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

Gas-phase [C2H5S](+) ions obtained by electron impact ionization from CH3SC 2H5 at 13 eV undergo three distinct low-pressure ion/molecule reactions wit h the parent neutral: proton transfer, charge transfer, and hydride abstrac tion. The kinetics of these reactions studied by FT-ICR techniques clearly suggests the [C2H5S](+) species to be a mixture of isomeric ions. While pro ton transfer and hydride abstraction are consistent with CH3CHSH+ and CH3SC H2+ reagent ions, the observed charge transfer strongly argues for the pres ence of thioethoxy cation, CH3CH2S+, predicted to be stable only in the tri plet 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 o f the CH3CH2S+ ions. Studies using CD3SC2H5 show that charge-transfer react ions are promoted by cations originating from a sulfur-methyl carbon bond c leavage. Ab initio calculations at several levels of theory predict that CH 3CH2S+ ions are only stable in the triplet state. Calculations along the fr agmentation pathway of the molecular ion reveal the tendency to generate tr iplet CH3CH2S+ ions upon cleavage of the sulfur-methyl carbon bond. Calcula tions were also carried out to determine the lifetime of triplet CH3CH2S+ u sing nonadiabatic RRKM theory. The exothermic or near thermoneutral spin-fo rbidden unimolecular isomerizations and dissociations were first characteri zed at different levels of theory, and the minimum energy crossing points ( MECPs) for all the channels were identified at the CCSD(T) level. The proba bility for surface hopping was then estimated from the spin-orbit matrix el ements. The calculated unimolecular dissociation rate constants predict tha t triplet CH3CH2S+ ions with less than 10 kcal mol(-1) of internal energy a nd at any level of rotational excitation should be long-lived, and strongly support the experimental observations.