Funnel region

Figure 6.13. Pericyclic funnel region of ethylene dimerization, showing two equivalent conical intersections corresponding to 1,3 and 2,4 diagonal interactions and the transition slate region at rectangular geometry (a = 0). The curves shown for a = 0 correspond to the van der Lugt-Oosterhoff model (by permission from Klessinger, 1995),...

Figure 6.16. The funnel region for butadiene isomerization a) cross section of the excited-slate potential energy surface for fi = a, where a, and a, correspond to rotations about the two C=C bonds of butadiene and p lo rotation about the single bond and b) optimized geometries of the three points of minimal energy within the funnel (by permission from Olivucci et al., 1993).

The reaction pathways both for single- and double-bond isomerization enter the funnel region at less than one-third of the way toward the products, suggesting that the majority of excited-state molecules should decay back to the ground-state reactant. In addition, the excited-state barrier for singlebond isomerization is smaller than that for double-bond isomerization, reducing the quantum yield for cis-trans isomerization (since the product of s-cis-s-trans isomerization is just a different conformer, not a new cis-trans isomer of the reactant). Thus the low experimental value of = 0.034 reported for the quantum yield of /ran.v-hexatriene from r/.v-hexatriene (Jacobs... 

Schematic surface diagram for a diabatic photoreaction. Excitation is followed by a geometry change on Sj toward the funnel region, designated by a box in the figure. At the minimum on S, relaxation to So occurs. Depending on the precise nature of this jump from one surface to another, the photochemistry can be productive (producing product) or non-productive (reforming starting material).

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