Polanyi’s rules

From the many readings available about Polanyi s life and scientific career two are recommended herein one is written by Eugen P. Wigner (1902-1995, Nobel Prize 1963) who was Polanyi s student in Berlin in the 1920s and lifelong friend [vi], and the other by his son John C. Polanyi (1929-, Nobel Prize in Chemistry 1986) who continued the family tradition regarding the study of reaction dynamics [vii]. Additional information on Polanyi can be found on the Polanyi Society home-page [viii]. See also Polanyi s rule. 

Polanyi s rule - Polanyi and co-workers [i-iii] established that for homologous reaction series a simple linear relationship exists between the - activation energy (Q) and the heat of reaction (A ) ... 

Today Polanyi s rule is used in the following form ... 

Polanyi s rule — Figure 1. Illustration of the relationship between Q and from [i]... 

One of the first attempts to compare the effect of different forms of energy on gas-phase reactions was done in a very general way by John Polanyi [70, 73]. When the reaction barrier is in the entrance or the exit channel of the reaction, translational or vibrational energy, respectively, is more efficient at promoting the reaction. These rules, known as Polanyi s rules, are not linked specifically to electron-transfer reactions. On the contrary, they were derived without reference to a specific reaction. As an illustration of these rules in an electron-transfer reaction, vibrational excitation of HCl gives easier access than translation to a late barrier in the K-hHCl reaction [123, 124]. 

For both these exothermic reactions, the potential barriers are early , that is, reactant-like , in accordance with Polanyi s rules . One consequence is that the transitional modes in the transition states have low frequencies and the partition function for the internal modes in the transition state (see earlier, eqn (1.16)) will be strongly temperature-dependent, providing at least a partial reason for the positive temperature-dependence of the pre-exponential factor in the expressions for the rate constants. 

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