Gas phase reactivity of cation radicals and protonated species from isomeric C4H8S thioethers

Four isomeric thioethers, 2,3-dimethylthiirane ( 1 ), 2-methylthietane ( 2 ), tetrahydrothiophene ( 3 ), and allyl methyl thioether ( 4 ), have been subjected to mass spectrometric analysis in the gas phase, under electron impact (El) and chemical ionization (CI) conditions. The metastable molecular ions M⁺′ generated from 1-4 under EI (70 eV) conditions give distinct patterns of unimolecular fragmentation, thus indicating that isomer interconversion reactions are slower than dissociation (a possible exception, to some extent, is the case of [M₂]⁺′ and [M₂]⁺′). The change of the relative intensities of some prominent peaks with increasing ion lifetime (decomposition within the ion source, the first, and the second field-free regions of the mass spectrometer) is pointed out. Metastable [MH]⁺ ions, generated from 1-4 in chemical ionization experiments with CH₄, all eliminate H₂ and H₂S, although in different relative proportions. In addition to these processes protonated 4 also undergoes loss of C₂H₄ and C₃H₆, likely from a C-protonated structure.