Hydrogen Atom Release Dynamics in Radical–Radical Reactions: Saturated vs Unsaturated

Abstract

The outstanding difference between the reactivities of saturated and unsaturated hydrocarbon radicals can be rationalized in terms of the characteristic geometry of the tight transition state and high forward and reverse activation barriers along the reaction coordinate on the potential energy surface, as shown in the schematic representation.

A Gas-Phase Investigation of Oxygen-Hydrogen Exchange Reaction of O(3P) + C2H5→ H(2S) + C2H4O

“…After considering the barrier height and reaction exothermicity along the reaction coordinate, the title reaction is, in fact, predicted to be a major channel, which is consistent with the gas-phase kinetic experiments. [5][6][7][8][9][10][11] Statistical Analyses and Reaction Dynamics. The partitioning of the energy into the H products and the microscopic kinetic rate constants were derived using the aforementioned ab initio calculations.…”

“…Consequently, the tight transition-state geometry, having a large 1. 8 On the other hand, in the case of the O + allyl reaction, the situation is totally reversed due to the longer C-H bond length in the loose, product-like transition state and the small exit-channel energy, resulting in the low f T value. 6 In the case of the medium transition state, the observed f T value can be attributed to the medium C-H bond length and reverse activation barrier, as shown in the O + iso-propyl reaction.…”

“…The crossed-beam apparatus used in this study was made in-house and has been described in detail elsewhere. 3,[5][6][7][8][9][10][11] In brief, a pulsed O( 3 P) radical beam was produced by irradiating 2 atm of He (ultra-high-purity helium: 99.999%) seeded with 12% NO 2 (99.5%, MG Industries) with 30 mJ pulses from a 355 nm Nd:YAG laser beam (Continuum Surelite II-10) near the throat of the supersonic expansion. A second C 2 H 5 radical beam was prepared by supersonic flash pyrolysis of the precursor, azoethane (C 2 H 5 -N=N-C 2 H 5 ), synthesized using the method of Renaud and Leitch.…”

“…4 By adopting the supersonic flash pyrolysis source, a series of combined crossed-beam and theoretical studies of the prototypal oxidation reactions of both π-conjugated hydrocarbon radicals such as allyl (C 3 H 5 ), propargyl (C 3 H 3 ), vinyl (C 2 H 3 ), and saturated hydrocarbon radicals such as t-butyl (t-C 4 H 9 ), ethyl (C 2 H 5 ), and iso-propyl (i-C 3 H 7 ) have recently been executed in our laboratory. [5][6][7][8][9][10][11] The analyses of the energy distributions of the nascent reaction products have demonstrated extraordinary reactivity and mechanistic characteristics, which have not previously been observed in the reactions of O( 3 P) + closed-shell hydrocarbon molecules. 12,13 Based on the authors' theoretical calculations, Leonori et al also studied the oxidation reaction of allyl radicals using a universal crossed-beam apparatus.…”

Probing the kinetic energy-release dynamics of H-atom products from the gas-phase reaction of O(3P) with vinyl radical C2H3

“…This work develops from this lab's recent progress in crossed-beam investigations of the reaction dynamics of O( 3 P) with a series of prototypical hydrocarbon radicals such as allyl (C 3 H 5 ), propargyl (C 3 H 3 ), t-butyl (t-C 4 H 9 ), ethyl (C 2 H 5 ) and iso-propyl (i-C 3 H 7 ). [4][5][6][7][8][9][10][11] All alkyl radicals were produced using a supersonic flash pyrolysis developed by Chen and coworkers. 12 The labile precursor molecules entrained in a molecular beam experienced a very short residence time inside a high-temperature tube, leading to rapid thermal decomposition into clean, jet-cooled radical beams without recombination and/or secondary dissociation.…”