Thermochemical and kinetic analysis of the allyl radical with O2 reaction system

Thermochemical properties and kinetics for the allyl radical (CH2=CHCH2center dot) + O-2 reaction system are analyzed to evaluate reaction paths and kinetics in thermal oxidation. (Delta H-f298(0)) are determined using isodesmic reaction analysis at the CBSQ//B3LYP/6-31G(d,p) composite and at density functional levels. Entropies (S-298(0)) and heat capacities (C-p(0)(T)) are determined using structures and vibrational frequencies 2 obtained at the B3LYP/6-31G(d,p) level of theory. Internal rotor contributions are included in S and C-p(T) values. The allyl radical adds to O-2 to form an energized peroxy adduct [CH2=CHCH2OO-]* with a shallow well, only 20 kcal/mol barrier to dissociation. Reaction channels of the peroxy adduct [CH2=CHCH-OO-]* include reverse reaction, stabilization, isomerizations via hydrogen shift with subsequent beta-scission or (RO)-O-center dot-OH bond cleavage. The peroxy adduct can also cyclize to four- or five-member cyclic peroxide-alkyl radicals, C*H2YCCOO and (YCCCOO)-C-center dot (Y = Uglic). All pathways in this allyl plus O-2 system are found to involve barriers that are higher than reverse reaction. The most accessible product (only 5 kcal/mol above reverse reaction) is the cyclization to a five-member ring, that reacts further with O-2, resulting in carbonyl and chain propagation products. Other important products are: allene + HO2 via molecular elimination and C'H=CHCH2OOH (via hydrogen shift), which undergoes beta-scission leading to C2H2 + CH2O + OH above 600 K, Rate constants are estimated as a function of pressure and temperature using quantum Rice-Ramsperger Kassel analysis for k(E) and master equation for falloff. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.