Activation Energy and Pre-Exponential Factors in the Reaction Rate Constant Expression

2 Activation Energy and Pre-Exponential Factors in the Reaction Rate Constant Expression 

Calculation of the reaction rate constant by the transition-state method requires knowledge of the activation energy Ea and of the activated complex partition function F . The accuracy of the theoretical potential energy surfaces is inadequate for the determination of Ea. The only exceptions are the diatomic molecules for which accurate theoretical potential curves are available and also some three-atom systems, especially H3. Therefore, either semiempirical methods or independent experimental information have to be used to obtain Ea [37, 236]. 

The situation with dissociation reactions is most favorable in this respect because, as a rule, it may be assumed that Ea for these reactions coincides with the reaction heat Q. For exchange reactions, the reaction path profile displays a potential barrier. Therefore, Q obtained from thermochemical data is not directly related here to the activation energy value. Nevertheless, it can be expected on the ground of qualitative theoretical arguments [230] that for similar reactions 

Consider now the calculation of the pre-exponential factor. Assuming x = 1 the preexponential factor will be equal to the product of the frequency factor kT/27rh and of the ratio of partition functions F /F. In the calculation of F, it is usually assumed that different motions in molecules can be separated. Then F may be represented as the product of the translational (F d, rotational vibrational (Fyib) and electronic (Fgi) partition 

Estimate now the pre-exponential factors for simple reaction types representing k in the form 

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