Isomerization and fragmentation of aliphatic thioether radical cations in the gas phase: ion–neutral complexes in the reactions of metastable ethyl propyl thioether ions

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Abstract
The dominant unimolecular reactions of the molecular ion of ethyl propyl thioether are (i) loss of a methyl radical from the ethyl or the propyl group, (ii) loss of an ethyl radical from the propyl group and (iii) elimination of a propene molecule and an allyl radical by transfer of one or two hydrogen atoms, respectively, from the propyl group to the sulphur atom. The loss of a methyl radical on the µs timescale involves only the propyl entity and is preceded by an isomerization of this group to an isopropyl group. Partial loss of the positional identity of the hydrogen atoms of the propyl group occurs during the reactions of the metastable ions, but incorporation of hydrogen or carbon atoms from the ethyl group into the formed neutral species does not occur. The reactions of the metastable ions are discussed in terms of cleavage of a C–S bond assisted by a 1,2-hydride shift in the incipient carbenium ion leading to an ion–neutral complex of a thioethoxy radical and a secondary propyl carbenium ion. The ion–neutral complex can recombine to form the molecular ion of ethyl isopropyl thioether prior to methyl radical loss, or react by proton transfer to give a complex of CH3CH2SH+˙ and CH2 CH–CH3 which may dissociate or undergo hydrogen atom transfer followed by elimination of an allyl radical. The partial loss of positional identity of the hydrogen atoms during the decomposition of the metastable ions is mainly a result of reversible proton transfer between the constituents, which competes favourably with 1,2-hydride shifts within the carbenium ion entity of the complex.