Fig. 4.

Schematic free energy diagrams for the Cl + MeCl’ S_N2 reaction in aqueous solution. (a) Schematic free energy diagram, indicating the activation free energy ΔG^‡. (b) Contour diagram indicating a vertical TS surface (perpendicular to the antisymmetric stretch coordinate– which in the TS neighborhood is x) rotated to give a better TS surface with reduced recrossing; the latter involves a solvent coordinate contribution. Recrossing trajectories for the original surface are shown. The solid line is a schematic minimum free energy path. Ref. [38] may be consulted for the original discussion.