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.

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