Saddle Points on the PES. Transition States

The determination of a transition state on the PES of a reacting system allows the kinetic parameters of a reaction to be calculated, using to this end standard relationships from the theory of absolute reaction rates. The geometry of the transition state structure pre-determines the stereochemical outcome of reaction. 

The quantum chemical calculations are the only source of direct information on the structure and the energetics of transition states. Although these cannot in principle be observed experimentally since the Schrodinger equations has no stationary solutions in the points not corresponding to minima on the PES, one may, according to the Bersuker theorem [28], still envisage an indirect experimental approach to a transition state structure. The fact is that over the saddle point of a PES, which corresponds to a transition state, there always exists such an excited state energy surface which at this point exhibits a minimum. Hence one may attempt to study this transition state by appropriate spectral methods. 

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