Chemical dynamics of the protonated water trimer analyzed by transition path sampling

Abstract

Using transition path sampling, we have identified the two most important classes of reaction pathways and computed microcanonical rate constants for proton transfer in a model of (H₂O)₃H⁺. The path sampling simulations visit the pathways in correct proportion despite the presence of a high potential energy barrier separating the two transition state regions. In both classes of pathways, transfer is driven by rearrangement of the oxygen ring rather than by distortion of chemical bonds. The potential experienced by the transferring proton is monostable throughout the transition, so that proton tunneling is not expected to play a large role in these dynamics. Transient times to traverse either transition state region are typically of the order of 0.1 ps. In contrast, proton residence times are 15 or more orders of magnitude longer for energies below the dissociation threshold. The computed residence times (or rate constants) vary by 25 orders of magnitude over the range of energies we have considered. Over this range, the energy scaling predicted by classical RRKM theory agrees well with that found from the computed rates.