Hammond s principle

It is often very difficult to obtain detailed information about such species, and the intermediates, of which the transition states are the immediate predecessors, are thus often taken as models for them, because detailed information about such intermediates is much more readily come by. This may be justified on the basis of Hammond s principle that in a sequence, immediately succeeding species that closely resemble each other in energy level are likely to resemble each other in structure also certainly the intermediate (6) in the sequence above is likely to be a better model for T.S., than is the starting material. We shall see a number of examples subsequently where o complexes are used in this way as models for the transition states that precede them (cf. p. 151). 

Hammond s principle states essentially that if two states along the reaction coordinate have nearly the same energy, their molecular structures will resemble each other very closely and thus their entropies will also be closely related. This principle can be used, with caution, to guess values of the entropy of the transition state. Thus, if an elementary step is exothermic, the structure of the transition state is expected to resemble that of the reactants rather than that of the products. The reverse is true for an endothermic step. 

Basic Mechanistic Concepts Kinetic versus Thermodynamic Control, Hammond s Postulate, the Curtin-Hammett Principle... 

SECTION 4.4. BASIC MECHANISTIC CONCEPTS KINETIC VERSUS THERMODYNAMIC CONTROL, HAMMOND S POSTULATE. THE CURTIN-HAMMETT PRINCIPLE... 

Hammond s postulate (Section 4.8) Principle used to deduce the approximate structure of a transition state. If two states, such as a transition state and an unstable intermediate derived from it, are similar in energy, they are believed to be similar in structure. 

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