Thermokinetic Proton Transfer and Ab Initio Studies of the [2H,S,O]+ System. The Proton Affinity of HSO

The [2H,S,O] + ionic system has been studied jointly with a selected ion flow tube (SIFT) and by coupled cluster ab initio methods. Experimentally, the [2H,S,O] + ions were generated by the reaction of SO + (X 2 Π r ) with C 2 H 6, and reacted at room temperature with CH 3 SH, n -C 3 H 6, HCO 2 H, o -C 6 H 4 F 2, CF 3 C(O)CH 3, CH 2 O, H 2 S, c-C 6 H 12, CS 2, C 2 H 4, and SO 2 comprising a series of reference bases with gas basicities ranging from 177 to 154 kcal mol -1 . Thermokinetic analysis of the proton-transfer efficiencies in these reactions yields a free energy of proton detachment of 163.6 ± 2.2 kcal mol -1 for HSOH + . The results of the ab initio study are used to determine the relationship of this proton detachment energy to the various isomeric forms of [2H,S,O] + and the two isomers HSO and SOH. The resulting best values for the gas-phase basicity and the proton affinity of HSO at 298 K are 163.6 (+4.6, −2.2) kcal mol -1 and 171.0 (+4.6, −2.4) kcal mol -1, respectively. Observation of the charge-transfer behavior of HSOH + with the selected neutral bases restricts the recombination energy of HSOH + to the range 9.5 ± 0.2 eV. Rate coefficients and product distributions for the reactions of HSOH + with the bases in the bracketing study are presented and discussed, as well as the results of the ab initio study and the interpretation of the thermokinetic data. Additionally, a revised determination of the gas-phase basicity, proton affinity, and enthalpy of formation of HS 2 at 298 K (from the proton-transfer reactions of HSSH + ) is presented, which increases those quantities by 0.8 kcal mol -1 to 170.6 ± 2.2, 178.8 ± 2.4, and 25.8 ± 2.5 kcal mol -1, respectively.