Photoisomerization reactions of 4-methoxy- and 4-hydroxy-6-methyl-α-pyrones: An experimental matrix isolation and theoretical density functional theory study

Abstract

Photoisomerization reactions were studied for 4-methoxy- and 4-hydroxy-6-methyl-α-pyrones isolated in low-temperature Ar matrices. Two types of photoproducts: Dewar valence isomer and conjugated aldehyde–ketene were photogenerated upon UV irradiation of the matrix-isolated compounds. After prolonged irradiation practically all the initial material was converted to the corresponding Dewar isomers, whereas the population of aldehyde–ketene photoproducts was marginal. The structure of the main photoproducts was positively identified by comparison of their experimental IR spectra with the spectra theoretically predicted for the Dewar isomers. Good agreement between the experimental observations and the theoretical calculations allowed for reliable assignment of the infrared bands in the spectra of the initial forms of the studied compounds as well as in the spectra of the Dewar isomers. For the 4-hydroxy-6-methyl-α-pyrone the presence of its minor tautomeric form in a low-temperature matrix was detected.