Assignment and vibrational analysis of the 600 nm absorption band in the phenoxyl radical and some of its derivatives

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Abstract
An experimental and theoretical study is reported of the 600 nm band system of phenoxyl and several methoxy substituted phenoxyls. These radicals, generated in freon, show a diffuse band in this region with a vibrational structure that is incompletely resolved but consistent with the 500 cm−1 "progression" observed earlier for phenoxyl in the vapour and in a rigid matrix. The low intensity of this band is considerably enhanced by ortho-methoxy substitution. To establish its assignment and analyze its structure, semi-empirical and ab initio quantum chemical calculations have been performed. It is found that, on the basis of the ordering of the states and the magnitude of their transition moments, all calculations favour a ππ* (2B2 → 2A2) rather than the earlier proposed nπ* assignment for this band. Of the methods used, only the (ab initio) CASSCF method is found to be able to simulate the observed vibrational structure adequately. This simulation is based on optimized structures and vibrational force fields calculated for the 1B2 and 1A2 states of phenoxyl. It identifies the dominant mode of about 500 cm−1 as ν6a the characteristic deformation mode of phenyl rings, but shows that higher members of the "progression" receive large intensity contributions from higher frequency modes. The chosen assignment and the assignments of higher-energy transitions reported in the literature are compared with those for the better known benzyl radical. It is shown that some of the phenoxyl assignments reported in the literature contain errors.

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