Marcus theory Hammond postulate

THE BELL-EVANS-POLANYIPRINCIPLE/HAMMOND POSTULATE/MARCUS THEORY... 

The Marcus equation allows AG for RX + Y —> RY + X to be calculated from the barriers of the two symmetrical reactions RX + X - RX + X and RY + Y — RY + Y. The results of such calculations are generally in agreement with the Hammond postulate. Marcus theory can be applied to any single-step process where something is transferred... 

The Bell-Evans-Polanyi Principle/Hammond Postulate/ Marcus Theory---------------------------------------------... 

The Hammond postulate is often accepted as a general principle an increase in reactivity is accompanied by a decrease in selectivity because the transition structure becomes closer to that of the reactant state as the energy barrier decreases. This idea has some truth for a hypothetical A to B reaction model where it is implicit that only a single bond change occurs moreover the Hammond postulate is predicted by the Marcus theory (above). The postulate often breaks down for reactions where more than one major bond change results in product formation. It should be emphasised that any discussion of the reactivity-selectivity relationship has to be confined to those reactions where there is no change in rate-limiting step or mechanism. 

Many reactions exhibit effects of thermodynamics on reaction rates. Embodied in the Bell-Evans-Polanyi principle and extended and modified by many critical chemists in a variety of interesting ways, the idea can be expressed quantitatively in its simplest form as the Marcus theory (15-18). Murdoch (19) showed some time ago how the Marcus equation can be derived from simple concepts based on the Hammond-Leffler postulate (20-22). Further, in this context, the equation is expected to be applicable to a wide range of reactions rather than only the electron-transfer processes for which it was originally developed and is generally used. Other more elaborate theories may be more correct (for instance, in terms of the physical aspects of the assumptions involving continuity). For the present, our discussion is in terms of Marcus theory, in part because of its simplicity and clear presentation of concepts and in part because our data are not sufficiently reliable to choose anything else. We do have sufficient data to show that Marcus theory cannot explain all of the results, but we view these deviations as fairly minor. 

The Bell-Evans-Polanyi Principle/Hammond Postulate/Marcus Theory... 

Using Eq. 7.64, show how Marcus theory nicely reproduces the predictions of the Hammond postulate by considering values of AG° that represen t exergonic and endergonic reactions. What kinds of reactions would you expect would not... 

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