Analysis of Tertiary Butyl Radical + O2, Isobutene + HO2, Isobutene + OH, and Isobutene−OH Adducts + O2: A Detailed Tertiary Butyl Oxidation Mechanism

Abstract

The reaction systems tert-butyl radical + O2, isobutene + HO2, isobutene + OH, and isobutene−OH adducts + O2, which are important to understanding the oxidation chemistry of tertiary butyl radical (C3C•), are analyzed. Thermochemical parameters are determined by ab initio−Mφller−Plesset (MP2(full)/6-31g(d)), complete basis set model chemistry (CBS-4 and CBS-q with MP2(full)/6-31g(d) and B3LYP/6-31g(d) optimized geometries), density functional (B3LYP/6-31g(d)), semiempirical MOPAC (PM3) molecular orbital calculations, and by group additivity estimation. Thermochemical kinetic parameters are developed for each elementary reaction path in these complex systems, and a chemical activation kinetic analysis using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) and master equation analysis for falloff is used to calculate rate constants as a function of pressure and temperature. An elementary reaction mechanism is constructed to model experimental data for oxidation of tert-butyl radical. Calculations for loss of tert-butyl precursor, 2,2,3,3-tetramethylbutane (C3CCC3), and production of isobutene and 2,2-dimethyloxirane from the mechanism are compared with experimental data reported in the literature. Reaction of tert-butyl radical (C3C•) with O2 forms an energized tert-butyl peroxy adduct C3COO•* which can dissociate back to reactants, dissociate to isobutene + HO2, or isomerize to tert-butyl hydroperoxide (C3•COOH). This isomer can dissociate to either isobutene + HO2 or 2,2-dimethyloxirane + OH, before it is stabilized. In the tert-butyl radical + O2 reaction system, dissociation of the [C3COO•]* adduct to isobutene + HO2 via HO2 molecular elimination is faster than the hydrogen shift to C3•COOH by a factor of 86:1 at 773 K and 60 Torr. The reaction barrier (reaction enthalpy difference between TS4 and C3•COOH) for the C3•COOH reaction to 2,2-dimethyloxirane + OH is calculated as 17.98 (19.06) kcal/mol at the CBS-q//MP2(full)/6-31g(d) level but is evaluated as 15.58 (18.06) kcal/mol by fitting experimental data. Data in parentheses are thermodynamic properties based on CBS-q//B3LYP/6-31g(d) calculation. Barriers for reactions of HO2 + isobutene → C3•COOH (HO2 addition at CD/C2 carbon atom of isobutene, CD = carbon double bond) and HO2 + isobutene → C2C•COOH (HO2 addition at CD/H2 carbon atom of isobutene) are respectively determined as 7.74 (7.38) and 10.69 (10.82) kcal/mol. 2,2-Dimethyloxirane is formed primarily by HO2 addition to isobutene. OH addition to isobutene results in adducts which further react with O2 to form acetone, formaldehyde, and the OH radical (Waddington mechanism) with these pathways also analyzed.