Reaction of the i-C4H5 (CH2CCHCH2) Radical with O2

The resonantly stabilized radical i-C4H5 (CH2CCHCH2) is an important intermediate in the combustion of unsaturated hydrocarbons and is thought to be involved in the formation of polycyclic aromatic hydrocarbons through its reaction with acetylene (C2H2) to form benzene + H. This study uses quantum chemistry and statistical reaction rate theory to investigate the mechanism and kinetics of the i-C4H5 + O2 reaction as a function of temperature and pressure, and unlike most resonantly stabilized radicals we show that i-C4H5 is consumed relatively rapidly by its reaction with molecular oxygen. O2 addition occurs at the vinylic and allenic radical sites in i-C4H5, with respective barriers of 0.9 and 4.9 kcal mol−1. Addition to the allenic radical form produces an allenemethylperoxy radical adduct with only around 20 kcal mol−1 excess vibrational energy. This adduct can isomerize to the ca. 14 kcal mol−1 more stable 1,3-divinyl-2-peroxy radical via concerted and stepwise processes, both steps with barriers around 10 kcal mol−1 below the entrance channel energy. Addition of O2 to the vinylic radical site in i-C4H5 directly forms the 1,3-divinyl-2-peroxy radical with a small barrier and around 36.8 kcal mol−1 of excess energy. The 1,3-divinyl-2-peroxy radical isomerizes via ipso addition of the O2 moiety followed by O atom insertion into the adjacent C−C bond. This process forms an unstable intermediate that ultimately dissociates to give the vinyl radical, formaldehyde, and CO. At higher temperatures formation of vinylacetylene + HO2, the vinoxyl radical + ketene, and the 1,3-divinyl-2-oxyl radical + O paths have some importance. Because of the adiabatic transition states for O2 addition, and significant reverse dissociation channels in the peroxy radical adducts, the i-C4H5 + O2 reaction proceeds to new products with rate constant of around 1011 cm3 mol−1 s−1 at typical combustion temperatures (1000−2000 K). For fuel-rich flames we show that the reaction of i-C4H5 with O2 is likely to be faster than that with C2H2, bringing into question the importance of the i-C4H5 + C2H2 reaction in initiating ring formation in sooting flames.