An attempt to detect thiirenium ions in the gas phase

Abstract

C_nH_2n–1S⁺ ions (n3–5,7,11) were generated in the gas phase from several CH₃SCR¹= CR²X precursors (R¹,R²= H, alkyl; X = Cl, Br)via electron-impact-induced X˙ loss. Their reactions, unimolecular and collision induced, were studied by ion kinetic energy spectrometry. The major fragementations are identified and the thermochemistry considered. Of particualr interest were unimolecular alkene elimination reactions, observed for n= 3–5, which lead to product ions of composition R¹(R²)CS⁺. C₄H₇S⁺ ions undergo elimination of C₂H₄ and C₃H₆ to give C₂H₃S⁺ and HCS⁺, respectively. Metastable peak shape analysis, analysis of model ions, and deuterium labelling experiments show that the two reactions occur via two different mechanisms. A general scheme is proposed for unimolecular decomposition of C_nH_2n–1S⁺ ions. Thiirenium ions could not be detected nor need be invoked as intermediates in the extensive rearrangements that precede some of the major reactions.