Rearrangements and tunneling splittings of protonated water trimer

Abstract

Stationary points and rearrangement mechanisms are characterized for protonated water trimer using a variety of basis sets and density functional theory to describe electron correlation. For the largest basis sets there are three distinct low-lying minima separated in energy by only a few wave numbers. Ten distinct transition states were found with barriers spanning nearly three orders of magnitude. Several of these mechanisms should produce observable tunneling splittings.

Alternative Low‐Energy Mechanisms for Isotopic Exchange in Gas‐Phase D2O–H+(H2O)n Reactions

“…However, the frequency of these processes was found to be at least an order of magnitude lower than that of the transformations presented in Figure 3, due to the larger barrier heights for the bifurcated processes. [38] These results, based on the OSS2 model potential, are in line with the barrier heights computed by Geissler et al [39] and by Wales, [40] respectively for the water shift (5.4 kcal mol…”

“…It was found that the experimental branching ratio disagrees with that computed using the statistical approach (complete H/D scrambling during the life time of the complex) for n = 1-4, thus indicating that the complex does not live long enough to permit complete H/D exchange. Moreover, the experimental branching ratio obtained for n = 1 was shown to be in good agreement with the prediction of a Rice-Ramsperger-Kassel-Marcus (RRKM) analysis based on QCISD(T)/6-311 2p) calculations of the Zundel minimum and of its bifurcated transition state [22] (Figure 1). Assuming that a similar mechanism (i.e.…”

“…In the first case, the system would pay a higher energetic price to reach the TS than the Zundel cation (or the trimer [39,40] ), so that one may expect the barrier for the bifurcation to be higher than for the transpositions. In the second case, the process would require a substantial synchronization in the motion of several water molecules, likely to reduce the probability of the event itself.…”

“…Assuming that a similar mechanism (i.e. transition state) is also accessible for the largest clusters, they proposed reaction schemes involving only a bifurcation-mediated H/D exchange [22] and a dissociation reaction; this model was found to produce a correct description of the branching ratio at high energy, although it disagrees somewhat with experiments at the lowest collisional energies.…”

“…Low energy barriers were always found, thus suggesting a fast water shift even at low energies. Based on these findings, mechanistic steps for the H/D scrambling are proposed; these only involve molecular shifts, which are expected to be faster than the tight bifurcation-mediated process [22] invoked in ref. [19].…”