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 H2 + CS2 and H2S + 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 H2S via a one-step decomposition of acid is found to be favored over that of CS2 whose reaction path involves a carbene intermediate. Carbene radical cations also participate as crucial intermediates in unimolecular transformation of ionized acids. Calculations on different hydrogenbonded dimers of acid and carbene suggest that, in the dimer form, while the carbene is not stable, elimination of both H2 and H2S is accelerated. The role of H2S and H2 as possible catalysts in the acid decomposition has also been investigated. H2S induces an efficient catalytic effect in different hydrogen transfer pathways. The standard heats of formation are estimated to be ΔH°f,298[HC(=S)SH] = 113 ± 8 kJ/mol and ΔH°f,298[HSC-SH] = 267 ± 12 kJ/mol based on CCSD(T)/6-311++G(3df,2p) calculations. Adiabatic ionization energies are predicted to be IEa(HS-C-SH) = 8.2 ± 0.3 eV and IEa(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.
