Conical intersection photochemical reaction path

Now let us turn to the situation where the reaction path does not lie in the plane X1X2. In this case we need three coordinates to define the course of a photochemical reaction through a conical intersection the reaction path X3 and the coordinates X1X2. In order to draw a picture similar to Fig. 7.2, we would need four dimensions. 

We have shown that an accessible conical intersection forms a bottleneck that separates the excited state branch of a nonadiabatic photochemical reaction path from the ground state branch, thus connecting the excited state reactant to two or more products on the ground state surface via a branching of the... 

Figure 16 (a) The three different electron recoupling patterns from the conical intersection shown in Figure 15. (b) The branching of the photochemical reaction path through a conical intersection. 

Figure 2.19 Schematic of a photochemical reaction path involving a conical intersection. The path starts on the excited-state potential energy surface and decays to the ground-state potential energy surface through a conical intersection, leading to the formation of different photoproducts 1, 2, and 3.

Fig. 13. The computational methods used for constructing a photochemical reaction path. The full path is computed by joining different MEPs, each one providing information on a specific part of the excited- or ground-state potential-energy surface. The IRD method is used to compute the steepest relaxation directions departing from the FC point (excited-state relaxation) or Cl (ground-state relaxation). The IRC method is used to compute the steepest-descent line defined by the computed IRDs. The CIO method is used to compute the lowest-energy conical intersection point directly. With TSO we indicate the standard transition structure optimization procedure.

In the preceding sections we show that, by postulating simple VB structures on a photochemical reaction path, one can deduce not only that a conical intersection may be involved but also the nature of the branching space of the conical intersection. For problems such as 3 orbitals with 3 electrons or 4 orbitals with 4 electrons it is simple to manipulate the VB matrix elements to make these deductions. By the time one gets to 6 orbitals with 6 electrons there are very many possibihties. So one has to leam " by extracting the VB structures from the ab initio data. For the 6 orbitals with 6 electron case, we use the MMVB method to do this. Once the more important structures are identified this way, we can perform the manipulations analytically to confirm the result by comparison with numerical data. Finally, for 8 orbitals with 8 electrons we were able to show that one may also extract the VB data from the MMVB method and come to understand the nature of the conical intersection. However, it is rather tedious to do the calculations analytically and this work has never been carried out. 

Our hypothesis for discussion in this section has been that the conical intersection can be characterized like any other reactive intermediate. On examining Figure 9.3 or 9.10, it is clear that a conical intersection divides the excited-state branch of the reaction path from the ground-state branch in a photochemical transformation. (We shall... 

Figure 6.6. Schematic representation a) of the transition state of a thermal reaction and b) of the conical intersection as a transition point between the excited state and the ground state in a photochemical reaction. Ground- and excited-state reaction paths are indicated by dark and light arrows, respectively (adapted from Olivucci et al., 1994b).

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