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

The scattering distributions of state-selected CH3 products are measured for the O(D-1(2)) 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(D-1(2)) with CH4 and CD4, respectively. The insertion pathway of the O(D-1(2)) 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.