Funnel diagonally distorted

Which of several possible products will be formed and whether photocycloaddition will occur at all will depend on the depth of the various minima on the excited surface S, on the height of the barriers between them, and on the partitioning between starting materials and products upon return to S . Additional complications result if the diagonally distorted funnels give rise to products of crossed cycloadditions. 

According to Figure 6.17, the height of the barrier between the excimer minimum and the pericyclic funnel depends both on their depths and on the relative placement of the two excited-state surfaces S and D. The depth of the diagonally distorted pericyclic funnel is determined by the nature of the biradical its dependence on molecular structure, on the head-to-head and head-to-tail orientation of the components, and on reaction medium can be discussed using the principles outlined in Section 4.4.1. 

Photodimerization often involves an excimer that can be treated as a su-permolecule. (Cf. Section 6.2.3.) Then, the state correlation diagram for the singlet process (Figure 7.27a) ordinarily calls for a two-step return from S, to So along the concerted reaction path. First, an excimer intermediate E is formed. Second, a thermally activated step takes the system to the diagonally distorted pericyclic funnel P" (cf. Section 4.4.1), and the return to So that follows is essentially immediate. The reaction will be stereospecific and concerted in the sense that the new bonds form in concert. However, it will not be concerted in the other sense of the word, in that it involves an intermediate E. ... 

It is likely that [2 + 2] and x[2 + 2] cycloadditions proceed through the same type of diagonally distorted pericyclic funnel (Section 4.4.1) with a preservation of the diagonal interaction, and eventual production of two di-... 

Next, we consider also the effects of rhomboidal distortions, which permit a diagonal interaction. These may either reinforce or counteract the effect of the substituents, depending on which of the two diagonals has been shortened. For a head-to-tail approach of two substituted ethylenes, there will be two funnels corresponding to 1,3-disubstituted critically heterosym-metric biradicaloids, one at less and one at more diagonally distorted geometry than for the unsubstituted ethylene. This is confirmed by the results of calculations shown in Figure 7.30. 

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