Cavity Ring-down Study of the Visible Absorption Spectrum of the Phenyl Radical and Kinetics of Its Reactions with Cl, Br, Cl₂, and O₂

Cavity ring-down spectroscopy coupled with pulsed laser photolysis was used to study the visible absorption spectrum (490−535 nm, 2 B 1 ← 2 A 1 transition) of the phenyl radical, C 6 H 5, in 10−50 Torr of argon diluent at 298 K. Absorption cross-sections were independent of total pressure over the range studied. At 504.8 nm, σ phenyl = (3.6 ± 1.6) × 10 -19 cm 2 molecule -1 (base e). Spectral simulation of the rotational structure of an origin band was performed using a model for a type C vibronic band. The vibronic spectrum was analyzed using normal-mode information from quantum chemical calculations employing hybrid density functional theory (B3LYP/aug-cc-pVDZ). The a 1 and b 1 vibrations were confirmed in the vibronic spectrum. Cavity ring-down spectroscopy was used to follow the loss of phenyl radicals and measure k (C 6 H 5 +Cl) = (1.2 ± 0.8) × 10 -10, k (C 6 H 5 +Br) = (7.0 ± 4.0) × 10 -11, and k (C 6 H 5 +Cl 2 ) = (2.96 ± 0.53) × 10 -11 at 298K, and k (C 6 H 5 +Cl 2 ) = ( ) × 10 -12 exp[(1000 ± 470)/T] cm 3 molecule -1 s -1 . Relative rate techniques were used to measure k (C 6 H 5 +Cl 2 )/ k (C 6 H 5 +O 2 ) = 2.1 ± 0.4 in 10−700 Torr of N 2 diluent at 296K. Combining the absolute and relative rate data gives k (C 6 H 5 +O 2 ) = (1.4 ± 0.4) × 10 -11 cm 3 molecule -1 s -1 . In 1 atm of air C 6 H 5 radicals have a lifetime of approximately 1.4 × 10 -8 s with respect to reaction with O 2 to give C 6 H 5 O 2 radicals. Results are discussed with respect to the spectroscopy and reactivity of C 6 H 5 radicals. Quoted uncertainties are 2 standard deviations from regression analyses.