Tests of transition state theory

TST predicts the trend of decreasing Arrhenius pre-exponential factor with increasing reactant size and molecular complexity that is revealed by experimental measurements of rate coefficients, and that SCT explained away by invoking the steric factor. This trend arises in TST through the internal degrees of freedom, which are accounted for in the partition functions, and which are not present in the structureless point masses of SCT. If electronic, vibrational, rotational, and translational dfs are independent, the molecular partition function factors into electronic, vibrational, rotational, and translational contributions, i.e., Q = e v r t-The orders of magnitude of the partition functions per df are v l-10 per vibrational or internal rotational df, per overall 

The above estimates show that TST is at least qualitatively correct, but they do not address the question of whether the theory is capable of providing useful estimates of rate coefficients for modeling purposes. A limitation of TST is that it cannot, by itself, provide any information on barrier heights. Historically, tests of the theory have been limited to comparison with experimental rate coefficients when a reliable experimentally determined activation energy is available. Because Eq in equation (80) is not the activation energy, some manipulation is necessary to relate theoretical quantities with the observed pre-exponential factor. See, for example. Pilling and Seakins [34] for appropriate relationships. The partition functions for the reactants can be evaluated from 

The obvious ambiguity of the historical approach is eliminated by employing modern electronic structure calculations, which are capable of giving stationary points to chemical accuracy, and reliable vibrational frequencies and moments of inertia not only for the reactants, but also 

---