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

Attempts have been made to resolve a discrepancy in the recent literature c oncerning 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 calc ulations including continuum treatments of the solvent effect, that the thi one for ms HC(=S)OH and CH3C(=S)OH are less predominant, irrespective of th e 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 bet ween polar and aprotic solvent molecules such as dimethyl ether and tetrahy drofuran and H atoms of the carboxylic function, giving strong hydrogen bon ded 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 h ere show that the continuum solvent effect does not significantly modify th e gas phase thermodynamic stability. In addition, the stability of the cycl ic 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 th an in S-H...O complexes and thus play a crucial role in stabilizing the thi one isomers, Specific solute-solvent interactions appear to play a key role in the thiol-thione equilibrium and need to be considered in similar cases .