Fragmentation Mechanisms of Butylmethylsilane, Butyldimethylsilane, and Butyltrimethylsilane Ions in EI Mass Spectrometry. 1. Experimental Approach

Abstract

The unimolecular dissociation of the butylmethylsilane, butyldimethylsilane, and butyltrimethylsilane ions in EI mass spectrometry has been investigated using the deuteriumlabeling technique and the B/E linked scanning method. The EI mass spectra of these alkylsilanes were characterized by the formation of silyl cation, [(CH₃)_nSiH_3-n]⁺ (n=1-3), due to the cleavage of an Si-C bond and the formation of [(CH₃)_nSiH_3-nCH₂=CH₂]⁺ due to the cleavage of a C_β-C_γ bond. The stabilization due to an Si-C hyperconjugation is found to be a driving force to produce [CH₃SiH₂CH₂CH₂]⁺ from butylmethylsilane ion and [(CH₃)₂SiHCH₂CH₂]⁺ from butyldimethylsilane ion, respectively. The present results show that the intensities of [(CH₃)_nSiH_3-n]⁺ and [M-CH₃]⁺, increase in the order of butylmethylsilane ^{butyldimethylsilane butyltrimethylsilane, and that of [(CH₃)_nSiH_3-nCH₂CH₂]+ increases in the reverse order. The EI mass spectra of the deuterium-labeled compounds show that [M-CH₃]+ is produced due to the cleavage of an Si-CH₃ bond. [M-H]+ in EI mass spectra of butylmethylsilane and butyldimethylsilane is produced due to the loss of a hydrogen atom connected to the Si atom. H₂ elimination from the butylmethylsilane ion occurs due to 1,4-H₂ elimination via a four-membered ring formation or 1,1-H₂ elimination due to a loss of two hydrogen atoms connected to the Si atom.}