Crystal structure and UV spectra of a 1,2-disubstituted benzimidazolium chloride.

1-(2-Hy-droxy-benz-yl)-2-(2-hy-droxy-phen-yl)-1H-benzimidazol-3-ium chloride, C20H17N2O2(+)·Cl(-), was prepared by reaction of salicyl-aldehyde with o-phenyl-enedi-amine in the presence of tri-methyl-silyl chloride acting as a source of HCl. As a result of steric hindrance, the cation in the crystal is far from planar: the benzimidazole ring system makes dihedral angles of 55.49 (9) and 81.36 (8)° with the planes of the phenolic groups. The crystal packing is dominated by O-H⋯Cl and N-H⋯Cl hydrogen bonds, which link the cations and anions into four-membered rings and then into chains along [100]. The title compound exhibits two transitions in the UV region, which are revealed in the solid state and solution spectra as an absorption maximum at 280 nm and a shoulder at 320 nm. According to the results of TD-DFT calculation, both transitions have a π-π* nature and the mol-ecular orbitals involved in these transitions are mostly localized on the benzimidazole ring system and on the phenyl ring attached to it at the 2-position.