Thermochemical and kinetic analysis on the reactions of neopentyl and hydroperoxy-neopentyl radicals with oxygen: part I. OH and initial stable HC product formation

Thermochemical properties for reactants, intermediates, products, and transition states in the neopentyl radical + O-2 reaction system are analyzed with ab initio and density functional calculations to evaluate reaction paths and kinetics for neopentyl oxidation. Enthalpies of formation (DeltaH(f)degrees(298)) are determined using isodesmic reaction analysis at the CBS-Q composite and density functional levels. The entropies (Sdegrees(298)) and heat capacities C-p(T) (0 less than or equal to T/K less than or equal to 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 Sdegrees(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 O-2 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 O-2. The DeltaH(f)degrees(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 + O-2 system are presented versus temperature and pressure.