Reaction barriers: Origin and evolution

The factors contributing to radical-molecule barrier heights and reactivity in atmospheric chemistry were analyzed. Variation in the excited state energies drived variation in the crossing height of two states related to the reactant and product configurations, which led to the reaction barrier. The tight correlation among transition-state energies, bending frequencies, and imaginary frequencies, causing a tight correlation between reactants and activation energies, also contributed to the barrier heights. The variation in frontier-orbital overlap drived variations in the coupling parameter when the depleted overlap reduced the coupling and increased the barrier height. The overlap-driven coupling changes caused enormous changes in reactivity.