Studies of the reaction of O₂⁺ with deuterated methanes

In the gas phase O₂⁺ reacts with methane at 300 K to produce a hydrogen atom and the CH₃O₂⁺ ion. The structure of this ion has recently been determined to be H₂COOH ⁺, methylene hydroperoxide ion. The reaction rate coefficients and product distributions have now been measured at 300 K for the CH _nD_4-n isotopes. The reaction shows both inter- and intramolecular isotope effects, e.g., CH₂D₂ reacts more slowly than methane and more rapidly than CD₄, but loses hydrogen or deuterium with equal probability. The ion readily transfers HO⁺ to alkenes, CS₂, and many other neutral molecules. The reaction with CS₂ has been used to investigate the isotopic distribution within mixed isotope product ions. In addition, the reaction rate coefficients for both CH₄ and CD₄ have been measured as functions of temperature between 20 and 500 K; in both cases a clear minimum is observed in the reaction rate coefficient near room temperature. A mechanism for the reaction is proposed which allows us to model the temperature dependence of the reaction rate coefficient over the entire range for which there are data.