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

• Published in: Combustion and flame 2023-11, Vol.257 (P1), p.112388, Article 112388
• Main Authors: Alarcon, Juan F., Ajo, Sergio, Morozov, Alexander N., Mebel, Alexander M.
• Format: Article
• Language: English
• Subjects: Allyl; Oxidation; Potential energy surface; Rate constant; RRKM-Master equation; VRC-TST
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2022.112388

The C3H5O and C3H5O2 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//ωB97XD/6-311G(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 C3H5 + O reaction is found to be fast in the 300-2500 K temperature interval, with the total rate constant of 1.8-1.0 × 10−10 cm3 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 CH2CHCH2O 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 C2H3 + formaldehyde (32%-26%) formed through the C-C bond β-scission in CH2CHCH2O and two minor products C2H4 + 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 ∼1500 K. The C3H5 + O2 reaction studied in the 200-2500 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 low-temperature 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 cm3 molecule−1 s−1) in the low-temperature regime when the reaction produces the peroxy association complex CH2CHCH2OO, but slow at higher temperatures when it leads to bimolecular products. Under combustion relevant conditions, the C3H5 + O2 rate constant is by 5 to 3 orders of magnitude lower than that for C3H5 + O, but since O2 concentrations in flames could be as much as 3 orders of magnitude larger than O concentrations, C3H5 + O2 cannot be ruled out as a significant allyl radical sink. Above 1500 K, the C3H5 + O2 reaction can form vinoxy radical + formaldehyde (40%-15%) via the five- or four-membered ring closure an