The chemical dynamics of the reactions of O(3P) with saturated hydrocarbons. I. Experiment

Abstract

The chemical dynamics of the reactions of O(3 P) with saturated hydrocarbons. II. Theoretical model Molecular beam-laser induced fluorescence experiments have probed the nascent internal state distributions and excitation functions of OH formed in the technologically important reactions OCP) + RH---+OH + R·. RH is a saturated hydrocarbon and R· is an alkyl radical. A variety of RH have been investigated corresponding to abstraction of primary, secondary, and tertiary hydrogens. The OH rotational state distribution is nearly identical for all RH and decrease rapidly from its peak at the lowest rotational level. This demonstrates that reaction occurs when OCP) is collinear to a C-H bond in the hydrocarbons. The vibrational state distribution of OH depends markedly on the type of hydrogen abstracted, with vibrational excitation increasing dramatically across the series primary to secondary to tertiary. This is interpreted as a shift from a repulsive towards a more attractive surface across the series. Partitioning into the OH spin doublets shows that these reactions are midway between the adiabatic and diabatic limits with respect to the spin-orbit surfaces. Excitation functions measure the dynamic thresholds, and are in good agreement with activation energies obtained from thermal rate constants. The excitation functions for v = I OH exhibit a sharp decrease at energies considerably above threshold. This suggests that excitation of internal modes of R· occurs only at high collisions energies. All of these results are interpreted qualitatively in terms of a simple, but general, triatomic model for the interaction of Oep) with RH, i.e., where R· can be considered a structureless particle.

Dynamics of OH Production in the Reaction of O(1D2) with Cyclopropane

“…Thus, for the reactions examined here, there was no significant difference between the populations of the fine structure levels in υ" = 1, and there was a slight difference favoring the production of the lower level ( 2 П 3/2 ) in υ" = 0. This observation is in contrast to the previously reported pronounced preferential population of the lower-lying f 1 state (about two times larger population) of the OH product produced in the reaction of ground state atomic oxygen (O( 3 P J )) with hydrocarbons, 16 a reaction that is known to proceed via a direct abstraction mechanism.…”

The dynamics of the reaction of O(1D) with HBr studied by crossed molecular beams and time-resolved Fourier transform spectroscopy

“…The presence of translationally hot o(~P), however, may not be important to the results reported here, for several reasons: first, it is a minor constituent (~1 0 % ) of the photodissociation products. Second, the reactive cross sections of processes such as this decrease at the extremely high relative translational energy of the o(~P) atom (33,34) and finally, increased reagent translational energy causes reduced product vibrational excitation in reactions such as this (35). This latter effect, in the case of the reaction reported here, causes the OH from the o(~P) reaction to be created predominantly in the lowest vibrational levels.…”

A Theoretical Investigation of the Gas‐Phase Oxidation Reaction of the Saturated tert‐Butyl Radical

“…The transfer mechanism involves a collinear transition state, which has been commonly observed in the reactions of O( 3 P) with saturated, closed-shell hydrocarbon molecules. [11][12][13][14] The entrance barrier is estimated to be negligible, due to both the highly attractive character and the unusually small CÀH bond dissociation energy (only 35.5 kcal mol À1 ) of t-C 4 H 9 .…”

“…Such purely dynamic features have not been observed in the oxidation reactions with closed-shell hydrocarbon molecules. [11][12][13][14] In this work, we describe an ab initio investigation of the oxidation reaction of O( 3 P) with the saturated tert-butyl hydrocarbon radical (t-C 4 H 9 ), which was mainly motivated by our recent progress in a related gas-phase crossed-beam study. [15][16][17][18] The organic tert-butyl radical is a well-known important intermediate in combustion processes such as hydrocarbon cracking reactions.…”

“…This stands in sharp contrast with the substantial barriers reported in the previous systematic studies of the reactions of O( 3 P) with closed-shell hydrocarbon molecules. [11][12][13][14] In general, for a series of primary, secondary, and tertiary hydrocarbons, as the reaction exothermicity increases, the barrier to the abstraction of the H-atom from hydrocarbon molecules is gradually decreased from 7 to 3 kcal mol…”