Potential energy bimolecular nucleophilic substitution

For example, for a bimolecular nucleophilic substitution (Sn2) reaction like Cl- + CHoBr — CICHo-l-Br, the potential energy has a double-well shape, i.e., two minima separated by a central barrier. The minima for this reaction reflects the stability (an effect that is also well known within classical electrostatics) of the ion-dipole complexes Cl- CHoBr and CICH3 Br . For other indirect (or complex mode) reactions one finds two saddle points separated by a well on the path from reactants to products. The existence of a well along the reaction path implies that the collision may be sticky , and a long-lived intermediate complex can be formed before the products show up. Examples of complex mode reactions are H + O2 — OH + O (with the intermediate H02), H+ + D2 and KC1 + NaBr. 

Gas-phase SN2 nucleophilic substitution reactions are particularly interesting because they have attributes of both bimolecular and unimolecular reactions.1 As discovered from experimental studies by Brauman and coworkers2 and electronic structure theory calculations,3 potential energy surfaces for gas-phase SN2 reactions of the type,... 

The mechanism proposed by Hughes and Ingold, called by them substitution nucleophilic bimolecular (8 2) is shown as an equation in Mechanism 8.1 and as a potential energy diagram in Figure 8.1. 

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