In collision theory, the detailed nature of the interactions between reactants was not considered. The detailed interactions are best represented through the potential energy surface which will be considered in the next lecture. From the Arrhenius equation, only those collisions with the minimum required energy can lead to the products. Consider one such path which is represented in Fig 33.1.
Figure 33.1 Potential energy of reaction as a function of the reaction coordinate.
The ordinate is the potential energy (PE) and the absissa is the reaction coordinate. At a specific state (configuration of the reactants), the potential energy is maximum, the slope is zero and the PE falls to lower values in both the forward and the reverse direction. All the structures in the vicinity of this transition state may be considered as the "activated complex", which is very reactive. A motion along the "forward" direction will lead to the products. The activated complex theory or the transition state theory provides a way to calculate the rate constant for the reaction. The assumptions involved in the transition state theory are:
1)
The electronic motion (which can only be described only quantum mechanically) may be separated from the motion of the nucleii and a classical description of the nuclear motion is used to evaluate the rate constant.
2)
The energy distribution of the reactants is described by the equilibrium Boltzmann distribution throughout the reaction.
3)
The activated complex which has crossed the transition state can not return to the reactant configuration.
4)
In the transition state, motion along the reaction coordinate may be separated from all other motions.
5)
The activated complex is also distributed according to the Boltzmann distribution even when the reactants and products are not in equilibrium. This is often called the quasi equilibrium postulate.
