Reaction dynamics simulations of the identity SN2 reaction H2O + HOOH2+ → H2OOH+ + H2O. Requirements for reaction and competition with proton transfer

Abstract

Despite the fact that the transition structure of the gas phase S_N2 reaction H₂O + HOOH₂⁺ → HOOH₂⁺ + H₂O is well below the reactants in potential energy, the reaction has not yet been observed by experiment. Variational transition state RRKM theory reveals a strong preference for the competing proton transfer reaction H₂O + HOOH₂⁺ → H₃O⁺ + HOOH due to entropy factors. Born–Oppenheimer reaction dynamics simulations confirm these results. However, by increasing the collision energy to around 7.5 eV the probability for nucleophilic substitution increases relative to proton transfer. These observations are explained by the presence of the key common intermediate HOO(H)⋯H–OH₂⁺ which leads to effective proton transfer, but can be avoided with increasing collision energy. However, the S_N2 probability remains below 0.2 since successful passage through the TS requires optimum initial orientation of the reactants, excitation of the relative translational motion and good phase correlation between the O–O vibration and the motion of the incoming water.