Infrared spectra of the hydrogen bond in benzoic acid crystals: temperature and polarization effects

This paper deals with experimental studies and with quantitative interpretation of the polarized IR crystalline spectra of benzoic acid and of its deuterium isotopomers: C6H5COOD, C6D5COOH and C6D5COOD. The spectra were measured in the vO-H and in the vO-D band frequency ranges at temperatures of 298 and 77 K. The intensity distribution in the bands was quantitatively reproduced on the basis of the 'strong-coupling' model, when assuming that the isolated (COOH)2 and (COOD)2 cycles were the source of the spectral properties of the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. Another band shaping mechanism, i.e. a vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations in the centrosymmetric dimers of hydrogen bonds was also considered. A new kind of the 'long-range' isotopic effects H/D in the spectra was indicated, depending on the influence of the aromatic ring hydrogen atoms onto the vO-H band fine structure patterns. The role of the aromatic rings was also discussed, in order to explain mechanisms of extremely effective promotion of the forbidden transition, as well as of the Fermi resonance impact on the crystalline spectra.