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Quantitative Hammond Postulate BY NOAMAGMON

Summary: Quantitative Hammond Postulate
Department of Physical Chemistry,
The Hebrew University of Jersualem, Jerusalem, Israel
Received 11th January, 1977
In this paper four assumptions are made : (1) Pauling's relation (2) Bond order conservation,
(3) An approximation for the potential energy along the reaction coordinate and (4)a symmetry
principle. The main results which followare : (a)An expressionfor thebond order of the transition
state, a result which is compared with potential energy surfaces,BEBO and Marcus' theories. (b)A
theoreticalfit for (nonlinear) Bronsted plots over a mild range of ApK. In addition, other theories
which relate kinetics to thermodynamics (in particular: BEBO, Marcus' theory and Thornton's
model)are analysed. Two empiricalrelations of Mok and Polanyi and the Evans-Polanyi relation
are derived by using the symmetry principle. Marcus' theory may be related to a ``weak sym-
metry principle" ;this theory is given a new proof by Thornton's model of the inverted parabola.
Thus these theories appear to have much in common; it is possible that they all follow from one
general theory.
Consider a chemical reaction
R - , T S + P
where reactants R pass through a transition state TS to products P. A typical graph


Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem


Collections: Chemistry