Structure and electronic spectra of 2-phenylbenzimidazoles. Quantum chemical analysis of the intramolecular hydrogen bond

The electronic structure of the 2-(2'-hydroxyphenyl)benzimidazole (HPBA) and 2-(2'hydroxyphenyl)-N-methylbenzimidazole (HPMBA) with an intramolecular hydrogen bond (IHB) and 2-phenylbenzimidazoie (PBA) molecules were calculated in the ground and singlet excited states. The anomalous Stokes shift of HPBA 9800 cm -I (calculated 12,232 cm -l) is due to transport of a proton in the excited state with the formation of the ketone form HPBAK, which is accumulated in the ground state under the action of irradiation in the JV region. Under the same conditions a form of HPBA winhout an IHB, which fluoresces in the blue region, is accumulated. Two stable structures of HPBAK in the excited state were found by calculation planar and more stable perpendicular with rotation of the phenyl ring through 90 ~ . The experimental Stokes shift of PBA wihtout an IHB is 5300 cm -I (c~lculated 3897 cm-l).