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

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 photo-excitation and density-functional theory (DFT) calculations. Two rotamers (anti and syn) for 2- and 3-pyridinecarboxaldehyde (2- and 3-PCA, respectively) and one rotamer for 4-pyridinecarboxaldehyde (4-PCA) were identified upon photo-excitation. Most of the observed bands of each rotamer have been assigned. Both the infrared data and the results of the DFT calculations agree that the syn rotamer is a less stable isomer for 2- and 3-PCA. Formation of an intramolecular CH⋯N hydrogen bond in the anti rotamer of 2-PCA results in a shortening of the aldehyde CH bond length. The CO bond length is shortened in the syn rotamer due to the repulsion between the N and aldehyde O atoms. With 2-PCA, both photoinduced rotational isomerism and photolysis were observed upon UV irradiation.