Conical intersection structures

Now let us return to our discussion of the conical intersection structure for the [2+2] photochemical cycloaddition of two ethylenes and photochemical di-Jt-methane rearrangement. They are both similar to the 4 orbital 4 electron model just discussed, except that we have p and p overlaps rather than Is orbital overlaps. In Figure 9.5 it is clear that the conical intersection geometry is associated with T = 0 in Eq. 9.2b. Thus (inspecting Figure 9.5) we can deduce that... 

Figure 11 Computed Sj/S0 conical intersection structure for benzene. The relevant geometrical parameters are in angstrom units. The —(CH)3— kink is framed. (From Ref. 25).

The all-tnms-hepta-2,4,6-trieniminium cation (2), a retinal protonated Schiff base model, may undergo trans cis isomerization of the double bond at either position 2 or 4. Thus, the photochemistry is dominated by the structure of the competitive excited state reaction paths leading to distinct conical intersection structures. 

Figure 26 Plot of the 7t-electron density for the degenerate S0 and Sj states at the conical intersection structure (Cl). The arrows indicate the number of electrons migrated from the CH2CHCH— allyl fragment to the —CHCHNH2 fragment. (From Ref. 37.)...

Figure 7.2. Schematic representation of the conical intersection structure for cis-trans isomerization of c -hexatriene and of some of the possible bond-making processes that might occur along different ground-state relaxation paths (by permission from Olivucci et al.. 1994a).

Similar organic chromophores (e.g. conjugated hydrocarbons) have similar conical intersection structures. 

Figure 2.22 Top view of the ab initio optimized conical intersection structure CIchd- The relevant geometrical parameters are given in A.

A converged vinoxy X-A conical intersection was successfully located at 12.8 kcal/mol above the A-state equilibrium level,with geometry shown in the X-AJ column in Table 7.8. A B - conical interaction was similarly located at 12.2 kcal/mol above the B-state equilibrium, with geometry shown in the final column of Table 7.8. Figure 7.8 shows ball-and-stick diagrams of the CASNBO-optimized equilibrium and conical intersection structures for visual comparison. 

Table 7.13 shows details of the NBOs and occupancies for and B Col conical intersection structures. Com-... 

Table 1. List of photochemical reactions where a conical intersection structure has been shown to be related to the structure of the photoproducts.

It is also possible to find MECI for 2 -f 2 and 2 + 2 structures for this cycloaddition. Such structures He on the same seam (see F re 3.1) as the 2s + 2s structure just discussed. Thus the conditions of Eqns (3.6) and (3.7) hold, however, Q changes. (Q is the coulomb energy corresponding to the energy of the system if all the Ky were zero.) Thus the 2s + 2 and 2a 4" 2a conical intersections are much higher in energy. The same type of conical-intersection structure is also found for the ultrafast deactivation of an excited cytosine-guanine base pair in DNA. ... 

II = A — B (a) X and (b) X2 and (rhs, Cj) for I = A + B and ll = A — B + C — E (see Figure 3.18). Note that the D2h conical intersection structure (left-hand side) is a rather idealized structure and is very high in energy, while the Cs structure is the lowest energy point on the conical intersection MECI) and corresponds to the conical intersection on the reaction path shown in Figure 3.15. 

Now let us turn to a much more difficult problem the 8 orbitals with 8 electrons photochemical addition of ethylene to benzene (Figure 3.23). At first sight this system would seem intractable for the VB approach. And, in fact, it is not easy to get analytical results such as those obtainable for the 6 orbitals with 6 electrons problem. Accordingly, our strategy, similar to the 6 orbitals with 6 electrons case, is to analyze the computed conical-intersection structures obtained at the ab initio level using the MMVB method and thus deduce the VB analysis. The end result rationalizes a complicated reaction mechanism (more than 10 MECI were located) quite simply as we will now discuss. The details can be found in our recent... 

In O Fig- 39-7 we plot the branching plane vectors (Xi and X2) at the conical intersection of 3db-PSBll model. The conical intersection structure features one highly twisted double bond (about 92°) and involves two electronic configurations, an ionic and a covalent state, that differ for the transfer of one electron between the C5-C4-C3- and -C2-Q-N fragments. 

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