Two Examples of Pericyclic Funnels

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. 

Sections 6.2.1.1 and 6.2.1.2 describe the pericyclic funnel in more detail for two particularly important and illuminating examples the cycloaddition of two ethylene molecules and the isomerization of butadiene. We rely heavily on the results of recent ad initio calculations (Bernardi et al., 1990a Olivucci et al., 1993). 

we consider the face-to-lace addition of two ethylene molecules. The orbital correlation ilia lam Ibr the liigh-syninieti y path analogous to Ihc rec- 

In the case of the [2 + 2] photocycloaddition of substituted ethylenes, the two situations remain equivalent for head-to-head addition (1,2 arrangement of the substituents) but are no longer equivalent for head-to-tail addition (1,3 arrangement of the substituents), since the latter affects the energy difference between the two localized forms of the nonbonding orbitals. Ac- 

Monomolecular photochemistry of butadiene is rather complex. Direct irradiation in dilute solution causes double-bond cis-trans isomerization and rearrangements to cyclobutene, bicyclo[l.l.01butane, and l-methylcyclo-propene (Srinivasan, 1968 Squillacote and Semple, 1990). Matrix-isolation studies have established another efficient pathway, s-cis-s-trans isomerization (Squillacote et al., 1979 Arnold et al., 1990, 1991). 

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