Abstract
Density functional theory (DFT) calculations are applied to optimise the structure of three possible tautomers of some 5H-1,3-oxazine-4,6-diones and 5H-1,3-thiazine-4,6-diones and their transition states using B3LYP/6-311++G** level of theory. Then, important molecular parameters, IR frequencies and energetic parameters are calculated. In all cases, tautomer 2 is about 0.7–8.7 kcal/mol more stable than tautomer 1 and about 10.4–14.4 kcal/mol more stable than tautomer 3. Our results confirm the available experimental data approving the higher stability of tautomer 2. Furthermore, barrier energies and reaction rate constants of the hydrogen exchange between each pair of tautomers in the presence of one, two and three water molecules are calculated. The computed activation barriers in the presence of water molecules are about 23.8–76.2 kcal/mol lower than those in the gas phase.
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