Metastable ion study of organosilicon compounds. Part V—tetramethoxysilane and trimethoxymethylsilane

The fragmentations of tetramethoxysilane ((CH₃O)₄Si (1)) and trimethoxymethylsilane ((CH₃O)₃SiCH₃ (3)) induced by electron impact were investigated by mass-analysed ion kinetic energy (MIKE) spectrometry and a deuterium-labelling study. These molecular ions begin to fragment by the loss of CH₃ or CH₃O. These fragmentations are followed by the loss of an aldehyde molecule (H₂CO), as commonly observed in the mass spectra of alkoxysilanes. Almost complete scrambling of the methoxy hydrogens takes place in the metastable molecular ion, [1]⁺˙, prior to the decomposition. On the other hand, a moderate extent of scrambling of the hydrogens takes place in [3]⁺˙. The fragmentations of [1]⁺˙ and [3]⁺˙ were compared with those of the corresponding carbon analogues, tetramethoxymethane ((CH₃O)₄C (2)) and 1,1,1-trimethoxyethane ((CH₃O)₃CCH₃ (4)), respectively.