Reaction path sampling of the reaction between iron(II) and hydrogen peroxide in aqueous solution

Abstract:

Previously, we have studied the coordination and dissociation of hydrogen peroxide with iron(II) in aqueous solution by Car-Parrinello molecular dynamics at room temperature. We presented a few illustrative reaction events, in which the ferryl ion ([Fe(IV)O]$^{2+}$) was formed either by a rebound mechanism or by a two-step mechanism via an iron(IV)dihydroxo intermediate, depending on the starting configuration, which was from either separated reactants or H$_2$O$_2$ already coordinated to the Fe$^{2+}$ ion, respectively. In the present work, we test if the illustrative reaction events are indeed representative ones. This is done by generating two sequences of 10 reactive pathways each, using the transition path sampling technique, taking as the initial trajectory the previous reactive pathway which followed the rebound mechanism. Along the generated sequence of reaction pathways, we observed (a) decreasing lifetimes of the intermediate OH. radical, and (b) a change in the reaction mechanism towards the two-step mechanism in which (c) the H-bonded wire through the solvent, along which the OH. radical jumps towards termination, becomes as short as a single H$_2$O molecule. These trends are rationalized from the point of view that the solvent is not relaxed around the separated reactants in the initial pathway, due to the artificial constraints imposed on the system in order to create a reaction event. During the transition path sampling, the solvent environment relaxes and incorporates H$_2$O$_2$ in its hydrogen bonded network. This leads to fast OH. radical transfer and termination along the established H-bond wires in the solvent, which is in favor of the two-step mechanism.