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

Abstract

Cavity ring-down spectroscopy coupled with pulsed laser photolysis was used to study the visible absorption spectrum (490−535 nm, 2B1 ← 2A1 transition) of the phenyl radical, C6H5, 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, σphenyl = (3.6 ± 1.6) × 10-19 cm2 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 a1 and b1 vibrations were confirmed in the vibronic spectrum. Cavity ring-down spectroscopy was used to follow the loss of phenyl radicals and measure k(C6H5+Cl) = (1.2 ± 0.8) × 10-10, k(C6H5+Br) = (7.0 ± 4.0) × 10-11, and k(C6H5+Cl2) = (2.96 ± 0.53) × 10-11 at 298K, and k(C6H5+Cl2) = ( ) × 10-12 exp[(1000 ± 470)/T] cm3 molecule-1 s-1. Relative rate techniques were used to measure k(C6H5+Cl2)/k(C6H5+O2) = 2.1 ± 0.4 in 10−700 Torr of N2 diluent at 296K. Combining the absolute and relative rate data gives k(C6H5+O2) = (1.4 ± 0.4) × 10-11 cm3 molecule-1 s-1. In 1 atm of air C6H5 radicals have a lifetime of approximately 1.4 × 10-8 s with respect to reaction with O2 to give C6H5O2 radicals. Results are discussed with respect to the spectroscopy and reactivity of C6H5 radicals. Quoted uncertainties are 2 standard deviations from regression analyses.