Temperature, H/D isotopic and Davydov-splitting effects in the polarized IR spectra of hydrogen bond chain systems: 1,2,4-Triazole and 3-methyl-2-oxindole crystals

•The polarized IR spectra of 124Tzl and 3MetOx were quantitatively analyzed.•The in- and inter-chain exciton coupling mechanisms were analyzed.•We found and explained temperature-dependent Davydov-splitting effects.•The H/D isotopic self-organization effects were identified.

The spectral properties of two different hydrogen-bonded crystalline systems, 1,2,4-triazole and 3-methyl-2-oxindole, containing molecular chains in their lattices, were examined by polarized IR spectroscopy, aided by the calculations utilizing the “strong-coupling” model. The experimental and theoretical approaches have shown that the individual crystal spectral properties in IR remain in a close relation with the electronic structure of the individual molecular systems. A vibronic coupling mechanism involving the hydrogen bond protons and the electrons occupying the π-electronic orbitals in the molecules determines the way in which the vibrational exciton coupling between the hydrogen bonds in the crystals occurs. For the associating systems, which molecules contain large delocalized π-electronic systems coupled directly with H-bonds, strong exciton interactions involving the vibrationally excited hydrogen bonds in the chains prefer a “tail-to-head”–type Davydov-coupling widespread via the π-electrons. A weak through-space exciton coupling involves two closely-spaced hydrogen bonds belonging to two different adjacent chains in the case, when large π-electronic systems in the molecules are absent. The relative contribution of each exciton coupling mechanism in the chain system spectra generation is temperature-dependent. The two competing individual Davydov-coupling mechanism are responsible for the appearance in the polarized spectra of temperature-dependent Davydov-splitting effects differentiating the spectral properties of the two crystalline systems. The H/D isotopic ‘‘self-organization’’ phenomenon, depending on a non-random distribution of protons and deuterons in the crystal hydrogen bridges was also analyzed.

Graphical abstractThis paper presents the results of experimental and theoretical study of the polarized IR spectra of two systems with hydrogen bond molecular chains in the crystal lattices, and of theirs deuterium derivative crystals.Figure optionsDownload full-size imageDownload as PowerPoint slide