Concertedness and solvent effects in multiple proton transfer reactions: The formic acid dimer in solution

The issue of multiple proton transfer (PT) reactions in solution is address ed by performing molecular dynamics simulations for a formic acid dimer emb edded in a water cluster. The reactant species is treated quantum mechanica lly, within a density functional approach, while the solvent is represented by a classical model. By constraining different distances within the dimer we analyze the PT process in a variety of situations representative of mor e complex environments. Free energy profiles are presented, and analyzed in terms of typical solvated configurations extracted from the simulations. A decrease in the PT barrier height upon solvation is rationalized in terms of a transition state which is more polarized than the stable states. The d ynamics of the double PT process is studied in a low-barrier case and corre lated with solvent polarization fluctuations. Cooperative effects in the mo tion of the two protons are observed in two different situations: when the solvent polarization does not favor the transfer of one of the two protons and when the motion of the two protons is not synchronized. This body of ob servations is correlated with local structural and dynamical properties of the solvent in the vicinity of the reactant. (C) 2000 American Institute of Physics. [S0021-9606(00)51121-0].