Intrahydrogen bonding and transition states between enol and enethiol tautomers in β-thioxoketones

Problem statement: Hydrogen bonding is one of the most important concepts in chemistry because it is crucial to understand many different interactions both in gas phase and in condensed media. A particular subset is presented by the intramolecular hydrogen bonds where two ends of the same molecule interact, resulting in a ringlike structure. Approach: The study involved theoretical investigation of proton transfer and transition states in β-thioxoketones. Both structures of the enol and enethiol tautomers of β-thioxoketones were geometrically optimized then the transition states were obtained. The calculations were done at the B3LYP hybrid and 6-31G(d,p), 6-31+G(d,p), 6- 31++G(d,p), 6-311G(d,p), 6-311+G(d,p) and 6-311++G(d,p) basis sets. Results: The calculated energies of the enethiol tautomer are higher than those of the enol tautomer for all the thioxoketones studied in this work. The S...O distances are in the order: enethiol > enol > transition state in addition the hydrogen bonding in the enol tautomer is stronger than that in the enethiol tautomer. Conclusion: The interconversion within the tautomeric mixture in these compounds is preferable in the enethiol → enol direction.