Application of Marcus theory to group transfer

An important extension of Marcus theory is to reactions of the following type, which may be called displacement reactions  

Such a comparison is often possible for electron-transfer reactions. The expected relation is derived in Section 9.1.3.1 below in its simplest form it is Equation (9.41). An exactly 

II is of interest that the slopes of the Br0nsted plots of these reactions where X and Y are NO2, Cl , Br and I are close to 0.5 (the expected value if the transition state is midway between reactants and products), but smaller for OH and CN which kinetically are poor nucleophiles, i.e., have relatively high energy-barriers and hence product-like transition states (in accordance with the Hammond postulate). A similar pattern is observed for the a-deuterium isotope effects for methyl transfers, for which the cross-relation predicts that the fractionation factors will be related by the equation the view that the theoretical 

F re 8.8 Test of Marcus cross-relation for methyl-transfer reactions in aqueous solution. Comparison of observed values of activation free energy with calculated values (AG. ) for 28 reactions. From 

A related refinement of the picture of the transition-state region is concerned with the question whether the electronic and atomic motions concerned in the bond changes all take place synchronously or whether in the transition state some of them are more advanced than others. The relative extent of individual bond changes in a transition state is called the balance . Clearly any imbalance will affect the electron distribution and therefore the reactivity of the system. Theoretical agrument shows that a measure of the imbalance is the ratio of the Brpnsted coefficient for bond formation to that for bond fission. 

---