Anti-Hammond effects

The energetic consequences of the CM and PES models are also the same. Stabilization of both parallel and perpendicular structures leads to stabilization of the transition state, in line with predictions of the CM model. We see then that perturbations on reactant and product configurations are equivalent to the PES parallel (Hammond) effects, while perturbations on intermediate configurations are equivalent to the PES perpendicular (anti-Hammond) effects. Thus the CM and PES approaches are, at least for this kind of application, analogous. 

Hammond behaviour may be observed if the intersecting parabolae are of different curvature. The physical interpretation of this difference in curvature is that bond-making and bond-breaking processes are not necessarily synchronous as is assumed in conventional Marcus theory. While such a modification in the theory may overcome the inherent problem of treating anti-Hammond effects it does make application of the Marcus theory more difficult by the introduction of additional unknowns into the free energy relationship of (112). 

Anti-Hammond effect 591 Antimutator strain of T4 phage 390,... 

The structures of the intermediate and transition state can be perturbed by radical mutations that cause significant decreases in stability.61 Interestingly, there is a mixture of the Hammond effect, as found for CI2 in Figure 19.7, and the very rare anti-Hammond effect (discussed in detail in referance 61). Molecular dynamics simulations are consistent with the experimental data and show that radical mutations in the major helix cause it to be more unfolded in the transition state for unfolding (the anti-Hammond effect) but the j8 sheet is less unfolded (Hammond Effect).62... 

In certain cases, only anti-Hammond effects will be obtained as a result of some perturbation, and, as a result, a simultaneous increase in both selectivity and reactivity is expected to be induced (More O Ferrall, 1973). The elimination reactions of the compounds listed in Table 7 fall into such a category (Hudson and Klopman, 1964). 

Ando and coworkers conducted isotope effect studies (entry 11) on the direct displacement reaction of benzyl arenesulfonates with dimethylaniline (DMA)38. They found that an electron-withdrawing substituent in the substrate (Y = 3-Br) caused the TS to shift to a later position along the reaction coordinate, which is consistent with that predicted by the Thornton rule (or anti-Hammond effect). The anilinolysis of phenylethyl arenesulfonates (entries 12 and 13) proceeds also by an SN2 mechanism. The reaction was found to proceed by a dissociative SN2 mechanism with a relatively small degree of aryl participation. The fraction of the phenonium ion intermediate captured by the aniline nucleophile in the aryl-assisted pathway has been shown to increase with a stronger nucleophile, and a four-center TS in an intermolecular SNi mechanism is suggested for the aryl-assisted pathway39,40. Under the same reaction conditions, benzylamine nucleophiles react at a rate ca two times faster than that of anilines. 

Konda SSM, Brantley JN, Varghese BT, Wiggins KM, Bielawski CW, Makarov DE (2013) Molecular catch bonds and the anti-Hammond effect in polymer mechanochemistry. J Am Chem Soc 135 12722-12729... 

Perpendicular to the diagonal the transition state shifts toward the corner that is lowered and away from the corner that is raised (referred to as an anti-Hammond effect). 

V. Daggett, A. J. Li, and A. R. Fersht, Combined molecular dynamics and phi-value analysis of structure-reactivity relationships in the transition state and unfolding pathway of barnase Structural basis ofHammond and anti-Hammond effects. J.Am. Chem. Soc. 120, 12740-12754 (1998). 

---