Infrared spectra of benzoxazoles exhibiting excited state proton transfer

Infrared spectra of solid state 2-(2′-hydroxyphenyl)benzoxazole, 2,5-bis(2-benzoxazolyl)phenol and 2,5-bis(2-benzoxazolyl)hydroquinone were studied in the region 400–4000 cm−1. The measured spectra were compared with the results of density functional theory calculations of normal modes performed for different conformations of the isolated molecules. The calculated frequencies and intensities are in good agreement with the experimental values except the region 670–780 cm−1 which comprises OH torsion modes, CH out-of-plane bending vibrations and torsions of the aromatic and oxazole rings. The experimental infrared spectra can be well simulated by the spectral patterns of rotamers possessing OH⋯N intramolecular hydrogen bonds. In the case of 2,5-bis(2-benzoxazolyl)phenol a number of bands are assigned to the vibrations of the conformer formed by rotation of the non-hydrogen-bridged benzoxazole moiety.