AN ABINITIO STUDY OF HYDROGEN-ATOM ABSTRACTIONS FROM SUBSTITUTED METHANES BY SUBSTITUTED METHYL RADICALS

The reactions CH3X + .CH2Y --> .CH2X + CH3Y (X, Y = H, F, Cl, OH, NH2, CN) have been studied using ab initio molecular orbital theory at the UHF/3-21G and UHF/6-31G** levels. The Marcus relation can be used to predict the barrier heights of the cross reactions based on the information from the identity reactions and the changes in energy for the cross reactions. This method predicts the barrier heights accurately with two exceptions (X = CN and Y = NH2 or OH); the average absolute error is 0.98 kcal/mol when compared to the ab initio barrier heights. Charge-transfer states have been found to play a significant role in the reactions where X = CN and Y = NH2 or OH and a minor role in a few other reactions.