Conical intersection MECIs

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. 

The position and energy of the minimum energy conical intersection (MECI) are given by... 

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. ... 

At the top left-hand side of Figure 3.19 we show the branching space for a conical intersection between A - - B and A — B" along with a MECI of... 

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... 

---