Thiol−Thione Tautomerism in Thioformic Acid:  Importance of Specific Solvent Interactions

Abstract

Attempts have been made to resolve a discrepancy in the recent literature concerning the thiol−thione tautomerism of monothioformic acid. Whereas Kato et al. (J. Am. Chem. Soc. 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 forms 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.

Substituent effects on the tautomerism of monochalcogenocarboxylic acids XC(O)YH (X=H, F, NH2, OH, CN, and CH3; Y=S, Se, and Te): A theoretical study

“…At all levels of theories, the calculated free energies show a thermodynamic preference of the keto isomer over the enol isomer, regardless of the electronic nature of the substituents and the acid derivatives. These results are consistent with the previous experimental and theoretical observations [6,7].…”

“…By self-consistent reaction filed (SCRF) calculations; they found that the equilibrium of these intramolecular reactions was only slightly influenced by the solvents of various dielectric constants. Nguyen and coworkers found that the tautomeric barrier of thioformic acid HC(@O)SH in the CH 3 OCH 3 is increased by about 1.5 kcal/mol relative to the reactant complex [7]. Interestingly, the work of Ren et al showed that the tautomerism involving CH 3 OCH 3 not only gives the preference of the enol form in the tautomeric equilibrium, but also significantly lowers the tautomeric barriers by >8 kcal/mol in THF solution [8].…”

Experimental and theoretical investigation of 5-para-nitro-benzylidene-thiazolidine-2-thione-4-one molecule

“…In liquid phase, the two tautomeric forms (thione and thiol) can convert into each other by a direct proton transfer mechanism, a similar case with that reported by Delaere and coworkers [56]. However, a solvent assisted proton transfer reaction [58] is also plausible due to the fact that NH donor and C@S acceptor H bonding sites are available for stable complexes formation which are predicted to greatly decrease the energy barrier height [48].…”

“…The thione-thiol tautomeric equilibrium has a significant importance in biochemistry and it has attracted both, experimental and theoretical interest, different tautomers in solid and liquid phase being commonly reported [19,[46][47][48][49][50][51][52][53][54][55][56][57]. For this compound, the presence of thiol tautomers in both syn and anti conformers can be supposed because the barrier to rotation amounts to 0.96 kcal/mol at B3LYP/6-31G(d) level of theory, in gas-phase.…”

The hydrogen bond donor and acceptor ability of thioformic acid

“…H 2 S complex with phenolic OH group as H-bond donor and sulfur as acceptor to be half as strong as p-CR.H 2 O complex. Delaere et al [37] studied interactions of thiol and thione forms of TFA with polar aprotic solvent dimethyl ether by employing ab initio MO and B3LYP calculations using 6-31G** and 6-311? ?G** basis sets.…”