Transition States
A simple and powerful way to connect the calculated barriers to experimental rates is by means of classical transition state theory. The central assumption here is that the transition structure is in rapid equilibrium with the reactants.
One can work out a useful relation to estimate reaction rates: At room temperature, a barrier of ca 18 kcal/mol corresponds to a rate constant of one second. Another useful rule of thumb is that for every increase (or decrease) of 1.4 kcal/mol in the barrier, the rate decreases (or increases) by one order of magnitude. From this we can immediately see that the accuracy of the theoretical methods used here does not allow for accurate determination of rate constants. An error of 3 kcal/mol in the calculated barrier, which is a quite small error in this context, corresponds to an error of two orders of magnitude in the rate.
Practically, locating a transition state structure is a bit tricky and relies to a certain degree on experience and chemical intuition. In cases where a transition state is difficult to optimize, a linear transit scheme can be employed. In this scheme, one (or several) degree of freedom is chosen as a reaction coordinate and the value of this is kept fixed in steps while all other degrees of freedom are minimized. This actually contains most of the necessary information to judge a reaction mechanism energetically, provided that the reaction coordinate chosen is the correct one.
Figure:
Optimized transition state structure for the C-C bond formation step in Benzylsuccinate Synthase.