The SN2 nucleophilic substitution reaction, X- + RY arrow right XR + Y-, is a paradigm reaction in organic chemistry . The modern understanding of the SN2 reaction mechanism is based on work of Hughes and Ingold , who proposed that the nucleophile (X-) approaches the carbon atom that bears the leaving group (Y-). As a result, the bond between the carbon atom and the leaving group becomes weakened. As this bond breaks and a new bond forms between the nucleophile and the carbon atom, the configuration of the carbon atom is inverted. Analyses of gas-phase reaction rates led to the suggestion of a potential energy surface (PES) with two wells connected by a central barrier transition state . Electronic structure calculations have confirmed this picture for some SN2 reactions , but recent studies have shown that the actual reaction dynamics may be considerably more complex (see the figure) (-).