ATMOSPHERIC OXIDATION MECHANISM OF N-BUTANE - THE FATE OF ALKOXY RADICALS

The atmospheric oxidation mechanism of n-butane is investigated by mea ns of density. functional theory and ab initio calculations. Calculati on of energies of reactants, transition states, and stable intermediat es predicts the detailed pathways leading to experimentally observed p roducts 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 specula tion. Complete basis set model chemistries CBS-4 and CBS-q were used w ith B3LYP/6-31G(d,p) optimized geometries to calculate energies of ove r 70 stable species and transition states. Energies based on density f unctional theory were obtained at the B3LYP/6-311+G(3df,2p)//B3LYP/6-3 1G(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 delta- hydroxy-1-butoxy radical, another 1,5-H shift is expected to be the pr imary route to 4-hydroxy-1-butanal. 4-Hydroperoxy-1-butanal can be for med after 1,6-H shift in chemically activated 4-hydroxy-1-butylperoxy radicals. Whereas beta-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.