Keto-enol tautomerization of cyanuric acid in the gas phase and in water and methanol

The keto-enol tautomerism of cyanuric acid, both in the gas phase and in water and methanol, has been studied at the B3LYP/aug-cc-pvDZ level of theory in this paper. The harmonic frequencies of all the structures are calculated. The results show that the transition states of the tautomerism are four-membered ring conformations in gas phase, whereas six-membered ring conformations in water and methanol. In the first proton transfer, the activation energy ΔE# is 69.9 and 62.0 kJ/mol for water and methanol, respectively, which is much lower than that in gas phase (189.7 kJ/mol). Water and methanol molecules produce an important catalytic effect in the tautomerism, especially for methanol system. The AIM and NBO analyses show that there exists an H-bonding complex between the cyanuric acid and the water (methanol). By viewpoints of the reaction enthalpy and reaction rate, keto-enol tautomerism in water and methanol systems is easier than that in gas phase. The keto-enol tautomerisms are endothermic both in gas phase and in water and methanol, so the enol forms are less stable than the keto ones.