Thermochemical and kinetic analysis on the reactions of O₂ with products from OH addition to isobutene, 2-hydroxy-1,1-dimethylethyl, and 2-hydroxy-2-methylpropyl Radicals: HO₂ formation from oxidation of neopentane, Part II

Unimolecular dissociation of a neopentyl radical to isobutene and methyl radical is competitive with the neopentyl association with O₂( ³Σ_g^-) in thermal oxidative systems. Furthermore, both isobutene and the OH radical are important primary products from the reactions of neopentyl with O₂. Consequently, the reactions of O₂ with the 2-hydroxy-1,1-dimethylethyl and 2-hydroxy-2- methylpropyl radicals resulting from the OH addition to isobutene are important to understanding the oxidation of neopentane and other branched hydrocarbons. Reactions that correspond to the association of radical adducts with O ₂(³Σ_g^-) involve chemically activated peroxy intermediates, which can isomerize and react to form one of several products before stabilization. The above reaction systems were analyzed with ab initio and density functional calculations to evaluate the thermochemistry, reaction paths, and kinetics that are important in neopentyl radical oxidation. The stationary points of potential energy surfaces were analyzed based on the enthalpies calculated at the CBS-Q level. The entropies, S^o ₂₉₈, and heat capacities, C_P(T), (0 ≤ T/K ≤ 1500), from vibration, translation, and external rotation contributions were calculated using statistical mechanics based on the vibrational frequencies and structures obtained from the density functional study. The hindered internal rotor contributions to S^o298 and C_P(T) were calculated by solving the Schrödinger equation with free rotor wave functions, and the partition coefficients were treated by direct integration over energy levels of the internal rotation potentials. Enthalpies of formation (ΔH_f^o₂₉₈) were determined using isodesmic reaction analysis. The ΔH_f^o₂₉₈ values of (CH₃)₂C·CH₂OH, (CH₃) ₂C(OO·)CH₂OH, (CH₃)₂C(OH) C·H₂, and (CH₃)₂C(OH)CH ₂OO· radicals were determined to be -23.3, -62.2, -24.2, and -61.8 kcal mol^-1, respectively. Elementary rate constants were calculated from canonical transition state theory, and pressure-dependent rate constants for multichannel reaction systems were calculated as functions of pressure and temperature using multifrequency quantum Rice-Ramsperger-Kassel (QRRK) analysis for k(E) and a master equation for pressure falloff. Kinetic parameters for intermediate and product formation channels of the above reaction systems are presented as functions of temperature and pressure.