JMSSJ On-line, Vol. 43 (1995) No. 1, pp. 53-63
Fragmentation Mechanisms of Butylmethylsilane, Butyldimethylsilane, and Butyltrimethylsilane Ions in EI Mass Spectrometry. 1. Experimental Approach
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    Takae TAKEUCHI,*a) Noriko MURATA,a) Yoshiko YOKOTA,a) Yoko KYOYA,a) Kaoru IWAI,a) and Masao YAMAMOTOa)

    *a) Department of Chemistry, Faculty of Science, Nara Women’s University, (Kitauoyanishi-machi, Nara 630, Japan)
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)nSiH3-n]+ (n=1-3), due to the cleavage of an Si-C bond and the formation of [(CH3)nSiH3-nCH2=CH2]+ due to the cleavage of a Cβ-Cγ 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)nSiH3-n]+ and [M - CH3]+, increase in the order of butylmethylsilane butyldimethylsilane butyltrimethylsilane, and that of [(CH3)nSiH3-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.
Key words: EI mass spectrum of butylmethylsilane, EI mass spectrum of butyldimethylsilane, EI mass spectrum of butyltrimethylsilane, Deuterium-labeling technique, Fragmentation mechanism, Si-C hyperconjugation, σ-π conjugation, Metastable decomposition, B/E linked scanning method
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