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 the 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.
The gas-phase methylenation reaction between CH(3)S(+)=CH(2) and alkylbenzenes, aniline, phenol and alkyl phenyl ethers, which yields [M + CH](+) and CH(3)SH, has been studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques and computational chemistry at the DFT level. The methylthiomethyl cation is less reactive than methoxymethyl and, unlike the latter, is unreactive toward benzene. The calculations suggest that reaction with toluene should proceed primarily by addition at the para and ortho positions resulting in a benzyl-type ion. Reaction with aniline-2,3,4,5,6-d(5) reveals that elimination of CH(3)SD is kinetically favored by a factor of 5 over elimination of CH(3)SH. Experiments with C(6)H(6)ND(2) and theoretical calculations suggest that methylenation at the nitrogen atom is energetically favorable and likely, but the observed results may reflect some H/D scrambling, which occurs after attack at a ring position. By comparison, reaction with phenol-2,3,4,5,6-d(5) reveals that methylenation followed by elimination of CH(3)SD is kinetically favored by a factor of 3.8 over elimination of CH(3)SH. For phenol, the theoretical calculations suggest that attack by CH(3)S(+)=CH(2) at the para or ortho position is the only low-energy pathway for methylenation. However, a low-energy pathway for hydrogen scrambling is predicted by the calculations originating from the exit complex, [CH(3)SH(...) CH(2)=C(6)H(4)=OH](+), of reaction at a ring position.
Gas-phase [C, H(3), S](+) ions obtained by electron impact from (CH(3))(2)S at 14 eV undergo two distinct low-pressure ion-molecule reactions with the parent neutral: proton transfer and charge exchange. The kinetics of these reactions studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques clearly suggests the [C, H(3), S](+) species to be a mixture of isomeric ions. While proton transfer is consistent with reagent ions displaying the CH(2)SH(+) connectivity, the observed charge exchange strongly argues for the presence of thiomethoxy cations, CH(3)S(+), predicted to be stable only in the triplet state. Charge exchange reactions are also observed in the reaction of these same [C, H(3), S](+) ions with benzene, toluene and phenetole. For these substrates, the CH(2)SH(+) ions can promote proton transfer and electrophilic methylene insertion in the aromatic ring with elimination of H(2)S. The results obtained for the different substrates suggest that the fraction of long-lived fraction of thiomethoxy cations obtained at 14 eV by electron ionization of dimethyl sulfide amounts to ~(22 -/+ 4)% of the [C, H(3), S](+) fragments.
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