Theoretical study of intermolecular proton transfer reaction in isolated 5-hydroxyisoxazole-water complexes

A systematic investigation in isolated 5-hydroxyisoxazole-water complexes (5-HIO · (H2O)nn = 1-3) is performed at the DFT level, employing B3LYP/6-31G(d, p) basis set. Single-point energy calculations are also performed at the MP2 level using B3LYP/6-31G(d, p) optimized geometries and the 6-311++G(d, p) basis set. The computational results show that the keto tautomer K2 is the most stable isomer in the gas phase, and the tautomer K1 to be the next most stable tautomer. Hydrogen bonding between HIO and the water molecule(s) will dramatically lower the barrier by a concerted multiple proton transfer mechanism. The proton transfer process of 3WEcis ↔ 3WK1 and 2WEtrans ↔ 2WK2 is found to be more efficient in two tautomerization, and the barrier heights are 7.03 and 14.15 kcal/mol at B3LYP/6-31G(d, p) level, respectively. However, the proton transfer reaction between Ecis and K1 cannot happen without solvent-assisted.