Abstract
Polarized IR spectra of 2-furanacetic acid and of 2-furanacrylic acid crystals were measured at 293 K and 77 K in the vO−H and vO−H band frequency ranges. The corresponding spectra of the two individual systems strongly differ, one from the other, by the corresponding band shapes as well as by the temperature effect characterizing the bands. The crystal spectral properties remain in a close relation with the electronic structure of the two different molecular systems. We show that a vibronic coupling mechanism involving the hydrogen bond protons and the electrons on the π-electronic systems in the molecules determines the way in which the vibrational exciton coupling between the hydrogen bonds in the carboxylic acid dimers occurs. A strong coupling in 2-furanacrylic acid dimers prefers a “tail-to-head-” type Davydov coupling widespread by the π-electrons. A weak through-space coupling in 2-furanacetic acid dimers is responsible for a “side-to-side-” type coupling. The relative contribution of each exciton coupling mechanism in the dimer spectra generation is temperature and the molecular electronic structure dependent. This explains the observed difference in the temperature-induced evolution of the compared spectra.
Highlights
Infrared spectroscopy still constitutes a basic tool in the research of the hydrogen bond dynamics
Complex fine structure patterns of these bands are considered as the result of anharmonical coupling mechanisms involving the proton stretching vibrations and other normal vibrations occurring in associated molecular systems, mainly the low-frequency X· · · Y hydrogen bridge stretching vibrational motions [1,2,3,4,5]
By measuring of polarized IR spectra of spatially oriented molecular crystals, characterized by a rich diversity of hydrogen bond arrangements met in their lattices, the most complete information has been be obtained about the coupling mechanisms involving hydrogen bonds in these systems
Summary
Infrared spectroscopy still constitutes a basic tool in the research of the hydrogen bond dynamics. By measuring of polarized IR spectra of spatially oriented molecular crystals, characterized by a rich diversity of hydrogen bond arrangements met in their lattices, the most complete information has been be obtained about the coupling mechanisms involving hydrogen bonds in these systems It appeared that the investigation of spectra of even so simple mutually interacting hydrogen bond aggregates like cyclic dimers (e.g., carboxylic acid dimers) allowed to reveal new H/D isotopic effects, namely, the H/D isotopic self-organization effects. The investigation results presented constitute a part of results obtained in the frames of a wider project, which assumed measuring of crystalline spectra of other carboxylic acids, mainly of furan and thiophene derivatives Our choice of these model molecular systems was strongly supported by advantageous well-developed νO−H and νO−D band contour shapes in the IR spectra of these systems. The understanding of the temperature effects and the generation mechanism of the intensity distribution patterns in the νO−H and νO−D bands in the spectra of diverse carboxylic acid crystals are of the particular interest and importance in this project
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