q MASS SPECTRAL STUDY OF TRIMETHYLSILYLMETHYL- SUBSTITUTED CHLOROSILANES THESIS Presented to the Graduate Council of the North Texas State University in Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE By Keith Randal Pope, B.S. Denton, Texas December, 1982 Pope, Keith Randal, Mass Spectral Study of Trimethylsilyl- methyl substituted Chlorosilanes. Master of Science (Chemistry), December, 1982, 47, pp., 5 tables, bibliography, 22 titles. The mass spectra of the compounds [Me 3 SiCH2 nSiCl 4 n (n=1-3) were studied in detail. MIKES and CID spectra were used in conjunction with the observance of metastable processes to develop consistent fragmentation schemes. Particular attention is drawn to the formation of charged and neutral species containing the silicon-carbon double bond, including 2-silaallene, under conditions of electron impact. TABLE OF CONTENTS Page LIST OF TABLES. ............ ii LIST OF ILLUSTRATIONS. ................. iii Chapter I. INTRODUCTION.o..........................1 II. RESULTS AND DISCUSSION ....... ..... 9 III. EXPERIMENTAL .......................... 39 APPENDIX. ............................. ........ 45 BIBLIOGRAPHY,*........................... ........ 46 i LIST OF TABLES Table Page I, [Me 3 SiCH2 InSiCl 4 -n . 9 II. Mass Spectrum of Trimethylsilyl- methyltrichlorosilane (I). ......... 11 III. Metastable Peaks Observed in the Mass Spectra of Compounds I-III. 13 IV. Mass Spectrum of Bis(trimethylsilylmethyl)- dichlorosilane (II)..................... 16 V. Mass Spectrum of Tris(trimethylsilylmethyl)- chlorosilane (III) .. ....... 27 ii LIST OF ILLUSTRATIONS Figure Page 1 Inverse Nier-Johnson Geometry with field-free regions A,B,C....... .......... 5 2 Mass Spectrum of Trimethylsilylmethyl- trichlorosilane (I)............. 12 3 Mass Spectrum of Bis(trimethylsilylmethyl)- dichlorosilane (II),.................. 17 4 Unimolecular MIKES Spectrum of m/e 257.0. .0 20 5 CID Spectrum of m/e 257........... .. 21 6 Unimolecular MIKES Spectrum of m/e 149. .0 22 7 CID Spectrum of m/e 149......... ........ 23 8 CID Spectrum of m/e 93....0. ... 25 9 Unimolecular MIKES Spectrum of m/e 93 . .0. .0 26 10 Mass Spectrum of Tris(trimethylsilylmethyl)- chlorosilane (III) . 28 11 Unimolecular MIKES Spectrum of m/e 309. .0.0.0 30 12 CIDSpectrum of m/e 309*. .0 .. 31 13 Unimolecular MIKES Spectrum of m/e 201. .0.0.0 32 14 CID Spectrum of m/e 201..0 . ...... 33 iii CHAPTER I INTRODUCTION The synthesis and reactivity of chemical species containing the silicon-carbon double bond, silenes, have been the subject of great interest for nearly a decade.1 Silenes are accessible under a wide variety of conditions including thermal decomposition of substituted monosilacyclo- butanes, 2 photochemical decomposition of polyorganosilanes, 3 and elimination of metal halides from a-lithiohalosilanes.4 Evidence also exists for the generation of silenes by electron impact, presently the most common method of ioni- zation in mass spectrometers. One such case is the mass spectra of substituted monosilacyclobutanes, reported by Auner and Grobe,5 in which prominent peaks in the spectra may be ascribed to ions containing the silicon-carbon double bond. These investigators have used mass spectral fragmen- tation patterns to characterize condensate from the pyrolysis of 1,3-disilacyclobutane under high vacuum at 6000C and again propose the formation of silenes under conditions of electron impact.6 A subsequent paper suggested the formation of silenes in the mass spectral decomposition of methylcyclo- pentadienyl silacyclobutane. Fragmentation patterns have also provided evidence for the formation of neutral species 1 2 incorporating double bonds between silicon and the chalcogens (O,S,Se,Te)8 and charged species containing double bonds between silicon and oxygen.9 Of particular interest is the possibility of correlating mass spectral fragmentation patterns with mechanisms involved in the high energy generation of silenes. The method of generating silenes by electron impact would seem to be a good method for studying the energetics of this highly reactive moiety if the genetic relationships between the observed fragments could be determined unambiguously. Indeed, typical mass spectra are obtained using 50-70 ev electrons, whereas the energies necessary to ionize most 10 organic compounds are in the neighborhood of 10 ev0. Under typical conditions, then, the ions produced initially in a mass spectrometer are in a highly excited state. This results in a large number of daughter fragments from the parent ion, but rarely are recombinations observed in mass spectrometry. Thus, most processes occurring under these conditions are unimolecular decompositions. The unambiguous mapping of mass spectral fragmentation patterns is not, however, a trivial matter. Although it is often possible to narrow the possibilities for ion genisis based upon structural arguments, it is difficult to absolutely prove that two fragments are related by a one-step process, as illustrated in Scheme 1.11 3 M0 M+ -- > + + Scheme 1 In principle, the ion Mt may be the parent molecular ion or any ion which results from some sequence of fragmen- tations of the parent ion, and the dissociation of M+l to + 0 anywhere produce M2 and the neutral species M3 may occur along the path of the ion beam from source to detector. If the ion M+ is long-lived, then fragmentation may occur after it has been accelerated by an electric potential towards the magnetic focusing sector; in particular, this may happen in a field-free region (FFR). The product M2 may 12 then give rise to a so-called "metastable peak."12 These peaks are readily identifiable because they are broader than normal peaks, much less intense, and need not occur at 13 integral mass values. The maximum intensity of the meta- stable peak corresponds to a mass M dependent upon the masses of the parent and daughter ions according to the well- known formula, * 2 M = M2/M A magnetic sector is necessary to observe such metastable peaks. 14 Although the presence of such a peak is consistent with the process in Scheme 1, it does not define the masses M and M2 independently. 4 A detailed analysis of mass spectral fragmentation patterns is facilitated by the recently developed techniques of mass analyzed ion kinetic energy spectrometry (MIKES) and collision induced decomposition (CID).15,16 The MIKES technique is performed on a double focusing mass spectrometer with inverse Nier-Johnson geometry as shown in Figure 1.17 The relevant features of this configuration are the presence of a magnetic sector followed by an electric sector with three interposing field-freE regions. The field strength of the magnetic sector can be varied to allow ions of a particular mass to pass intc the second FFR. The electric focusing sector can then be decoupled and tuned to obtain a spectrum of fragmentations Occurring in the second FFR. The important result is that the origin of the daughter ions analyzed by the electric sector is clear. In practice, the number of fragmentations occurring in region B of Figure 1 will be small in relation to the number occurring in the ion source. Thus MIKES spectra will detect only a few fragmentations. To overcome this difficulty, ions entering region B can be collisionally activated. If a slight pressure of some inert gas is present in region B of the spectrometer, enteri-n-g ions will collide with the gas molecules and may dissociate. Helium is usually preferred as a target gas because of its inertness, and because small target particles tend to fayor excitation of the incident ions to a dissociative electronic state.18 Most importantly, 5 MAGNETIC SECTOR ELECTRIC SECTOR - - ~------B---------~ N / / I I I I I I I I I A C ION SOURCE DETECTOR Figure 1: Inverse Nier-Johnson Geometry with field-free regions A,B,C. 6 ions which have added stability can be induced to dissociate through collisional activation. Therefore, the CID technique, in conjunction with a MIKES scan, can be used to probe frag- mentation pathways to a greater extent than would be possible with conventional mass spectrometry. The major disadvantage in using this method appears to be a decreased resolving power due to the scattering effects of collisional processes. If a high resolution mass spectrum is available to compare with the MIKES and CID data, poor resolution is not a great barrier to the illucidation of fragmentation pathways. In general, the MIKES and CID techniques provide information which is necessary for the detailed analysis of mass spectral fragmentation patterns. 7 REFERENCES 1. Bush, R. D.j Golino, C. M. and Sommer, L. H., J. Amer. Chem. Soc. 1, 96, 7105 (1974). 2. Barton, T. J., Kilgour, J. A. and Marquardt, G.,' J. Organometal. Chem., 85, 317 (1975). 3. Ishikawa, M., Pure Ap. Chem., 50(1), 11 (1978). 4. Jones, P. R. and Lim, T. F. 0., J. Amer. Chemi. Soc., 99(6), 2013 (1977). 5. Auner, N. and Grobe, J., J. Organometal. Chem., 188(1), 25 (1980). 6. Auner, N. and Grobe, J.., Zeit. Anorg. Allg. Chem., 459, 15 (1979). 7. Auner, N. and Grobe, J., J. Organometal. Chem., 190(2), 129 (1980). 8. Drake, J. E., Glavincevski, B. M. and Wong, C., J. Inorg. Nucl. Chem., 42, 175 (1980). 9. Larsen, *G. L., Oliva, A. and Tsai, R. S.-C.,, Org. Mass Spectrom. , 14, 364 (1979). 10. Schlunegger, U. P., Advanced Mass Spectrometry, New York, Pergamon Press, 1980. 11. Bowen, R. D., Howe, I. and Williams, D. H., Mass Spectronmetry, 2 ed., New York, McGraw-Hill, 1981. 12. Beynon, J. H. and Cooks, R. G., Res./Dev, 22, 26 (1971) . 13. Reference 11, p. 56. 8 14. Reference 11, p. 54. 15. Reference 10, p. 13. 16. Reference 12, p. 26. 17. Reference 10, p. 32. 18. Dong, P., Durup, J. and Yamaoka, H., Chem. Phys. , 51, 3465 (1969). CHAPTER II RESULTS AND DISCUSSION Reports published as early as 1968 suggested that a silene might be generated by reacting an organohalosilane with tert-butyllithium at low temperatures in the presence of a chelating amine.1 To investigate this possibility more thoroughly, the compounds [Me3 SiCH2 InSiC4-n were prepared by a method similar to that of Sommer and Murch.