Isomerization-assisted Proton Transfers in MeOH-(H2O)2H+

We carried out Path Integral Molecular Dynamics simulations that describe microscopic properties of two isomerization processes taking place in the MeOH(H2O)2H+ trimer, at T = 50 K. In particular, we focused attention on the free energies associated with the exchange of the connective/dangling characteristics of a pair of protons located at key subunits of the trimer. In one of the processes, the isomerization produces a modification in the topology of the connectivity pattern of the cluster, from branched-like to chain-like motifs. In contrast, along the other transformation, reactant and product states are both chain-like and equiv- alent. Changes in associated free energies were computed following reversible paths described in terms of order parameters involving angular degrees of freedom. As a common feature, along both isomerizations, we registered concomitant migrations of the excess charge. The strongly confining, single-well character- istics of the potential energy surface along the asymmetric stretch coordinate promote compact, ∼ 0.35 ˚A long structures for the isomorphic polymer associated with the itinerant proton which persist along the com- plete isomerization path. These observations suggest the absence of tunneling contributions to the resulting mechanisms that control the charge transfer process. Estimates for the corresponding isomerization rates are also computed.