Temperature-Dependent Branching Ratios of Deuterated Methoxy Radicals (CH₂DO•) Reacting With O₂

Abstract

The methoxy radical is an intermediate in the atmospheric oxidation of methane, and the branching ratio (<i>k</i>_1a/<i>k</i>_1b) (CH₂DO• + O₂ → CHDO + HO₂ (1a) and CH₂DO• + O₂ → CH₂O + DO₂ (1b)) strongly influences the HD/H₂ ratio in the atmosphere, which is widely used to investigate the global cycling of molecular hydrogen. By using the FT-IR smog chamber technique, we measured the yields of CH₂O and CHDO from the reaction at 250–333 K. Kinetic modeling was used to confirm the suppression of secondary chemistry. The resulting branching ratios are well fit by an Arrhenius expression: ln­(<i>k</i>_1a/<i>k</i>_1b) = (416 ± 152)/<i>T</i> + (0.52 ± 0.53), which agrees with the room-temperature results reported in the only previous study. The present results will be used to test our theoretical understanding of the role of tunneling in the methoxy + O₂ reaction, which is the prototype for the entire class of alkoxy + O₂ reactions