Barriers of hydrogen abstraction from primary, secondary, and tertiary alkane sites by O(3P)

We present an extensive study of the barriers of hydrogen abstraction from primary, secondary, and tertiary sites of acyclic alkanes by ground-state oxygen atoms. Our studies include the characterization of the lowest-energy transition states of the O(3P) reactions with methane, ethane, propane, isobutane, and isopentane using high-level ab initio methods. The order of the calculated barriers heights is primary > secondary > tertiary, in agreement with the trends gleaned from kinetic measurements. Analysis of the transition-state geometry reveals a shift toward more reagents-like structures in the primary --> secondary --> tertiary sequence, which concurs with the expectation from Hammond's postulate. Using the ab initio data, we calculate thermal rate constants via transition-state theory. Our highest-level calculations indicate that the room-temperature relative reactivities of primary, secondary, and tertiary alkane sites in hydrogen-abstraction reactions by ground-state oxygen atoms are 1, 29, and 422, respectively. These results are used to interpret recent experiments on the reactions of O(3P) with liquid alkanes.