The effect of strong coupling between vibrations in hydrogen bonds on the polarized spectra of the mercaptobenzothiazole crystal: an “anomalous” isotopic effect

Abstract

This paper attempts to give a theoretical interpretation of the IR spectra of hydrogen bonds in mercaptobenzothiazole crystals. A model composed of a simple hydrogen-bonded mercaptobenzothiazole dimer is considered. It is shown that the main spectral properties of a mercaptobenzothiazole crystal and the unexpected isotopic effects in the spectra can be reproduced as resulting from specific counting between the low- and the high-frequency hydrogen bond vibrations in the dimer. It is suggested that such a coupling arises from the dependence of the resonance interaction energy in the dimer on the totally symmetric normal coordinate describing the low-frequency, hydrogen bond stretching vibration of the dimer. Analysis of the ν N—H bandshape for mercaptobenzothiazole using the proposed model allow us to assume that some effects observed in the spectra result in a deviation from the vibrational selection rules for centrosymmetric dimers, which influence the intensity distribution in the band.