Qualitative Models for FERs

A general group transfer reaction (19.1a) may be considered as the sum of the two half-reactions 19.1b and 19.1c (in which homolysis or heterolysis is deliberately not specified). 

Bell [5] considered the case of X = H, representing proton transfer between a Bronsted acid and a base, and noted that, if A l- and were, respectively, proportional to log k and log K, Eq. (19.2) was equivalent to the Bronsted relation (Eq. (19.3)) where k is the rate constant and K is the equilibrium constant for the acid-base reaction of Eq. (19.4). 

These simple considerations yield several corollaries, sometimes known together as the Bell-Evans-Polanyi (BEP) principle [14]. First, there is an approximately linear relation between the barrier height and the reaction energy this is the basis of the Bronsted relation (and other LFERs). Second, the proportionality constant a in Eq. (19.2) tends to be smaller for exothermic reactions (but larger for endothermic reactions). Third, the position of the crossing point between the curves lies closer to the reactants for more exothermic reactions this is the basis of the Hammond postulate, that the TS for a more exothermic reaction more closely resembles the reactants (and that for a more endothermic reaction more closely resembles the products). 

Murdoch [15] pointed out that the quantitative barrier expressions given by Lon-don-Eyring-Polanyi-Sato [16], Johnston and Parr [17], Marcus [6, 18], Murdoch 

If the interpolation functions in Eq. (19.5) take the values = AE J4AE fi- and fi this relationship between the barrier and the overall energy change for reaction becomes the Marcus relation, Eq. (19.6), whose range of applicability is 

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We begin by reviewing briefly some of the qualitative models for FERs before surveying some examples of simulated FERs for proton transfer (PT) reactions. These include molecular orbital (MO)-based studies of potential energy surfaces (PES) for gas-phase reactions and valence bond (VB)-based studies of free energy changes for reactions in condensed phases. 

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