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Thermochemical properties for reactants, intermediates, products, and transition states in the neopentyl radical + O2 reaction system are analyzed with ab initio and density functional calculations to evaluate reaction paths and kinetics for neopentyl oxidation. Enthalpies of formation (Δ H f ° 298) are determined using isodesmic reaction analysis at the CBS-Q composite and density functional levels. The entropies (S°298) and heat capacities C p (T) (0 ≤ T/K ≤ 1500) from vibrational, translational, and external rotational contributions are calculated using statistical mechanics based on the vibrational frequencies and structures obtained from the density functional study. Potential barriers for the internal rotations are calculated at the B3LYP/6-31G(d,p) level, and hindered rotational contributions to S°298 and C p (T)'s are calculated by using direct integration over energy levels of the internal rotation potentials. The kinetic analysis on reactions of neopentyl with O2 is performed using enthalpies at the CBS-Q calculation level. The reaction forms a chemically activated neopentyl peroxy adduct with an energy of 38.13 kcal mol-1. The energized adduct can be stabilized, dissociate back to reactants, or isomerize to the hydroperoxy-neopentyl radical. The isomer can dissociate to 3,3-dimethyloxetane + OH, to isobutene + CH2O + OH, to methyl + 2-methyl-2-propenyl-hydroperoxide, isomerize back to the neopentyl peroxy radical, or further react with O2. The Δ H f ° 298 values for the neopentyl, neopentyl peroxy, and hydroperoxy-neopentyl radicals are calculated to be 10.52, −27.61, and −9.43 kcal mol-1, respectively, at the CBS-Q level. Rate constants to products and stabilized adducts (isomers) of the chemically activated neopentyl peroxy are calculated as functions of pressure and temperature using quantum Rice−Ramsperger−Kassel (QRRK) analysis for k(E) and a master equation analysis for the pressure falloff. An elementary reaction mechanism is constructed to model the experimental OH formation profile; the concentrations of initial products 3,3-dimethyloxetane and isobutene are also calculated by the model and compared with the experimental results. Kinetic parameters for intermediate and product-formation channels of the neopentyl + O2 system are presented versus temperature and pressure.

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The rate coefficients of H-abstraction reactions of butene isomers by the OH radical were determined by both canonical variational transition-state theory and transition-state theory, with potential energy surfaces calculated at the CCSD(T)/6-311++G(d,p)//BH&HLYP/6-311G(d,p) level and CCSD(T)/6-311++G(d,p)//BH&HLYP/cc-pVTZ level and quantum mechanical tunneling effect corrected by either the small-curvature tunneling method or the Eckart method. While 1-butene contains allylic, vinylic, and alkyl hydrogens that can be abstracted to form different butene radicals, results reveal that s-allylic H-abstraction channels have low and broad energy barriers, and they are the most dominant channels which can occur via direct and indirect H-abstraction channels. For the indirect H-abstraction s-allylic channel, the reaction can proceed via forming two van der Waals prereactive complexes with energies that are 2.7-2.8 kcal mol(-1) lower than that of the entrance channel at 0 K. Assuming that neither mixing nor crossover occurs between different reaction pathways, the overall rate coefficient was calculated by summing the rate coefficients of the s-allyic, methyl, and vinyl H-abstraction paths and found to agree well with the experimentally measured OH disappearance rate. Furthermore, the rate coefficients of p-allylic H abstraction of cis-2-butene, trans-2-butene, and isobutene by the OH radical were also determined at 300-1500 K, with results analyzed and compared with available experimental data.

Unimolecular dissociation of a neopentyl radical to isobutene and methyl radical is competitive with the neopentyl association with O2 ((3)Sigma(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 O2((3)Sigma(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(degrees)298, and heat capacities, C(p)(T), (0