Ab-initio study of initial atmospheric oxidation reactions of C₃ and C₄ alkanes

Second-order, Møller-Plesset (MP2)-unrestricted Hartree-Fock calculations with full geometry optimization in the 6-31G(d, p) basis set were carried out to study the initial atmospheric oxidation reactions of alkanes. All structures in the initial hydrogen abstraction reaction by an OH radical and the subsequent addition of molecular oxygen to the alkyl radical were characterized for alkanes with three and four carbon atoms. The reaction paths for the formation of the peroxyl radicals were obtained and discussed in the light of similarities along series involving primary, secondary, and tertiary hydrogens. A 0.999 correlation was found between the height of our barriers for the OH abstraction of a primary hydrogen atom from alkanes containing one to four carbon atoms and the optimally estimated activation energies for this reaction recently presented. From the slope and the intersection at zero activation energy an equation was obtained that yields scaled values of the activation energies to account for the tunnel effect and for the error due to the basis set and the method employed. We present new results for the abstraction of the less favored primary hydrogens in propane, butane, and isobutane, which should be important at high temperatures. Negative net activation energies were obtained for the addition of molecular oxygen to all the alkyl radicals formed in the first reaction. The structure of the peroxyl radicals is discussed; and very good correlations are observed for similar compounds, regardless of the length of the carbon chain. A revision of some experimental values is suggested. Single point density functional calculations at the MP2 geometries were also performed with the B3LYP functional for comparison. The observed trends are exactly the same for the two methods. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 845-856, 1999.