Unimolecular reaction of organosilicon ions in electron impact mass spectrometry .1. The fragmentation of 1,1,2,2-tetramethyldisilane, bis(dimethylsilyl) methane, and 1,2-bis (dimethylsilyl) ethane ions

The electron impact (EI) mass spectra of three organosilicon compounds with two silicon atoms (CH3)(2)SiH-(CH2)(n)-SiH(CH3)(2) (n = 0 similar to 2) were measured and the fragmentation mechanisms were investigated by the deuterium-labelled technique and the B/E linked-scanning technique. In the EI mass spectrum of 1,1,2,2-tetramethyldisilanes, the main fragmentation produced m/z 73 ions via a four-center transition structure. In bis(dimethylsilyl)methane, methyl radical loss from molecular ion M(+.) was the main process, and most of the fragment ions were produced by the secondary decomposition of [M-CH3](+). In 1,2-bis(dimethylsilyl)ethane, Si-C bond cleavage occurred simultaneously with the Si-Si bond formation producing m/z 103 ions. In this process, a four-center transition state was involved. The optimized geometries of the neutral molecule and the molecular cation of bis(dimethylsilyl)methane were calculated using the ab initio MO method.