Potential energy surfaces related to thioxy-hydroxy-carbene (HSνCνOH) and its radical cation

Abstract

Thioxy-hydroxy-carbene (HS–C–OH) and its radical cation have been generated in the gas phase upon dissociative ionization of ethyl thioformates and characterized by various MS-MS-MS experiments. In the present study, their energies and unimolecular rearrangements have further been determined with the aid of abinitio molecular orbital calculations. Potential energy surfaces for both neutral [CH₂OS] and ionized [CH₂OS]⁺ species constructed at the QCISD(T)/6-311++G(d,p)//(U)MP2/6-31(d,p) level confirm that in both states, the carbene form is kinetically stable. While HS–C–OH is 179 kJ mol^-1 less stable than the thiol acid HC(O)SH, [HS–C–OH]⁺ becomes even more stable than [HC(O)SH]⁺ and lies at 77 kJ mol^-1 higher in energy than the thion acid [HC(S)OH]⁺, the most stable ion isomer. While it is not involved in the unimolecular chemistry of neutral thioformic acids, carbene plays a key role in that of ionized isomers. Some thermochemical parameters of HS–C–OH are estimated as follows: heat of formation ΔH_f⁰=63 kJ mol^-1 at 0 K and 57 kJ mol^-1 at 298 K; ionization energy E_i=8.6 eV, and single–triplet gap ΔE_S-T=-156 kJ mol^-1 in favour of the singlet state. For thioformic acid, its heat of formation is evaluated to be ΔH_f,298⁰[HC(O)SH]=-124 kJ mol^-1 and proton affinity PA[HC(O)SH]=773 kJ mol^-1, with errors of ±10 kJ mol^-1.

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