Thermochemical and kinetic analysis of the allyl radical with O 2 reaction system

Abstract

Thermochemical properties and kinetics for the allyl radical (CH 2 CHCH 2 ) + O 2 reaction system are analyzed to evaluate reaction paths and kinetics in thermal oxidation. ( Δ H f 298 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 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 [CH 2 CHCH 2OO ]* with a shallow well, only 20 kcal/mol barrier to dissociation. Reaction channels of the peroxy adduct [CH 2 CHCH 2OO ]* include reverse reaction, stabilization, isomerizations via hydrogen shift with subsequent β-scission or R O–OH bond cleavage. The peroxy adduct can also cyclize to four- or five-member cyclic peroxide-alkyl radicals, C H 2Y CCOO and Y CC COO (Y = cyclic). 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 + HO 2 via molecular elimination and C H CHCH 2OOH (via hydrogen shift), which undergoes β-scission leading to C 2H 2 + CH 2O + 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.