Conclusion

We have successfully applied the reaction path generation technique used in transition path sampling in order to obtain relaxed reaction pathways of the reaction between pentaaqua iron(II) and hydrogen peroxide in aqueous solution. The initial reaction pathway which was the subject of this relaxation procedure showed a rebound mechanism in which the hydrogen peroxide O-O bond dissociated at H$_2$O$_2$ coordination to the iron(II) center, after which the leaving OH. radical rebounded to abstract the intermediate Fe$^{\rm {III}}$-OH hydrogen, forming the ferryl ion and a water molecule. We generated two sequences of ten reaction pathways from this initial path in order to obtain pathways which have lost the memory of the artificial construction of the initial pathway. Along these sequences, we found a shift to a new mechanism in which the leaving OH. jumps via a solvent water molecule and abstracts the hydrogen from a water ligand forming a dihydroxo iron(IV) complex and a water molecule. The mechanistic change can be rationalized as the result of the proper reestablishing of the solvent structure around the reactants, which had been broken up in the artificial generation of the initial pathway. It is to be noted that the ``mechanism'' which emerges from these transition path sampling experiments is precisely the one found in our earlier AIMD simulation, starting from coordinated H$_2$O$_2$ [144], and was indeed already indicated by gas phase calculations [145]