Vibrational spectroscopy of 2-pyridone and its clusters in supersonic jets: Structures of the clusters as revealed by characteristic shifts of the NH and C=O bands

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Abstract
Vibrational spectroscopy of the key functional vibrations of 2-pyridone and its hydrogen-bonded clusters with water, methanol, dioxane, dimethylether, as well as its dimer, has been carried out by using infrared-ultraviolet (IR–UV) and stimulated Raman–UV double resonance methods combined with fluorescence detection. The characteristicspectral changes upon the cluster formation have been observed for the NH and C=O stretching vibrations of the bare molecule and also for the OH stretching vibrations of the solvent molecules. The clusterstructures were investigated by comparing the observed spectra with the simulated ones of the energy-optimized structures obtained by ab-initio molecular orbital calculations. It was found that the “ring-type” hydrogen-bonded structure is appropriate for the cluster with water or methanol, while the “linear-type” hydrogen-bonded structure is appropriate for the cluster with dioxane or dimethylether. The symmetric form of 2-pyridone dimer was confirmed by the observed spectra, as well as the ab-initio calculation. A clear correlation between the observed frequency shifts of the NH stretching vibrations and the calculated NH⋯O hydrogen-bond angles was obtained indicating that the hydrogen-bond angle distortion reduces the local hydrogen-bond strength. Also it was found the blue shifts of the origin bands of the S 1 ←S 0 electronic transition strongly depends on the type of the clusterstructures.

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