Hammond effect

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,... 

Figure 2.3 The basis of the Hammond postulate and Hammond effect. The Gibbs free energy curves for a substrate and product (heavy lines) intersect on the reaction coordinate at the position of the transition state. A change of structure in S, away from the seat of reaction, destabilizes it, displacing its energy curve (lighter line) higher. The point of intersection moves closer to the substrate.

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... 

An extensive review with many examples125 shows that the reactivity-selectivity principle cannot be used to predict the selectivity of a reaction except in unique systems where one reaction is close to or diffusion controlled. The relative importance of the Hammond effect and the frontier-orbital effects determines the reactivity-selectivity relationship that will be found in a particular system. The review also concludes that the Hammond-Leffler a-value cannot be used as an indicator of transition-state structure. 

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. 

The Hammond and Cordes-Thornton coefficients can be utilised to characterise transition states in terms of structure reactivity surfaces as follows. The Hammond coefficient is defined so that it is positive when an increase in energy of the products (relative to reactants) accompanies an increase in the first derivative. This describes the generalisation (Hammond effect) that endergonic reactions tend to have product-like transition structures. A positive Hammond coefficient is expected for a fundamental process where structural change affects the energy of one end of a reaction coordinate, as a consequence of the maximum (negative curvature) at the saddle-point in the direction along the reaction coordinate. 

Both quadratic and quartic equations for a fundamental process satisfactorily model the Hammond effect whereby the transition structure moves towards that of the reactant as the fundamental process becomes more exergonic. 

Very similar results are found for reactions of quinuclidines with phos-phorylated pyridine, which follow a Brpnsted slope of Pnuc = —0.1. The less reactive phosphorylated 4-morpholinopyridine follows a slope of (3nuc = -0.01 (12). The changes in (3nuc for these phosphorylated pyridines and phosphate esters with a poorer leaving group (lg) represent Hammond effects that are described by the cross-interaction coefficient, pxy, of equation 3. This cross coefficient is larger than the direct coefficient that describes... 

Obopile, M. R.B. Hammond. Effects of delayed harvest on soybean seed quality following bean leaf beetle (Coleoptera Chrysomelidae) pod injury. J.Kansas Entomol. Soc. 2001, 74, 40—48. 

Another suggestion (, 10) has been that the existence of the limiting rate might be explained by what has been called the Hammond Effect. As one increases the reactivity along a series of suitable substrates the transition state will become... 

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... 

Along the diagonal the transition state shifts toward the corner that is raised in energy and away from the corner that is lowered in energy (referred to as a Hammond effect). 

The effect of making X more electron withdrawing is to stabilize the complexes in the corners B and D relative to A and C. As a result the transition state is shifted towards corner A ("Hammond effect") and toward corner B ("Thornton effect") as shown in Fig. lb. The net result of the two effects is to shift the transition state in the direction of more proton transfer and a should increase. This is again borne out by the results a similar trend in a was found for the phenoxide ion departure from substituted benzaldehyde methyl phenyl acetals (13). 

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). 

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