Proton-Transfer Mechanism in 2-Thioxoimidazolidin-4-one: A Competition between Keto/Enol and Thione/Thiol Tautomerism Reactions

The proton transfer in 2-thioxoimidazolidin-4-one (1,3-IM) via two keto/enol and thione/thiol intramolecular mechanisms was investigated by using DFT (B3LYP) and ab initio (MP2) methods in the gas phase. A conspicuous consistency was found between results of B3LYP and MP2 calculations. The change in structural parameters for direct keto/enol tautomerization is greater than thione/thiol transformation on going from ground state to transition state. At both level of calculations, the computed energy barrier for direct and water-assisted keto/enol tautomerization is higher than the corresponding thione/thiol. The energy barriers for direct proton-transfer tautomerization reactions are significantly greater than H2O-assisted tautomerization. The NBO results show that the change of electronic charge of hydrogens involved in migration in direct proton transfer is much greater than in the water-assisted mechanism.