Isomerization-assisted proton transfers in MeOH-(H2O)2H+

We carried out Path Integral Molecular Dynamics simulations that describe the 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 cluster’s connectivity pattern, from branched-like to chain-like motifs. In contrast, along the other transformation, reactant and product states are both chain-like and equivalent. 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 proton. The strongly confining, single-well characteristics of the potential energy surface along the asymmetric stretch coordinate promote compact, ∼0.35 Å long structures for the isomorphic polymer associated with the itinerant proton, which persist along the complete isomerization path. These observations suggest the absence of tunneling contributions to the resulting mechanisms that control the proton transfer process. Estimates for the corresponding isomerization rates are also computed.