The RRKM Mechanism for Unimolecular Gas Phase Reactions

The formation of the transition state from the excited molecule is referred to as a microcanonical process, while the formation of the transition state in conventional TST in Chapter4 and in VTST in Chapter 6 is referred to as canonical process. The terms microcanonical and canonical in statistical mechanics refer respectively to processes at constant energy and processes at constant temperature. 

The rate constants 8ki, k2, ka, k3 in the equations above have meaning for example, Equation 14.1 means 

Here [A] is the concentration of A. and t refers to time. 8ki (E) is a differential rate coefficient for the reaction in which a molecule is excited to an energy between E and E + 8E. Thus, Equation 14.5 is the equation for the rate of appearance of A molecules with energy between E and E + 8E. 

After some rearranging, this equation yields the following result for [A ], 

Equation 14.11 explicitly contains the equilibrium assumption of transition state theory, i.e. that the transition state and the excited molecules are in chemical equilibrium. 

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