High-Pressure Raman Studies on Aqueous Protonated Thiazole: Presence of Charge-Enhanced C−H···O Hydrogen Bonds

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Abstract
Close interactions of the charge-enhanced C−H···O type have been analyzed both experimentally and computationally in the protonated thiazole−water system. The formation of a weak hydrogen bond was directly evidenced by a low-frequency shift of the hydrogen-bonded aromatic C−H stretch in the protonated thiazole moiety. For pure thiazole, the pressure dependence of the C−H bands yielded blue frequency shifts. The peak frequency of the aromatic C−H stretch band of protonated thiazole in a dilute D2O solution possesses an unusual nonmonotonic pressure dependence, which indicates enhanced C−H···O hydrogen-bond formation at high pressure. We performed density functional theory calculations to predict the frequency shift of the C−H stretching vibrations. The reorganization of the hydrogen-bonded network may be one of the factors to induce the blue frequency shift to the red frequency shift.

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