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

Abstract

Relative rate studies were carried out to determine the temperature dependent rate constant ratio <i>k</i>₁/<i>k</i>_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 <i>k</i>₂ 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 <i>k</i>₂ were in excellent agreement with the recommendations of the JPL Data Evaluation panel. The combined data (i.e., <i>k</i>₁/<i>k</i>₂ and <i>k</i>₂) allow the determination of <i>k</i>₁ as 1.3_–0.5^+0.9 × 10^–14 exp[−(663 ± 144)/<i>T</i>] 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, <i>k</i>_H/<i>k</i>_D, can be expressed in the Arrhenius form as <i>k</i>₁/<i>k</i>₃ = (1.7_–0.4^+0.5) exp((306 ± 70)/<i>T</i>). 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., <i>J. Phys. Chem. A</i> <b>2013</b>, <i>117</i>, 14230–14242.