The Infrared Spectrum of the Matrix-Isolated Phenyl Radical

We have measured the infrared absorption spectrum of C₆H₅, X̃ ²A₁, in an Ar matrix at 10 K. The experimental frequencies (cm^-1) and polarizations follow, a₁ modes: 3086, 3072, 3037, 1581, 1441, 1154, 1027, 997, 976, 605; b₁ modes: 972, 874, 706, 657, 416; b₂ modes: 3071, 3060, 1624, 1432, 1321, 1283, 1159, 1063, and 587. Three different methods have been used for the production of the phenyl radicals. Infrared absorption spectra of five deuterated isotopomers, C₆D₅, p-C₆H₄D, p-C₆HD₄, o-C₆H₄D, and m-C₆H₄D, were recorded to compare experimental frequency shifts with calculated (UB3LYP/cc-pVDZ) harmonic frequency shifts. The use of CO₂ or NO as internal standards enabled the experimental determination of absolute infrared intensities. The linear dichroism was measured with photooriented samples to establish experimental polarizations of each vibrational band. True gas-phase vibrational frequencies were estimated by considering the gas-to-matrix shifts and matrix inhomogeneous line broadening. The phenyl radical matrix frequencies listed above are within ±1% of the gas-phase vibrational frequencies. The C₆H₅ frequencies from this paper supersede our earlier values reported in J. Am. Chem. Soc. 1996, 118, 7400-7401. See also: http://ellison.colorado.edu/phenyl.