Effect of Mono- and Di-hydration on the Intramolecular Proton Transfers and Stability of Cyanuric Acid Isomers: A DFT Study

Abstract

Structural and thermodynamic properties of 10 isomers of cyanuric acid were studied in aqueous and gas phases, employing B3LYP/6-311 ++G(d,p) method. The aromaticities of these isomers were evaluated using nucleus-independent chemical shift (NICS) index. The calculations showed that as the number of the keto groups increases the stability of the isomers increases and the aromaticity decreases. Mono- and di-hydrations of the isomers did not change the stability trend, so that the tri-keto isomer was the most stable isomer among the hydrated and non-hydrated isomers. The activation energies (Ea) of the intramolecular proton transfers (tautomerisms) and energy barriers of H-rotations around its C-O axis in enolic isomers were calculated. The energy barriers were smaller than 45 kJ/mol for the H-rotations while the Ea values of the proton transfers were in the range of 130-210 kJ/mol. Effect of micro-hydrations on the transition state structures and the energy barriers of the tautomerisms were investigated. The mono- and di-hydrations lower the activation energies to 100-130 kJ/mol and 110-145 kJ/mol, respectively. Cyanuric acid (CA) is a triazine molecule which exists in both keto and enol forms. The activation energies of the keto↔enol tautomerisms in CA are about 130–210 kJ/mol. When the tautomerisms occur in the micro-hydrated isomers of CA, the water molecules catalyze the tautomerism and decrease the activation energies.