Effects of Dynamical Couplings in IR Spectra of the Hydrogen Bond in N-Phenylacrylamide Crystals

This article presents the investigation results of the polarized IR spectra of the hydrogen bond in N-phenylacrylamide crystals measured in the frequency range of the proton and deuteron, νN−H and νN−D, stretching vibration bands. The basic spectral properties of the crystals were interpreted quantitatively in terms of the “strong-coupling” theory. The proposed model of the centrosymmetric dimer of hydrogen bonds facilitated the explanation of the well-developed, two-branch structure of the νN−H and νN−D bands as well as the isotopic dilution effects in the spectra. The vibronic mechanism of the generation of the long-wave branch of the νN−H band ascribed to the excitation of the totally symmetric proton vibration was elucidated. The complex fine structure pattern of νN−H and νN−D bands in N-phenylacrylamide spectra in comparison with the spectra of other secondary amide crystals (e.g., N-methylacetamide and acetanilide) can be accounted for in terms of the vibronic model for the forbidden transition breaking in the dimers. On the basis of the linear dichroic and temperature effects in the polarized IR spectra of N-phenylacrylamide crystals, the H/D isotopic “self-organization” effects were revealed.