Conical intersections pericyclic reactions

Except in the case of very exothermic reactions (cf. Example 6.7), molecules that have traveled from the reactant geometry to the biradicaloid minimum will have little chance of reaching the product geometry on the same surface, because the return to Sg from the pericyclic minimum or a nearby conical intersection will be very efficient. 

The global term pericyclic funnel will be used to refer to the funnel or funnels in the S surface that occur at the critically heterosymmetric biradicaloid geometries reached near the halfway point along the path of a thermally forbidden pericyclic reaction, and the minima in S, that are encountered along one-dimensional cuts along reaction paths that miss the conical intersections (in particular, those along high-symmetry paths, which pass... 

Figure 6.11. Schematic correlation diagrams for ground-state-forbidden pericyclic reactions a) HMO model of Zimmerman (1966), b) PPP model of van der Lugt and Oosterhoff (1969), and c) real conical intersection resulting from diagonal interactions. The two planes shown correspond to the homosymmetric (y) and heterosym-metric (6) case. Cf. Figure 4.20.

Finally, in a remarkable series of recent papers, Bernard , Olivucci, Robb and their collaborators (1990-1994) demonstrated that the S -S touching actually is not avoided even in the low-symmetry case of real organic molecules, and they confirmed the earlier conjectures by computing the actual geometries of the funnels (conical intersections) in the S, surface at a reasonable level of ab initio theory. They also pointed out that still additional reactions can proceed through the same pericyclic funnel, such as the cis-trans isomerization of butadiene. 

Bernardi, F., Olivucci, M., Robb, M. A., The Role of Conical Intersections and Excited State Reaction Paths in Photochemical Pericyclic Reactions, J. Photochem. Photobiol. A 1997, 105, 365 371. 

Bemardi F, De S, Ohvucci M, Robb MA. Mechanism of Ground-state-forbidden photochemical pericyclic-reactions - evidence for real conical intersections. J Am Chem Soc. 1990 112 1737-1744. 

Bernardi, R, De, S., Olivucci, M., 8c Robb, M. A. (1990). The mechanism of ground-state-forbidden photochemical pericyclic reactions Evidence for real conical intersections. Journal of the American Chemical Society, 112(5), 1737-1744. 

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