Energy transfer in the activation step

The change over from first order kinetics, when the reaction step is rate determining, to second order kinetics, when activation is rate determining, depends on the total pressure, the complexity of the reactant and to a lesser extent on the temperature. 

Remember, activation is bimolecular, while reaction is unimolecular. 

If the molecule is small, there are fewer normal modes over which the critical energy can be spread, and it is much more likely that the required critical energy 

---