Properties of the potential energy surface relevant to transition state theory

2 Properties of the potential energy surface relevant to transition state theory 

During reaction the unit (X——Y——Z) changes from configuration a to configuration d. Any continuous path linking point a to point d is a physically possible route, and the route chosen by the vast majority of the units (X——Y——Z) will be the most probable one. 

For the activated intermediate displacements in all directions, even along the reaction coordinate, give increases in potential energy, in sharp contrast to the decrease along the reaction coordinate found for the activated complex. An intermediate is simply a normal but high energy molecule, in contrast to the critical arrangement of atoms of the activated complex. 

Note it is very important always to be aware of this very crucial distinction. 

If the PE is constant, then d(PE)/dr is zero, and the force is zero and there is no acceleration in the motion of the reaction unit as it passes through the critical configuration, i.e. through the region of the surface where the PE is constant. If there is no acceleration, then the reaction unit must pass over the barrier at a constant rate. This rate can be calculated from kinetic theory. 

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