The effect of the solvent on the ground state free energy differences of three enol conformers of 2-(2‘-hydroxyphenyl)benzimidazole and its keto tautomer has been examined by means of Monte Carlo simulations and continuum model calculations. In agreement with the experimental data, calculations show that the trans enol and keto forms are stabilized by polar solvents, leading to a conformational and tautomeric equilibrium with the closed cis enol conformer in water, the only single species in apolar solvents. Monte Carlo simulations have also been used to examine the influence of the solvent on the absorption band of the closed cis enol structure and the fluorescence band of the keto form generated by photoinduced intramolecular proton transfer. In concordance with the experimental spectra, absorption and fluorescence band maxima for the closed cis enol and keto forms, respectively, are found to be blue-shifted with increasing polarity and hydrogen bonding capacity of the solvent.

The intramolecular proton transfer of 2-(2′-hydroxyvinyl)benzimidazole (HVBI) and 2-(2′-hydroxyphenyl)-imidazole (HPI) in the ground state and in the 1 ππ*, 1 nπ*, and 3 ππ* excited states has been studied at the HF/CIS/D95** level of theory. Their rotamerism reaction in the ground and 1 ππ* excited states has been also analyzed. These systems are two different fragments of 2-(2′-hydroxyphenyl)benzimidazole (HPBI), containing an intramolecular hydrogen bond through a common NCCCO backbone. The comparison of the calculations on HVBI and HPI with the experimental results available for HPBI and the theoretical calculations done for HPBI, salicyaldimine, and 1-amino-3-propenal allow us to determine the influence that each functional group of HPBI has on its intramolecular proton transfer in different electronic states. It is found that the aromaticity of the phenol ring of HPBI exerts a great influence on the proton transfer in the ground state and the lowest-lying 1 ππ*, and 3 ππ* excited states. The aromatic character of the phenol ring explains the higher stability of the enol form with respect to the keto form in the ground state, while a change in its aromaticity is responsible for the shift in the relative stability of the two tautomeric forms in the 1 ππ*, and 3 ππ* excited states. The presence of the imidazole moiety stabilizes the keto form in the 1 nπ* excited state, exerting a significant influence on the proton transfer in this state.

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