Experimental and theoretical characterization of long-lived triplet state CH3CH2S+ cations

Gas-phase [C2H5S](+) ions obtained by electron impact ionization from CH3SC2H5 at 13 eV undergo three distinct low-pressure ion/molecule reactions with the parent neutral: proton transfer, charge transfer, and hydride abstraction. The kinetics of these reactions studied by FT-ICR techniques clearly suggests the [C2H5S](+) species to be a mixture of isomeric ions. While proton transfer and hydride abstraction are consistent with CH3CHSH+ and CH3SCH2+ reagent ions, the observed charge transfer strongly argues for the presence of thioethoxy cation, CH3CH2S+, predicted to be stable only in the triplet state. Charge transfer reactions only occur with substrates having an IE below 8.8 eV and thus yield an upper limit for he recombination energy of the CH3CH2S+ ions. Studies using CD3SC2H5 show that charge-transfer reactions are promoted by cations originating from a sulfur-methyl carbon bond cleavage. Ab initio calculations at several levels of theory predict that CH3CH2S+ ions are only stable in the triplet state. Calculations along the fragmentation pathway of the molecular ion reveal the tendency to generate triplet CH3CH2S+ ions upon cleavage of the sulfur-methyl carbon bond. Calculations were also carried out to determine the lifetime of triplet CH3CH2S+ using nonadiabatic RRKM theory. The exothermic or near thermoneutral spin-forbidden unimolecular isomerizations and dissociations were first characterized at different levels of theory, and the minimum energy crossing points (MECPs) for all the channels were identified at the CCSD(T) level. The probability for surface hopping was then estimated from the spin-orbit matrix elements. The calculated unimolecular dissociation rate constants predict that triplet CH3CH2S+ ions with less than 10 kcal mol(-1) of internal energy and at any level of rotational excitation should be long-lived, and strongly support the experimental observations.