dealing with the cyclic form 1 (and, by analogy, for the SizN system with 4) simply on the ground that, in the Si20 cluster, the oxygen atom strongly prefers to coordinate to two silicon atoms (as in 1) rather than to one (as in 3).
N+
Si; A
I
'
,
-
I:
m/z
Figure 2. N R mass spectrum of Si2N+ (xenon, 80% T; oxygen 80% T). SiO+'
Acknowledgment. Financial support of our work by the Deutsche Forschungsgemeinschaftand the Fonds der Chemischen Industrie is acknowledged. H.S.is grateful to the Alexander von Humboldt Foundation for a Max Planck Research Award which he shares with Professor Chava Lifshitz and which forms the financial basis for the collaboration between the Hebrew University of Jerusalem and the Technische Universitlt Berlin. References and Notes
Si+'
mlz
-
Figure 3. Collisional activation (CA) mass spectrum of Si20'+ (helium, 80% T). Si+' I
SiNt
mlz
-
Figure 4. CA mass spectrum of Si2N+ (helium, 80% T).
For the two systems we note the presence of very intense recovery signals which reflect both a high stability of the neutrals generated and also favorable Franck-Condon factors for the consecutive electron-transfer processes: Si2X+ + e- SizX Si*X+(X = 0,N). The presence of the strong fragment signals for Six+ (X = 0,N), in contrast to Si2*+,is in perfect agreement with the theoretically predicted behavior of Si2Ol2 with regard to dissociation: Evaporation of an oxygen atom to generate Si2 is largely disfavored as compared with the production of S i 0 and Si. Consequently, in the reionization step SiO*+and Si'+ions are generated more abundantly than Si2*+.13 Further information concerning the connectivities of the Si2X systems (X = 0, N) is provided by subjecting the Si2X+ ions to collisional activation (CA; helium, 80% T). The presence of intense signals due to Si2'+ in the CA spectra (Figures 3 and 4) clearly indicates that both Si2X+ions most likely contain a silicon-
-
+
(1) (a) Duley, W. W.; Williams, D. A. Inrersrel1arChemistry;Academic Press: New York, 1984. (b) Carlo, R.; Ginebreda, A. J. Chem. Educ. 1985, 62, 832. (2) Chiang, C.-M.; Zegarski, B. R.; Dubois, L. H. J. Phys. Chem. 1993, 97,6948. (3) (a) Yamada, C.; Saito, S.;Kanamori, H.; Hirota, E. Astrophys. J . 1985,290,L65. (b) Yamada, C.; Kanamori, H.; Moriguchi, H.; Tsuchiya, S.; Hisota, E. J . Chem. Phys. 1986,84,2573. (c) Ohishi, M.;Suzuki, H.; Tshikawa, S I . ; Yamada, C.; Kanamori, H.; Irvine, W. N.; Brown, R. D.; Godfrey, P. D.; Kaifu, N. Astrophys. J. 1991, 380,L39. (4) (a) Lembke, R. R.; Ferrante, R. F.; Weltner, W. J. Am. Chem. SOC. 1977,99,416.(b) Van Zee, R. J.; Ferrante, R. F.; Weltner, W. Chem. Phys. Lett. 1987,139,426. ( 5 ) DeKock, R. L.; Yates, B. F.; Schaefer 111, H. F. Inorg. Chem. 1989, 28, 1680. (6) Boldyrev, A. I.; Simons, J. J . Phys. Chem. 1993,97, 5875. (7) Reviews: (a) Terlouw, J. K.; Burgers, P. C.; van Baar, B. L. M.; Weiske, T.; Schwarz, H. Chimia 1986, 40, 357. (b) Wesdemiotis, C.; McLafferty, F.W. Chem. Rev. 1987,87,485.(c) Terlouw, J. K.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1987,26,805.(d) Schwarz, H. Pure Appl. Chem. 1989,61,685.(e) Terlouw, J. K. Adv. MassSpectrom. 1989,Il,984. (0 Holmes, J. L. Mass Spectrom. Rev.1989,8,513. (g) McLafferty, F. W. Science 1990,247, 925. (h) McLafferty, F. W. Int. J. Mass Spectrom. Ion Processes 1992,118/119,221. (8) Common to NRMS and many other mass spectrometry based techniques is that only gross structural features (Le., atom connectivities) can be inferred from the experiments. Information on detailed geometrical details or on spin states is, in general, not available. For a review, see: Holmes, J. L. Org. Mass Spectrom. 1985,20, 169. (9) (a) For many examples, see: Iraqi, M.; Schwarz, H. Chem. Phys. Lett. 1993,205, 183 and references therein. Also see: (b) Goldberg, N.; Iraqi, M.; HruSak, J.; Schwarz, H. Int. J. Mass Spectrom. Ion Processes 1993,125,267.(c) Srinivas, R.; Bohme, D. K.; Schwarz, H. J. Phys. Chem., in press. (d) Goldberg, N.; HruSBk, J.; Iraqi, M.; Schwarz, H. J. Phys. Chem., in press. (10) (a) Srinivas, R.; Siilzle, D.; Weiske, T.; Schwarz, H. Int. J. Mass Spectrom. Ion Processes 1990, 107,369. (b) Srinivas, R.; Siilzle, D.; Koch, W.; DePuy, C. H.; Schwarz, H. J . Am. Chem. SOC.1991, 113,5970. (c) Srinivas, R.; Bbhme, D. K.; Siilzle, D.; Schwarz, H. J . Phys. Chem. 1991,95, 9836. (11) Schenk, P. W.; Huber, F.; Schmeisser, M. In Handbuch der Pr¶tivenAnorganischen Chemie; Brauer, G.,Ed.; Enke Verlag: Stuttgart, 1978. (12) For the Si2N system no thermochemical data are available, which would permit an estimate on the energy requirements for the dissociations of interest. (13) No quantitative measurements, aimed at taking into account the cross sections of the electron-transfer steps for the various neutral species or corrections for detector responses for ions having different kinetic energies, were performed. Similarly, in principle the NR spectra may also contain contributions of collision-induceddissociationsof the recovery signals Si2X+. We note, however, that this is not very likely in the present case for the reason that collisional activation of Si2X+ (Figures 3 and 4) gives rise to abundant charge-stripping processes (Si2X+-c Si2X2+). Signals due to this reaction are absent in the NR mass spectra (Figures 1 and 2). (14) In contrast to the neutral S i t 0 clusters, the energy difference for the two competing processes to generate the pairs Sio'+/Si and Si2'+/0 is much smaller (24kcal/mol (ref 13) versus 112 kcal/mol (ref 6)) for the analogous decomposition of neutral Si20.
