Metastable and Collision-Induced Fragmentation Studies of All C,H,,Si+’ Isomers; A Systematic Study of Structure-Reactivity Relations Dovas A. Saulys, Cornelis E. C. A. Hop, and Donald F. Gaines Department of Chemistry, University of Wisconsin-Madison, Madison, Wiicmwin, USA zyxwvutsrqponmlkjihgfedcbaZYXWVUTS Metastable ion (MI) and collision-induced dissociation (CID) mass spectra have been recorded and compared for all nine C,H,,Si+’ isomers. The (Me),Si+; t-B&H;; s-BuSiH;, and (Me),EtSiH+’ isomers have unique MI and CID mass spectra. The MI mass spectra, including the kinetic energy release values, of (MeXi-Pr)SW;’ and (MeXn-R&H; are identical, which implies isomerization. MI data also suggest that a fraction of the Et,Sii;’ ions rearrange into branched (Me),EtSiH+’ ions and a fraction of the n-BuSiH;’ ions rearrange into branched s-BuSiH;’ ions. A comparison with the isomeric C,H;; pentanes reveals a crucial difference: H, loss occurs for n-BuSiH’,; i-BuSiH’,; s-BuSiH’,; (Me&Pr)SiH’,; (Me%Pr)SiH$‘, and Et,SiH;; but not for any of the CsH& isomers. Generation of four- or five-membered silicon containing rings is suggested for H, loss from the C4H,,Si” silanes. (I zyxwvutsrqponmlkjihgfedcbaZYXWVUT Am SOC Mass Spectrom 1w 4,5,537- 543) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA S ilanes and their organo-derivatives are of interest as precursors in chemical vapor deposition (CVD) processes used to prepare epitaxial films. Choice of precursor would be made more effective if it proved possible to relate precursor composition and structure to the mechanisms of thermal decomposition in the CVD process. This work is part of a larger effort in which the pyrolysis and film behavior of a complete set of organosilane isomers are being examined in an effort to understand the influence of precursor struc- ture on CVD behavior. Initial results indicate a corre- spondence between the type of fragments produced by thermal decomposition and those generated by elec- tron impact in a mass spectrometer for some silanes [l]. If the correlations between mass spectral data and pyrolysis behavior for these compounds are general, and can be subsequently related to particular features of epitaxial films, it should then prove possible to predict the CVD characteristics of a precursor from an examination of its mass spectra. Over the past two decades mass spectrometry has distinguished itself as an extremely powerful tech- nique for distinguishing isomeric ions. Two comple- mentary approaches are available to determine the structure of ions: (1) ion/ molecule reactions [2] and (2) metastable and collision-induced dissociation (CID) [ 31. The latter approach was used to examine the gas phase zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM Address reprint requests to C. E. C. A. Hop, Department of Chem- istry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706. behavior of Si(CH,): ions [4]. Surprisingly, the don+ nant metastable process is loss of C,H,, which must involve a complex rearrangement. In general, even- electron organosilicon cations commonly undergo complex rearrangement processes [4, 51. Specific ion/ molecule reactions can be used to distinguish iso- mers as well. For example, ion cyclotron resonance experiments showed that the reactions of the isomers Si(CH,),+ and SiH(CH&C2Hs)+ with methanol give rise to different product ions [6]. However, the reac- tions of the isomers Si(CH,),(n-C 3H7)1 and Si(CH&i-C3H7)+ with methanol produced product ions of the same mass and they could be distinguished only on the basis of their relative abundances. That collisional activation can also induce rearrangements has been shown in the case of isomerization of Si(C,H,)H; to the thermodynamically more stable isomer Si(CH&,H+ [7]. The ion structures were probed by ion/ molecule reactions with ethene-D,, which showed that while Si(C,H,)Hl undergoes multiple H/ D exchanges, Si(CH3)2H+ tends to undergo a sin- gle H/ D exchange. Odd-electron alkylsilane cations have received much less attention and most studies have focused on ioniza- tion and appearance energy measurements for tetra- methyl- and trimethylsilane [&lo]. These measure- ments allowed the determination of heats of formation of methylsilanes and other alkylsilanes. (Heats of for- mation of organosilicon compounds determined via static oxygen-bomb calorimetry are frequently inaccu- rate because of incomplete combustion and all pre-1970 8 1994 American Society for Mass Spectrometcy 1044~0305/94/$7.00 Received November 8,1993 Revised December 21,1993 A ccepted December 21,1993