Rate Constants and Kinetic Isotope Effects for Methoxy Radical Reacting with NO₂ and O₂

Relative rate studies were carried out to determine the temperature dependent rate constant ratio k₁/k_2a: CH₃O· + O₂ → HCHO + HO₂· and CH₃O· + NO₂ (+M) → CH₃ONO₂ (+M) over the temperature range 250–333 K in an environmental chamber at 700 Torr using Fourier transform infrared detection. Absolute rate constants k₂ were determined using laser flash photolysis/laser-induced fluorescence under the same conditions. The analogous experiments were carried out for the reactions of the perdeuterated methoxy radical (CD₃O·). Absolute rate constants k₂ were in excellent agreement with the recommendations of the JPL Data Evaluation panel. The combined data (i.e., k₁/k₂ and k₂) allow the determination of k₁ as 1.3_–0.5^+0.9 × 10^–14 exp[−(663 ± 144)/T] cm³ s^–1, corresponding to 1.4 × 10^–15 cm³ s^–1 at 298 K. The rate constant at 298 K is in excellent agreement with previous work, but the observed temperature dependence is less than was previously reported. The deuterium isotope effect, k_H/k_D, can be expressed in the Arrhenius form as k₁/k₃ = (1.7_–0.4^+0.5) exp((306 ± 70)/T). The deuterium isotope effect does not appear to be greatly influenced by tunneling, which is consistent with a previous theoretical work by Hu and Dibble. (Hu, H.; Dibble, T. S., J. Phys. Chem. A 2013, 117, 14230–14242.)