Abstract. Hybrid quantum mechanical/molecular mechanical electronic structure calculations reveal the transition state for CH bond cleavage in [(LCu)2 (-O)2]2+ (L=1,4,7-tribenzyl-1,4,7-triazacyclononane) to be consistent with a hydrogen-atom-transfer mechanism from carbon to oxygen. At the MPW1K/double-zeta effective core potential(+)|univeral force field level, 0K activation enthalpies for the parent, p-CF3, and p-OH substituted benzyl systems are predicted to be 8.8, 9.5, and 7.8kcal/mol. Using a one-dimensional Eckart potential to estimate quantum effects on the reaction coordinate, reaction in the unsubstituted system is predicted to proceed with a primary kinetic isotope effect of 22 at 233K. Structural parameters associated with the hydrogen-atom transfer are consistent with the Hammond postulate.