From structure D, the reaction can also proceed in the presence of environmental O2. Following path B (see Figure ) the reaction with oxygen results in a spin crossover event, with a barrier of ΔG ‡ = 8.0 kcal/mol for D1 MECP , culminating in the formation of D1 (with ΔG = -52.3 kcal/mol). D1 then either re-enters path A or can proceed via EE (ΔG = -53.0 kcal/mol), provided that the H2O2 generated during the initiation (path A) is in proximity. Continuing from EE, the structure undergoes a series of rearrangements, with the highest energy point represented by TS E-E1 at ΔG = -36.6 kcal/mol. The rate determining barrier for these steps is ΔG ‡ = 15.6 kcal/mol, the second HAT. In the final product of the reaction, F2, the 18 O-labelled oxygen atom is substituted -in line with the experimentally observed drop in 18 O-content.