Effects of strong inter-hydrogen bond dynamical couplings in the polarized IR spectra of adipic acid crystals

This paper presents the results of the re-investigation of polarized IR spectra of adipic acid and of its d₂, d₈ and d₁₀ deuterium derivative crystals. The spectra were measured at 77 K by a transmission method using polarized light for two different crystalline faces. Theoretical analysis concerned linear dichroic effects and H/D isotopic effects observed in the spectra of the hydrogen and deuterium bonds in adipic acid crystals at the frequency ranges of the ν_O–H and the ν_O–D bands. The two-branch fine structure pattern of the ν_O–H and ν_O–D bands and the basic linear dichroic effects characterizing them were ascribed to the vibronic mechanism of vibrational dipole selection rule breaking for IR transitions in centrosymmetric hydrogen bond dimers. It was proved that for isotopically diluted crystalline samples of adipic acid, a non-random distribution of protons and deuterons occurs in the dimers (H/D isotopic “self-organization” effect). This effect results from the dynamical co-operative interactions involving the dimeric hydrogen bonds.