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 isome r, dithiohydroxy carbene, in both neutral and ionized states have been inve stigated 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 u nimolecular decomposition of HC(=S)SH giving H-2 + CS2 and H2S + C=S, its r earrangement to HS-C-SH, and different simple bond cleavages have been exam ined. In both electronic states, the carbene is a stable isomer lying in a relatively deep potential well. Formation of H2S via a one-step decompositi on of acid is found to be favored over that of CS2 whose reaction path invo lves a carbene intermediate. Carbene radical cations also participate as cr ucial intermediates in unimolecular transformation of ionized acids. Calcul ations 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 H2S is accelerated. The role of H2S and H-2 as possible catalysts in the acid decomposition has also been investigated. H2S induces an efficien t catalytic effect in different hydrogen transfer pathways. The standard he ats of formation are estimated to be Delta H(f,298)degrees[HC(=S)SH] = 113 +/- 8 kJ/mol and Delta H(f,298)degrees[HS-C-SH] = 267 +/- 12 kJ/mol based o n 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[H S-C-SH] = 956 +/- 12 kJ/mol.