Excited-State Intramolecular Proton Transfer and Rotamerism of 2-(2‘-hydroxyvinyl)benzimidazole and 2-(2‘-hydroxyphenyl)imidazole

Abstract

The intramolecular proton transfer of 2-(2‘-hydroxyvinyl)benzimidazole (HVBI) and 2-(2‘-hydroxyphenyl)imidazole (HPI) in the ground state and in the 1ππ*, 1nπ*, and 3ππ* excited states has been studied at the HF/CIS/D95** level of theory. Their rotamerism reaction in the ground and 1ππ* 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 1ππ*, and 3ππ* 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 1ππ*, and 3ππ* excited states. The presence of the imidazole moiety stabilizes the keto form in the 1nπ* excited state, exerting a significant influence on the proton transfer in this state.