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/k2a: 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, kH/kD, 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.)