Free-Energy Landscape of the S N 2 Reaction CH 3 Br + Cl - → CH 3 Cl + Br - in Different Liquid Environments.

This work describes in detail the reaction path of the well-known S N 2 reaction CH 3 Br + Cl - → CH 3 Cl + Br - , whose reaction rate has a huge variation with the solvent in the gas phase and in protic and aprotic liquid environments. We employed the ASEC-FEG method to optimize for minima (reactants and products) and saddle points (transition states) in the in-solution free-energy hypersurface. The method takes atomistic details of the solvent into account. A polarizable continuum model (PCM) has also been employed for comparison. The most perceptive structural changes are noted in aqueous solution by using the ASEC-FEG approach. The activation energies in all solvents, estimated by means of free-energy perturbation calculations, are in good agreement with the experimental data. The total solute-solvent hydrogen bonds play an important role in the increased barrier height observed in water and are therefore crucial to explain the huge decrease in the kinetic constant. It is also found that the hydration shell around the ions breaks itself spontaneously to accommodate the molecule, thus forming minimum energy complexes.