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
.