AN AB-INITIO STUDY OF HYDROGEN ABSTRACTION BY FLUORINE, CHLORINE AND BROMINE ATOMS FROM ETHANE AND PROPANE

The H-abstraction reaction by F, Cl and Br radicals from ethane and propane is studied using ab initio UHF, UHF/MP2 and UHF/MP4 computational methods with the 6-31G* basis set. It is found that in all the cases examined the reaction proceeds through a transition state where the halogen atom, the abstracted H atom and the C atom are almost collinear. The geometries of the various transition states are not significantly modified by the dynamic correlation corrections included with the MP2 treatment. However the correlation energy contributions on reactants and transition states are essential to obtain values of activation energy in reasonable agreement with experimental results. Also the trend of reactivity, which decreases in the direction F --> Cl --> Br, and the selectivity toward primary and secondary hydrogens are in good agreement with experiment. Another interesting point concerns the performance of the effective potential basis set LAN1DZ, which is found to yield reliable geometries for the transition states. So, more accurate activation energies can be obtained cheaply by single-point computations with the 6-31G* basis set on LANL1DZ optimized geometries. A simple diabatic model is used to rationalize the trend of reactivity along the series F --> Cl --> Br and the greater reactivity of secondary versus primary hydrogens.