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Published June 12, 1997 | public
Journal Article

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


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.

Additional Information

T.P.W.J. gratefully acknowledges a "Forschungsstipendium der Deutschen Forschungsgemeinschaft". Generous allocation of CPU time on the IBM SP/2 by the Center for Advanced Computing Research (CACR) at the California Institute of Technology is acknowledged. We are obliged to Anthony M. Dean of Exxon Research and Engineering Company for providing us with the program CHEMACT. We thank Mr. Kiran Shekar for performing numerous GAUSS-IAN94 calculations and Mr. Bryan Hathorn for his assistance on RRKM theory. This research was supported in part by U.S. Environmental Protection Agency Center on Airborne Organics R-819714-01-0.

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