Quasi-Equilibrium and RRKM Theory

The recombination activation energy Er of a radical with a cation is low. As is shown, this means A El=Ea-Er Ea. 

In thermal reactions, the reaction rate depends on the activation-deactivation equilibrium and on the decomposition rate of the activated complex  

Values of 2 are difficult to deduce for thermal reactions in the condensed phase because the rates measured are always combinations of these three steps. In the gas phase, however, a 2 is directly measurable, but the values are not directly transferable to the reactions in the liquid phase, since the reacting species in the gas phase are not solvated. 

Two almost identical theories explaining the phenomena observed in the case of unimolec-ular reactions in the gas phase at high vacuum were proposed in 1952. One of them, the quasi-equilibrium theory (QET), was suggested by Rosenstock et al. [2] and applies to mass spectrometry. The other is named after the initials of its authors, RRKM, standing for Rice, Rampsberger, Kassel and Marcus [3], and deals with neutral molecules. 

Both are based on assumptions and postulates. The first assumption is that the rotation, vibration, translation and electronic movements are independent of each other. The second assumption states that the movement of the nuclei can be expressed by classical mechanics. However, some quantum mechanical corrections are used. 

---