Theoretical study on the mechanism and kinetics of the oxidation of allyl radical with atomic and molecular oxygen

The C 3 H 5 O and C 3 H 5 O 2 potential energy surfaces accessed by the oxidation reactions of allyl radical with atomic and molecular oxygen have been mapped out at the CCSD(T)-F12b/cc-pVTZ-F12// omega B97XD/6311G(d,p) level of electronic structure theory to unravel the reaction mechanism. Temperature- and pressure-dependent reaction rate constants have been evaluated using variable reaction coordinate transition state and RRKM-Master Equation theoretical kinetics methods. The C 3 H 5 + O reaction is found to be fast in the 30 0-250 0 K temperature interval, with the total rate constant of 1.8-1.0 x 10 -10 cm 3 molecules s -1 independent of pressure in the considered 30 Torr - 100 atm range. Acrolein + H formed by a simple O addition/H elimination mechanism via the CH 2 CHCH 2 O association complex are predicted to be the major reaction products with the branching ratio decreasing from 61% at 300 K to 44% at 2500 K. Other substantial bimolecular products include C 2 H 3 + formaldehyde (32%-26%) formed through the C-C bond beta-scission in CH 2 CHCH 2 O and two minor products C 2 H 4 + formyl radical HCO and allene + OH, where the latter, formed via direct H abstraction by O from the central carbon atom of allyl, becomes significant only at high temperatures above similar to 1500 K. The C 3 H 5 + O 2 reaction studied in the 20 0-250 0 K temperature range shows a peculiar kinetic behavior characteristic for a bimolecular reaction with a low entrance barrier, shallow association well, and high ensuing isomerization and decomposition barriers to bimolecular products. This behavior is described in terms of three distinct temperature regimes: the lowtemperature one with slightly negative dependence of the rate constant, the intermediate one where the rate constant sharply drops, and the high-temperature regime with an Arrhenius-like rate constant. The rate constant is relatively high (10 -13 -10 -12 cm 3 molecule -1 s -1 ) in the low-temperature regime when the reaction produces the peroxy association complex CH 2 CHCH 2 OO, but slow at higher temperatures when it leads to bimolecular products. Under combustion relevant conditions, the C 3 H 5 + O 2 rate constant is by 5 to 3 orders of magnitude lower than that for C 3 H 5 + O, but since O 2 concentrations in flames could be as much as 3 orders of magnitude larger than O concentrations, C 3 H 5 + O 2 cannot be ruled out as a significant allyl radical sink. Above 1500 K, the C 3 H 5 + O 2 reaction can form vinoxy radical + formaldehyde (40%-15%) via the five- or four-membered ring closure and opening mechanism starting from the initial peroxy intermediate, acrolein + OH (41%-49%) via the H shift from the attacked carbon to the terminal oxygen followed by the immediate O-O bond cleavage, and allene + OH (18-25%) via the H migration from the central CH group to the terminal O atom accompanied or followed by the C-O bond rupture. Modified Arrhenius expressions fitting the computed rate constants for both reactions have been generated and proposed for kinetic models. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.