Purely empirical estimation of rate parameters

Especially in the early stages of mechanism development, it is not likely to be practical to carry out quantum chemistry and transition state theory calculations for all the possible reactions in the mechanism. So, simpler empirical methods must be used to obtain initial estimates of the rate parameters. Perhaps the most useful description of these methods is found in the classic text. Thermochemical Kinetics, by Benson [42]. The following rules of thumb for estimating rate parameters closely follow what is presented in more detail there. For different classes of reactions, the framework of transition state theory can be applied to aid in the estimation of rate parameters. This is most 

Examples C2H6 2CH3, AICI3 AlCl2 + Cl, SiCU SiCls + Cl 

Examples 2CH3 C2H6, AICI2 + Cl AICI3, SiCl3 + Cl SiCU 

These are the reverse of unimolecular decomposition reactions. The decomposition reaction can be analyzed by the means discussed above, and then the rate constant for the association reaction can be obtained using the equilibrium constant. When doing this, it is important to check that the rate of the association reaction does not substantially exceed the gas-kinetic collision rate. If it does, then there is probably a problem with the decomposition rate constant, the equilibrium constant, or both. 

Eor bimolecular reactions, the entropy of activation is always negative, since two reactants combine into one transition state (converting translational and rotational degrees of freedom into vibrations). Eor 

---