Near-UV Absorption Spectrum of the Phenoxyl Radical and Kinetics of Its Reaction with CH3†.

Cavity ring-down spectroscopy combined with pulsed laser photolysis has been used to study the near-ultraviolet absorption spectrum (375−410 nm, 2̃2B1 ← X̃2B1 transition) of the phenoxyl radical (C6H5O•) in 10−20 Torr of nitrogen diluent at 298 K. By using a numerical fitting routine on the basis of a modeling of chemical reaction system, the absorption cross section of the phenoxyl radical was obtained, σ = (7.7 ± 2.3) × 10-18 cm2 molecule-1 (base e) at 394.4 nm. A spectrum simulation was carried out using available Franck−Condon integrals with a 400 cm-1 Lorentzian line width, which suggests a short-lived excited state. Time-dependent density functional theory (TD-UB3LYP/aug-cc-pVTZ) calculations supported the interpretation of the absorption band for the phenoxyl radical. The rate constant of the phenoxyl radicals with methyl radicals was derived, k(C6H5O + CH3) = (6.2 ± 2.6) × 10-11 cm3 molecule-1 s-1, at 298 K in 20 Torr of nitrogen diluent. [ABSTRACT FROM AUTHOR]