title: Barriers of hydrogen abstraction from primary, secondary, and tertiary alkane sites by O(3P).
abstract: 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.
authors.affiliation: Department of Chemistry, Virginia Tech, 107 Davidson Hall, Blacksburg, Virginia 24061-0212, USA.
authors.forename: Diego
authors.initials: D
authors.lastname: Troya