Theoretical Study of Dithioformic Acid, Dithiohydroxy Carbene and Their Radical Cations: Unimolecular and Assisted Rearrangements

The potential energy surfaces of dithioformic acid and its functional isomer, dithiohydroxy carbene, in both neutral and ionized states have been investigated using ab initio molecular orbital calculations at both (U)MP2 and (U)QCISD(T) levels with the 6-31G(d,p) and 6-311++G(d,p) basis sets. The unimolecular decomposition of HC( S)SH giving H 2 + CS 2 and H 2 S + C S, its rearrangement to HS−C−SH, and different simple bond cleavages have been examined. In both electronic states, the carbene is a stable isomer lying in a relatively deep potential well. Formation of H 2 S via a one-step decomposition of acid is found to be favored over that of CS 2 whose reaction path involves a carbene intermediate. Carbene radical cations also participate as crucial intermediates in unimolecular transformation of ionized acids. Calculations on different hydrogen-bonded dimers of acid and carbene suggest that, in the dimer form, while the carbene is not stable, elimination of both H 2 and H 2 S is accelerated. The role of H 2 S and H 2 as possible catalysts in the acid decomposition has also been investigated. H 2 S induces an efficient catalytic effect in different hydrogen transfer pathways. The standard heats of formation are estimated to be Δ H ° f,298 [HC( S)SH] = 113 ± 8 kJ/mol and Δ H ° f,298 [HS−C−SH] = 267 ± 12 kJ/mol based on CCSD(T)/6-311++G(3df,2p) calculations. Adiabatic ionization energies are predicted to be IE a (HS−C−SH) = 8.2 ± 0.3 eV and IE a (HC( S)SH)] = 9.0 ± 0.3 eV, and proton affinities are PA[HC( S)SH] = 802 ± 12 kJ/mol and PA[HS−C−SH] = 956 ± 12 kJ/mol.