Minimum energy spin crossings for an O-O bond formation reaction

The O-O bond formation reaction in the synthetic O2 formation catalyst, [(terpy)(H2O)MnIV(μ-O)2MnIII(H2O)(terpy)]3+ (terpy = 2,2′:6,2″-terpyridine), goes through a transition between two spin surfaces. Using DFT with the B3LYP functional, three different minimum energy crossing points have been located for this redox reaction. The system is predicted to perform the spin crossing close to (+0.1 kcal/mol) the low-spin MnIII-OOH product and this transition does not significantly affect the kinetics. The location of spin crossings in this system is relevant for discussions of O-O bond formation in the oxygen evolving complex of photosystem II.