Atmospheric chemistry of benzaldehyde: UV absorption spectrum and reaction kinetics and mechanisms of the C6H5C(O)O2 radical

Abstract

Flash photolysis–UV absorption and long pathlength FTIR–smog chamber studies of several reactions involving C₆H₅C(O) and C₆H₅C(O)O₂ radicals have been performed. It was determined that reaction of Cl atoms with C₆H₅CHO proceeds via abstraction of the aldehydic hydrogen to give benzoyl radicals. The sole atmospheric fate of benzoyl radicals is addition of O₂ to give peroxybenzoyl radicals. Reaction of C₆H₅C(O) radicals with molecular chlorine proceeds with a rate constant of (5.9±0.4)×10^-11 cm³ molecule^-1 s^-1 at 296 K and 1–700 Torr total pressure. The UV spectrum of C₆H₅C(O)O₂ radicals (245–300 nm) and the self reaction were investigated simultaneously, yielding σ_max=(2.0±0.1)×10^-17 cm² molecule^-1 at 245 nm and k₁₆=(3.1±1.4)×10^-13 exp[(1110±160) K/T] cm³ molecule^-1 s^-1, measured from 298 to 460 K. At 338 K, C₆H₅C(O)O₂ radicals react with NO with a rate constant of (1.6±0.4)×10^-11 cm³ molecule^-1 s^-1. At 296 K, C₆H₅C(O)O₂ radicals react with NO₂ with a rate constant of (1.1±0.3)×10^-11 cm³ molecule^-1 s^-1 to form C₆H₅C(O)O₂NO₂, which undergoes thermal decomposition at a rate of k_-4=(2.1_-1.5^+5.0)×10¹⁶ exp[-(13600±400)K/T] s^-1 in one atmosphere of air. At 296 K in 100–700 Torr of air k[C₆H₅C(O)O₂+NO]/k[C₆H₅C(O)₂+NO₂]=1.44±0.15. Relative rate methods were used to measure k[Cl+C₆H₅C(O)Cl]=(1.1±0.2)×10^-15 cm³ molecule^-1 s^-1 at 296 K. Uncertainty limits are all two standard deviations. Results are discussed with respect to the literature data and the atmospheric chemistry of benzaldehyde.