Transition State Structure For Hydrogen Abstraction From Methane by Bromine

The transition state structure is shown for the hydrogen abstraction from methane by bromine radical. The length of the C-H bond that is being broken is 160.0 pm while the other three C-H bonds are 108.3 pm long. The H-Br distance is 152.7 pm long in the transition state structure, only slightly longer than the distance of 143.6 pm found in HBr. The H-C-H(Br) angle is 99.2 degrees and is significantly less than the tetrahedral angle. Hence: This transition state is VERY LATE. The structure no longer looks like methane but the development of a methyl radical is almost complete and the H-Br bond also is very close to being formed completely. This activation energy for this reaction is 18 kcal/mol and the reaction is endothermic.


Figure. The UMP2(full)/6-31G* optimized structure of the transition state structure for
hydrogen abstraction reaction from methane by bromine.