Potential energy surfaces and profiles

Importantly, motion along the reaction coordinate at the saddle point has no restoring force, so the transition structure has only 3 N — 7 real vibrations. Indeed, passage through the saddle corresponds to conversion of the vibration along the reaction coordinate into a translation, so that ksT = hv, where v is frequency of this hypothetical vibration. At 25°C, v = 6 x 1012 s 1 equivalently, the lifetime of a transition structure is 1.7 x 10-13 s and methods capable of observing labile species on this time scale ( transition state spectroscopy ) have been developed, permitting many of the assumptions about their behaviour to be tested directly [1], 

The plot of PE against progress along the reaction coordinate provides the familiar reaction profile of the mechanistic chemist, with the PE maximum in the profile corresponding to the saddle point on the PE surface Fig. 9.1a shows the simplest case. Reactions where a single transition structure ( ) separates reactants and products are said to be single-step or concerted elementary processes. 

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