Kinetic study of the oxidation reaction of 4‐methylphenyl radical.

A kinetic study of the reaction of the 4‐methylphenyl radical (4‐C6H4CH3) with the oxygen molecule was conducted using experimental and theoretical approaches. The absorption spectrum for the λ = 266 nm photolysis of the 4‐C6H4CH3X (X = Cl, Br)/N2/O2 mixture was measured in the wavelength range of λ = 503‐512 nm using N2 as the buffer gas at a total pressure of 40 Torr using a cavity ring‐down spectroscopy apparatus coupled with a pulsed laser photolysis system. Based on the absorbance of the product of the 4‐C6H4CH3 + O2 reaction at λ = 504 nm, the reaction rate coefficient for the 4‐C6H4CH3 + O2 reaction was determined to be k = (1.21 ± 0.10) × 10−11 cm3 molecule−1 s−1 and k = (1.18 ± 0.21) × 10−11 cm3 molecule−1 s−1 using 4‐C6H4CH3Cl and 4‐C6H4CH3Br, respectively, as the radical precursor. And there was no pressure dependence in the total pressure range of 10‐90 Torr varying partial pressure of N2 buffer gas at T = 296 ± 5 K. The geometries, vibration frequencies, and potential energy surfaces of the reactants, major products, and transition states in the 4‐C6H4CH3 + O2 reaction were determined using the CBS‐QB3 method. The k value at the high‐pressure limit was calculated to be 1.26 × 10−11 cm3 molecule−1 s−1 using the variational transition‐state theory. The calculated value of k was consistent with the experimental value, which indicated that the 4‐C6H4CH3 + O2 reaction reaches the high‐pressure limit at 10 Torr. Therefore, the oxidation of the 4‐C6H4CH3 radical is almost 10 times faster than that of the benzyl radical, which has the same chemical formula, at the high‐pressure limit. [ABSTRACT FROM AUTHOR]