Photoreaction mechanisms of 2-bromophenols studied by low-temperature matrix-isolation infrared spectroscopy and density-functional-theory calculation

Photoreaction mechanisms of 2-bromo, 2-bromo-4-chloro and 2,4-dibromophenols in low-temperature argon matrices have been investigated by Fourier transform infrared spectroscopy. Density-functional-theory calculations are performed to assign the observed infrared bands of photoproducts, where a 6-311+G(d,p) basis set is used to optimize geometrical structures. It is concluded that 4-bromo-2,5-cyclohexadienone is mainly produced from 2-bromophenol, while formation of cyclopentadienylidenemethanone⋯HBr complex is a minor pathway, in contrast to the case of 2-chlorophenols. The branching ratio between the two reaction pathways is estimated from the absorbance changes of the infrared bands to be 5.4±0.3.