Multi-Excitation Entropy, Entropy-Enthalpy Relations, and their Impact on Catalysis

Catalyzed reactions involve large activation enthalpies, requiring a fluctuation in which many excitations, in the form of infrared vibrations or phonons, are needed to surmount the activation barrier. It was previously shown that this results in a multiexcitation entropy (MEE) term in the free energy of activation. For a series of related reactions, this leads to an isokinetic rule. We show here that this almost always implies that the series will also obey an isoequilibrium relation. This provides a necessary and sufficient condition for the Hammett substituent constant equation to be obeyed. Finally, we discuss the importance of MEE in catalysis. This has not generally been recognized.