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The molecular orbital (MP2) and density functional theory calculations (B3LYP) using 6-311??G(2d,2p) basis set have been carried out on adducts of the cis and trans conformers of two tautomeric forms of thioformic acid (TFA) with water. Thirteen adducts of TFA with water have been optimized. Similar calculations have been carried out on adducts of formic acid with water and seven energy minima have been obtained for them. Our findings indicate that the specific interactions with water play an important role in the conformational stability as thiol form of TFA is the most stable form in gas phase, while it is the thione form which is the most stable in 1:1 adduct. The H-bond acceptor ability of S and O at the thiocarbonyl and carbonyl positions in TFA and FA, respectively, has been compared and observed to be only slightly lower in the former. However, the H-bond donor ability of S-H has been observed to be nearly half to that of O-H. The contributors to stabilization energies of adducts are explored by analyzing geometrical variations, atomic charges, and electron delocalizations.

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