Lattice vibrations and disorder in crystalline benzoxazoles undergoing excited state intramolecular proton transfer: DFTB modeling

Structure and crystal lattice vibrations are calculated for 2-(2'-hydroxyphenyl)bezoxazole and bis(2,5-benzoxazolyl)hydroquinone by density functional based tight-binding methods. Despite lowering of the molecular symmetry, structure parameters of the molecules in crystal and forms of the internal vibrations are similar to those of isolated molecules. Weak interaction between the molecules in the molecular crystals leads to appearance of the external vibrations, splitting and mixing of the vibrations of the isolated molecules into internal crystal vibrations. External and internal vibrations are not separated well; contribution of the translations and librations is noticeable in the region below 150 cm. The magnitude of the splitting does not exceed 4 cm for the most vibrations. The internal vibrations that correspond to the out-of plane molecular vibrations demonstrate larger molecule-to-crystal frequency shift than in-plane modes, mostly to higher frequencies, whereas the modes involving torsion motion of the OH bond are shifted toward lower frequencies. Mixing occurs for the molecular vibrations with frequencies that are different by less than 16 cm. Calculations performed for model molecular clusters show that the defect caused by different molecule orientation has lower energy than the defect related to the formation of rotamers.