Theoretical Evaluation of Solvent Effects on the Conformational and Tautomeric Equilibria of 2-(2‘-Hydroxyphenyl)benzimidazole and on Its Absorption and Fluorescence Spectra

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.