Strong vibrational exciton coupling effects in polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals |
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| Institution: | 1. Faculty of Foundry Engineering, AGH University of Science and Technology, 30-059 Krakow, Poland;2. Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, 30-059 Krakow, Poland;3. Department of Chemistry, Hankuk University of Foreign Studies, Yongin, Kyunggi-Do 449-791, Republic of Korea;1. Department of Physics, SCE⬝Shamoon College of Engineering, Beer-Sheva 84100, Israel;2. Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel;3. Department of Medicine, University of Virginia, Charlottesville, VA 22901, USA;4. Surgery B Department, Soroka University Medical Center, Faculty of Health Sciences, Ben-Gurion University, Beer-Sheva 84101, Israel;1. Department of Neurobiology, Institute of Pharmacology, Polish Academy of Sciences, Smętna 12, 31-343 Kraków, Poland;2. Faculty of Foundry Engineering, AGH University of Science and Technology, ul. Reymonta 23, 30-059 Kraków, Poland;3. Institute of Nuclear Physics, Polish Academy of Science, 31-342 Krakow, Poland;4. Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Kraków, Poland;5. Department of Chemistry, Hankuk University of Foreign Studies, Yongin, Kyunggi-Do, 449-791, South Korea |
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| 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 ND 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” νNH and νND 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” νNH and νND 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 NH stretching and the low-frequency νN⋯S hydrogen bond stretching vibrations. |
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