Deuterium isotope effects in the polyatomic reaction of O(1D2) + CH4 → OH + CH3

Abstract

The scattering distributions of state-selected CH₃ products are measured for the O(¹D₂) reaction with CH₄ using a crossed molecular beam ion imaging method at collision energies of 0.9–6.8 kcal mol⁻¹. The results are compared with the reaction with CD₄ to examine the isotope effects. The scattering distributions exhibit contributions from both the insertion and abstraction pathways, respectively, on the ground- and excited-state potential energy surfaces. Insertion is the main pathway, and it provides a strongly forward-enhanced angular distribution of methyl radicals. Abstraction is a minor pathway, causing backward scattering of methyl radicals with a discrete speed distribution. From the collision energy dependence of the abstraction/insertion ratio, the barrier height for the abstraction pathway is estimated to be 0.7 ± 0.3 and 0.8 ± 0.1 kcal mol⁻¹ for O(¹D₂) with CH₄ and CD₄, respectively. The insertion pathway of the O(¹D₂) reaction with CH₄ has a narrower angular width in the forward scattering and a larger insertion/abstraction ratio than the reaction with CD₄, which indicates that the insertion reaction with CH₄ has a larger cross section and a shorter reaction time than the reaction with CD₄. Additionally, while the insertion reaction with CD₄ exhibits strong angular dependence of the CD₃ speed distribution, CH₃ exhibits considerably smaller dependence. The result suggests that, although intramolecular vibrational redistribution (IVR) within the lifetime of the methanol intermediate is restrictive in both isotopomers, relatively more extensive IVR occurs in CD₃OD than CH₃OH, presumably due to the higher vibrational state density.