Conical intersection excited-state reaction path

Figure 9.3. Cartoon of a classic double cone conical intersection, showing the excited state reaction path and two ground state reaction paths. See color insert.

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 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).

Bernardi, F., Olivucci, M., Robb, M. A., The Role of Conical Intersections and Excited State Reaction Paths in Photochemical Pericyclic Reactions, J. Photochem. Photobiol. A 1997, 105, 365 371. 

The energetics of the photoreaction of 2if-azirine as well as the thermal ground-state reaction as obtained from CASPT2 calculations are summarized in Figure 6.14. From this diagram it can be concluded that the photolysis of 2H-azirine to form nitrile ylide occurs from the nji -excited state by way of an S -Sq conical intersection. Because the reaction paths on the surface from the Franck-Condon geometry to the conical intersection as well as on the Sq surface from the conical intersection to the nitrile ylide are... 

Shemesh D, Domcke W (2011) Effect of the Chirality of Residues and gamma-Turns on the Electronic Excitation Spectra, Excited-State Reaction Paths and Conical Intersections of Capped Phenylalanine-Alanine Dipeptides. ChemPhysChem 12 1833-1840... 

Fig. 7. The excited-state reaction path of the penta-2,4-dienimminium cation intercepts the conical intersection point CI700 located ca. 5 kcal mol above the minimum-energy conical intersection CIq2o. FC— Cl7oo MEP ( i open diamonds and So full diamonds), Cl7oo Cl92o MEP (5i open circles and So full circles). The values of the relevant structural parameters are given in A and degrees.

In Figure 3.3 we show (1) the ground-state reaction path (CHD TSSd cZc-HT and a second TSs ), (2) the excited-state reaction path (FC CHD Mins,), and (3) the conical-intersection seam (Colntu CoInxs2 Coinxs, Coinmin Colnq,). The complete minimum energy So and Si reaction paths (X3) for the conrotatory... 

As we have previously underlined, the low-lying conical intersections could be only provided through the computation of the photochemical minimum energy path (MEP). However, some cases have been documented where excited state reaction path does not necessarily hit the lowest energy point belonging to the intersection space (IS) and the decay may not occur in this region. One of these cases concerns the excited state relaxation path of the 3db-PSBll... 

Accordingly, the reaction path then proceeds via the Ag excited state on the excited state PES until the conical intersection region is reached, passing through an excited state minimum. At the conical intersection, the molecule drops down to the ground... 

Part of the explanation that follows concerns the reasons why a conical intersection may be found in a particular system. This part could be skipped over in a first reading, as it is more mathematical. Nevertheless, it is closely connected with an explanation of the shape of a conical intersection, which in turn determines the crossing s accessibility on the excited state, and the subsequent reaction paths on the ground... 

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... 

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