Barrier heights vs IP-EA for H Atom Abstraction from alkanes by
OH (red),O (blue),H (black), and Cl(green).
Methane reactions are circled.
The underlying image shows the orbital interactions controlling the barrier height.
For years we have been pursuing the question of what fundamental process controls barrier heights in radical molecule reactions. The answer lies in an avoided curve crossing between the reactant ground state and an ionic state. When the reactants are separated the ionic state lies at an energy equal to IP-EA above the ground state. In the product, the identities of the two states are reversed, and the transition state is found at the crossing point. Look for two papers coming soon to a journal near you; one is under review at J. Phys. Chem, and one is about to be submitted there.