Polarized Infrared Spectra of the H(D) Bond in 2‐Thiophenic Acid Crystals: A Spectroscopic and Computational Study

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Abstract
Polarized IR spectra of the hydrogen bond in 2‐thiophenic acid crystals, isotopically neat and of mixed H/D isotopic content, are measured at 298 and 77 K in the “residual” νOH and νOD band frequency ranges. This crystalline system provides spectra in these band frequency ranges that differ considerably in intensity distribution from the spectra of other H‐bonded centrosymmetric dimeric species. This change in the spectral properties of the crystals is probably due to the influence of the sulfur atoms from the thiophene aromatic rings, which are directly linked to the (COOH)2 or (COOD)2 cycles. The magnitude of this effect correlates with the net electronic charge distribution at the 2‐ and 3‐positions of substituted thiophene rings, which in a different way influences the electron charge density in the hydrogen bonds of the two thiophenic acid isomers. The experimental results for spectral structures are compared to predictions obtained with theoretical calculations involving the combined effects of anharmonicities, Davydov coupling, Fermi resonances, and direct and indirect relaxations within the framework of the linear response theory. Numerical results show that mixing of all these effects allows satisfactory reproduction of the main features of the experimental IR line shapes of crystalline H‐ and D‐bonded 2‐thiophenic acid at room and liquid‐nitrogen temperatures.

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