Atmospheric Oxidation Mechanism of n-Butane:  The Fate of Alkoxy Radicals

Abstract

The atmospheric oxidation mechanism of n-butane is investigated by means of density functional theory and ab initio calculations. Calculation of energies of reactants, transition states, and stable intermediates predicts the detailed pathways leading to experimentally observed products of n-butane oxidation. Also serving as a model system for the oxidation of larger alkanes, quantitative information is obtained for elementary reaction steps that heretofore have been subject to speculation. Complete basis set model chemistries CBS-4 and CBS-q were used with B3LYP/6-31G(d,p) optimized geometries to calculate energies of over 70 stable species and transition states. Energies based on density functional theory were obtained at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level of theory. The principal pathway following formation of the 1-butyl radical from hydroxyl (OH) attack on n-butane is found to be 1,5-H shift of the 1-butoxy radical. After conversion to the δ-hydroxy-1-butoxy radical, another 1,5-H shift is expected to be the primary route to 4-hydroxy-1-butanal. 4-Hydroperoxy-1-butanal can be formed after 1,6-H shift in chemically activated 4-hydroxy-1-butylperoxy radicals. Whereas β-scission in 1-butoxy is an endothermic process, fragmentation of 2-butoxy into C₂H₅ and CH₃CHO is predicted to be the major degradation pathway of the secondary butyl radicals.