The relative stabilities and probable proton transfer in isolated and monohydrated 2-hydroxyimidazole molecules have been studied in gas phase using ab initio (MP2/6-31＋G*) method. The results show that the keto tautomer is the relatively stable structure in isolated and monohydrated states. Monte Carlo simulation with free energy perturbation technique has been used to study the proton transfer reaction in water. The calculated free energy of activation of direct proton transfer reaction in water was found to be higher than that in gas phase by 1.71 kJ•mol^－1 though the free energy of tautomerism in water is lower than that in the gas phase by 5.55 kJ•mol^－1. The reduction of free energies of activation and tautomerism in the water-assisted proton transfer path was found to be significant due to the solvent effect of water. All the results show that the water-assisted proton transfer reaction of 2-hydroxyimidazole could occur easily in gas phase and water and they could be observed easily experimentally.

Key words: 2-hydroxyimidazole, tautomerism, proton transfer, ab initio calculation, Monte Carlo simulation