Recent computational and experimental studies of organic reactions that show sequential transition structures on the potential energy surfaces are reviewed. The specific focus is on pericyclic and pseudopericyclic reactions. A distinction is made between symmetric and unsymmetric systems; in the former, the intrinsic reaction coordinate connects the two transition structures. The importance of the change in the reaction coordinate for establishing the existence of the second transition structure and the influence of the energy of the second transition structure on the barrier height of the first and emphasized. In this latter context, corners on a reaction pathway are also significant. Lastly, the merging of two sequential transition structures is shown to give an “effective monkey saddle” transition structure.
Keywords: Energy Landscapes, Born-Oppenheimer approximation, The Murrell-Laidler theorem, Diels-Alder reaction, thermal electrocyclic ring, facile retro [2+2] reaction, Berson correlation, monkey saddle transition state, Intrinsic reaction coordinate (IRC)
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