we see that the formation of the new H-H bond (BE 23 ) strongly overlaps the disappearance of the old H-H bond (BE 12 ), so that the change in the sum of these bond energies (∆E 12 +∆E 23 ) is very small. We could reasonably describe the mechanism as the bond moving from the 1, 2 to the 2, 3 hydrogen pair. The small barrier (9.1 kcal/mol) for this reaction results from the 1, 3-antibonding interaction of the unpaired electron originating on the hydrogen atom. The reaction of methane with a methyl radical: