Strong vibrational exciton coupling effects in polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals

Abstract

This paper presents the results of investigation on polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals. The spectra were measured in the frequency range of the N H and N D bond stretching vibrations, for two different crystalline forms, having developed ab or bc crystal faces. The spectra exhibited extremely strong vibrational exciton coupling effects characterized by a large Davydov- splitting ( correlation field splitting), whose existence was confirmed by a strong difference between the polarized spectra of the two forms of 2-thiopyridone crystals. It was shown that extremely strong exciton interactions involving the translationally non-equivalent hydrogen bonds in the unit cell are responsible for these effects. Isotopic dilution in the crystals caused the disappearance of the spectral effects, ascribed to the inter-dimer exciton couplings, and the simultaneous retaining of the dimeric character of the “ residual” ν N H and ν N D bands. This spectral behavior of the isotopically diluted crystals was interpreted as the result of the dynamical co-operative interactions involving the hydrogen bonds in the lattice. These interactions lead to a non-random distribution of the protons and deuterons in the cyclic hydrogen bond dimeric systems and in consequence to the so-called H/D isotopic “ self- organization” effects in the crystal spectra. The fine structure of the “ residual” ν N H and ν N D bands is also influenced by such non-conventional spectral effects as the selection rule breaking for IR transitions, as well as the “ reversal” exciton coupling effect for centrosymmetric hydrogen bonded dimers. This statement is supported by model calculations of the analyzed band shapes. They are performed in terms of the “ strong- coupling” theory which assumes a strong anharmonic coupling involving different frequency hydrogen bond normal vibrations in the dimers, namely the high-frequency N H stretching and the low-frequency ν N⋯S hydrogen bond stretching vibrations.