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

Abstract

The radical-radical reaction mechanisms and dynamics of ground-state atomic oxygen [O(3P)] with the saturated tert-butyl radical (t-C4H9) are investigated using the density functional method and the complete basis set model. Two distinctive reaction pathways are predicted to be in competition: addition and abstraction. The barrierless addition of O(3P) to t-C4H9 leads to the formation of an energy-rich intermediate (OC4H9) on the lowest doublet potential energy surface, which undergoes subsequent direct elimination or isomerization-elimination leading to various products: C3H6O + CH3, iso-C4H8O + H, C3H7O + CH2, and iso-C4H8 + OH. The respective microscopic reaction processes examined with the aid of statistical calculations, predict that the major addition pathway is the formation of acetone (C3H6O) + CH3 through a low-barrier, single-step cleavage. For the direct, barrierless H-atom abstraction mechanism producing iso-C4H8 (isobutene) + OH, which was recently reported in gas-phase crossed-beam investigations, the reaction is described in terms of both an abstraction process (major) and a short-lived addition dynamic complex (minor).

A combined crossed-beam and ab initio study of the atom–radical reaction dynamics of O(3P) + C2H5→ C2H4 + OH: analysis of nascent internal state distributions of the OH product

“…Particularly, the entrance barrier along the abstraction reaction coordinates was calculated to be negligible, unlike the substantial barrier reported in the reactions of O( 3 P) with closed-shell hydrocarbon molecules, which are in the range of 3.0-7.0 kcal mol À1 . 17,18 This absence of an activation barrier can be attributed to unusually small C-H bond dissociation energy (BDE = 36.2 kcal mol À1 ), high reaction exothermicity of À67.3 kcal mol À1 , and the highly attractive character of the abstraction process. 25 Here, H-atom abstraction from the CH 2 group of C 2 H 5 was not taken into account due to unfavorable endothermic reaction pathways to the formation of OH + 1 CH 3 CH and 3 CH 3 CH (DH = 8.0 and 5.6 kcal mol À1 , respectively).…”

“…The crossed molecular beam apparatus employed in this study has been described in detail elsewhere, and herein only a relevant account is presented. 1,[12][13][14][15][16][17][18][19][20] The home-made apparatus designed for studies of gas-phase reactive scattering processes consisted of two source chambers and a scattering chamber that were pumped by two 6 in. (1400 l s À1 ) and one 10 in.…”

“…This work stems from this lab's recent progress in gas-phase crossed-beam studies. [12][13][14][15][16][17][18][19][20] Investigation of radical reaction dynamics were performed in a crossedbeam configuration to explain the mechanistic and dynamic characteristics of the reactions of O( 3 P) with a series of prototypal hydrocarbon radicals such as allyl (C 3 H 5 ), propargyl (C 3 H 3 ), and tert-butyl (t-C 4 H 9 ). Leonori et al also studied the oxidation reaction of allyl radicals by means of the crossedbeam apparatus.…”