(643d) Thermochemistry and Kinetics of the Acetonyl Radicals + NO2 : a Theoretical Study

Thermochemical properties for important species and transition state structures in the acetonyl radicals with NO2 reaction systems are analyzed to evaluate reaction paths and kinetics. The initial association reactions form nitro alkanes (RNO2) or alkylnitrites (RONO) adducts. Enthalpies of formation (ΔHf°298) are determined using isodesmic reaction analysis at the CBS QB3 composite and density functional levels. Entropies (S°298) and heat capacities [Cp°(T)] are determined using geometric parameters and vibration frequencies obtained at the B3LYP/6-311G(2d,d,p) level of theory. Internal rotor contributions are included in S and Cp(T) values in place of torsion frequencies. Detailed potential energy surfaces for the reactions are presented, with intramolecular isomerization high pressure rate parameters calculated from transition state theory. The chemically activated R? + NO2 systems are modeled using quantum Rice-Ramsperger-Kassel (QRRK) theory, with Master Equation analysis for falloff. At atmospheric pressure and temperature both association paths of the R? + NO2 --> reactions proceed almost primarily to stabilization with a fraction of reaction to lower energy products. At combustion temperatures, the R? + NO2 --> R-ONO* adduct primarily dissociates to RCO? + NO via chemical activation reaction and the HNO molecular elimination reaction. Rate constants for abstraction of H from the above carbonyl species by NO to form acetylformaldehyde plus HNO also studied.