Vibrations of the phenoxyl radical

Phenoxyl radical (C6H5O) was prepared photochemically in low-temperature ar gon matrices. The infrared absorption spectra were obtained for C6H5O and f or the isotopically labeled species C6D5O and 1-(CC5H5O)-C-13-C-12. All but one IR-active fundamental vibrations were detected, most of them not previ ously observed. Combination of results from IR linear dichroism measurement s on photooriented samples, determination of absolute IR intensities with t he help of internal standards, analysis of isotopic shifts, and quantum che mical predictions (B3LYP/cc-pVTZ) led to a detailed assignment of phenoxyl radical vibrations. Significant frequency shifts are observed with respect to previously reported data based on resonance Raman studies in polar solut ions. For some vibrations, these shifts reflect environment-induced structu ral changes, such as increase of the quinoid character of the phenoxyl radi cal in polar media. In particular, the frequency of the CO stretching vibra tion, readily observable in both IR and Raman experiments, is extremely sen sitive to the environment and can thus be used to probe its polarity.