Matrix-isolation infrared spectra of 2-, 3- and 4-pyridinecarboxaldehyde before and after UV irradiation

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Three structural isomers of pyridinecarboxaldehydes (2-, 3- and 4-pyridinecarboxaldehyde) have been investigated in detail with matrix-isolation infrared spectroscopy in the 3000.600 cm.1 region, combined with the UV photoexcitation and the density-functional theory (DFT) calculations. Two rotamers (anti and syn) were identified for 2- and 3-pyridinecarboxaldehyde (2- and 3-PCA, respectively) upon the photoexcitation and most of the observed bands of each rotamer have been assigned. Both of the infrared data and the results of the DFT calculations strongly support that the syn rotamer exists as a less stable isomer for 2- and 3-PCA. It is shown that the formation of the intramolecular C.H...N hydrogen bond for the anti rotamer of 2-PCA results in the shortening of the aldehyde C.H bond length, while for the syn rotamer the C=O bond length is shortened due to the repulsion between the N and aldehyde O atoms. With 2-PCA, the photoinduced rotational isomerism and photolysis were observed simultaneously upon the UV irradiation. Keywords: 2-, 3- and 4-Pyridinecarboxaldehyde, Rotamer, Matrix-isolation infrared spectroscopy, UV photoexcitation, Energetics, Density-functional theory calculation

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