Study of polarized IR spectra of the hydrogen bond system in crystals of styrylacetic acid

We have investigated the polarized IR spectra of the hydrogen bond system in crystals of trans-styrylacetic acid C6H5CHCHCH2COOH, and also in crystals of the following three deuterium isotopomers of the compound: C6H5CHCHCH2COOD, C6H5CHCHCD2COOH and C6H5CHCHCD2COOD. The spectra were measured at room temperature and at 77 K by a transmission method. The spectral studies were preceded by determination of the X-ray crystal structure. Theoretical analysis of the results concerned linear dichroic effects, the H/D isotopic and temperature effects, observed in the solid-state IR spectra of the hydrogen and of the deuterium bond, at the frequency ranges of the νOH and the νOD bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of the “strong-coupling” theory, when based on a hydrogen bond dimer model. This model sufficiently explained not only a two-branch structure of the νOH and the νOD bands, and temperature-induced evolution of the crystalline spectra, but also the linear dichroic effects observed in the band frequency ranges. A vibronic mechanism was analyzed, responsible for promotion of the symmetry-forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers. It was found to be of minor importance, when compared with analogous spectral properties of arylcarboxylic acid, or of cinnamic acid crystals. These effects were ascribed to a substantial weakening of electronic couplings between the hydrogen bonds of the associated carboxyl groups and the styryl radicals, associated with the separation of these groups in styrylacetic acid molecules by methylene groups in the molecules.