UV-light induced conformational changes of 2-pyridinecarboxylic acid in low-temperature argon matrices

Conformational changes of 2-pyridinecarboxylic acid (2-picolinic acid) upon UV irradiation have been investigated by low-temperature matrix-isolation infrared spectroscopy. Only one conformer having an intramolecular hydrogen bond of COOH center dot center dot center dot N in the pyridine ring is found to exist in argon matrices after deposition before UV irradiation, in contrast to a reported feature in the CCl4 solution. The hydrogen bond is broken by UV irradiation, resulting in two less stable conformers; one increases immediately at the early irradiation stage while the other increases slowly during the prolonged irradiation time. The conformations around the OC-OH and C-COOH bonds of the photoproducts are identified by assignments of the IR bands measured in an argon matrix with an aid of the density-functional-theory calculation. The proposed photoreaction pathways of the conformational changes based on this observation are supported by a kinetic analysis of the IR absorbance changes with the reactants and the photoproducts against the irradiation time. (C) 2015 Elsevier B.V. All rights reserved.