Thiol-thione tautomerism in thioformic acid: Importance of specific solvent interactions

Attempts have been made to resolve a discrepancy in the recent literature concerning the thiol-thione tautomerism of monothioformic acid. Whereas Kato ct al. (J. Aln. Chem. Sec. 1996, 118, 1262) reported that the thione acid [RC(=S)OH] exists predominantly in polar solvents at very low temperatures, Jemmis et al. (J. Phys. Chem A 1997, 101, 7389) found, from ab initio calculations including continuum treatments of the solvent effect, that the thione for ms HC(=S)OH and CH3C(=S)OH are less predominant, irrespective of the solvent polarity. However, both groups of authors have suggested specific solvation as a possible reason for the discrepancy and called for further studies. Indeed, the present work points out that specific interactions between polar and aprotic solvent molecules such as dimethyl ether and tetrahydrofuran and H atoms of the carboxylic function, giving strong hydrogen bonded O-H...O complexes, are likely to tip the balance in favor of the thione acids. This fact is supported by calculations carried out using ab initio molecular orbital methods [HF, MP2, CCSD(T)] and density functional theory (B3LYP) with the 6-31G** and 6-311++G** basis sets. The results presented here show that the continuum solvent effect does not significantly modify the gas phase thermodynamic stability. In addition, the stability of the cyclic dimeric forms has been examined, suggesting the thiol form is still the most stable, although the greatest stabilization occurs in the thione form. Overall, hydrogen-bonded interactions in O-H...O complexes are stronger than in S-H...O complexes and thus play a crucial role in stabilizing the thione isomers, Specific solute-solvent interactions appear to play a key role in the thiol-thione equilibrium and need to be considered in similar cases.