A systematic study of polarized IR spectra of the hydrogen bond in formic acid crystals

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In this paper, we present the results of our investigation of the polarized IR spectra of the hydrogen bond in formic acid crystals, as well as in crystals of the three deuterium derivatives of the compound: DCOOH, HCOOD, and DCOOD. These spectra were measured by a transmission method, using polarized light at 273 K and also at the temperature of liquid nitrogen for the “ bc” crystalline face. Our studies mainly focused on spectral effects corresponding to the intensity distribution, the influence of temperature, the linear dichroism, and isotopic substitution of deuterium in formic acid molecules measured in the frequency range of the proton and the deuterium stretching vibration bands, ν O–H and ν O–D, respectively. The basic spectral properties of the hydrogen bond IR spectra of formic acid crystals were interpreted in a quantitative way in terms of the “strong-coupling” theory. On the basis of this theoretical model, we succeeded in explaining the fine structure patterns of the ν O–H and ν O–D polarized sub-bands, the temperature, the dichroic, and the H/D isotopic effects in the spectra. The spectra were interpreted as a result of vibrational exciton interactions involving two adjacent hydrogen bonds in one chain of the associated molecules. The hydrogen bonds from one unit cell belonging to different chains were found uncoupled. In the spectra of isotopically diluted crystalline samples of HCOOH and DCOOH we found no characteristic changes in the “residual” ν O–H band shapes, which could be attributed to a random distribution of protons and deuterons in open chains of the hydrogen bonded molecules. This fact proves the existence of strong dynamical co-operative interactions in the hydrogen bond chain system in the formic acid crystal lattices responsible for a non-random distribution of protons and deuterons in the chains.