Exploring the dynamics of hydrogen atom release from the radical–radical reaction of O(3P) with C3H5

Abstract

The gas-phase radical-radical reaction dynamics of O(3P) + C3H5 --> H(2S) + C3H4O was studied at an average collision energy of 6.4 kcal/mol in a crossed beam configuration. The ground-state atomic oxygen [O(3P)] and allyl radicals (C3H5) were generated by the photolysis of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. Nascent hydrogen atom products were probed by the vacuum-ultraviolet-laser induced fluorescence spectroscopy in the Lyman-alpha region centered at 121.6 nm. With the aid of the CBS-QB3 level of ab initio theory, it has been found that the barrierless addition of O(3P) to C3H5 forms the energy-rich addition complexes on the lowest doublet potential energy surface, which are predicted to undergo a subsequent direct decomposition step leading to the reaction products H + C3H4O. The major counterpart C3H4O of the probed hydrogen atom is calculated to be acrolein after taking into account the factors of barrier height, reaction enthalpy, and the number of intermediates involved along the reaction pathway. The nascent H-atom Doppler profile analysis shows that the average center-of-mass translational energy of the H + C3H4O products and the fraction of the total available energy released as the translational energy were determined to be 3.83 kcal/mol and 0.054, respectively. On the basis of comparison with statistical calculations, the reaction proceeds through the formation of short-lived addition complexes rather than statistical, long-lived intermediates, and the polyatomic acrolein product is significantly internally excited at the moment of the decomposition.

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

“…Experimental Studies: The lab-made crossed-beam apparatus for investigating gas-phase reaction dynamics has been described in detail elsewhere. [1,[5][6][7][8][9][10][11][12][13][14][15][16][17] It consists of two radical-source chambers, each pumped by a 6-inch (1400 L s À1 ) baffled diffusion pump, and a scattering chamber, pumped by a 10-inch (3000 L s À1 ) baffled diffusion pump. The average base pressure in the scattering chamber was kept below 2.0 10 À6 Torr.…”

“…It builds on previous gas-phase crossed-beam studies. [5][6][7][8][9][10][11][12][13][14][15][16][17] Radical-radical reaction dynamics were investigated as part of ongoing efforts to elucidate the mechanistic and dynamic attributes of the reactions of O( 3 P) with a series of prototypal alkyl radicals, such as allyl, propargyl, tert-butyl (t-C 4 H 9 ), and vinyl, and to identify the general principles that determine their reactivity and mechanisms. Some hydrocarbon species showed intrinsic stability due to resonance stabilization or hyperconjugation.…”

Primary photodissociation pathways of epichlorohydrin and analysis of the C–C bond fission channels from an O(P3)+allyl radical intermediate

“…[1][2][3][4][5][6][7][8][9][10][11] The complexity of the competition between bond fission and isomerization pathways is surprising for such a small system, composed of only four heavy atoms and five hydrogen atoms. Previous studies have sought to probe the product branching and dynamics of this reaction.…”

The reaction of allyl radicals with oxygen atoms—rate coefficient and product branching

“…Their RRKM analysis of the fate of the excited allyloxy radical shows a variety of possible isomerization and subsequent decomposition pathways with only minor differences in the barrier heights [17]. A crossed-beam study of Joo et al [18] provides details of the mechanism of H atom formation. In a recent communication Leonori et al [19] reported detailed mechanistic information on the acrolein channel and found evidence for one or more C-C bond-breaking channels.…”

“…Since for the allyl radical the unimolcular C-C and C-H bond fission reactions and the reaction with molecular oxygen are significantly slower than for saturated alkyl radicals [11][12][13], the reactions of allyl radicals with atoms (O, H) and other radicals (OH, alkyl) become dominant under high-temperature conditions. Several experimental and theoretical studies have been performed to determine the rate and the products of the title reaction so far [14][15][16][17][18][19]. According to these investigations the reaction allyl + O mainly proceeds via a highly energized allyloxy intermediate, which subsequently undergoes fast decomposition, in analogy to the reactions of alkyl radicals with O atoms [20][21][22][23][24][25][26].…”