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 + CS, 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 hydrogen-bonded 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[HS−C−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.