Conical intersection ground-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 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... 

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... 

Figure 9 Potential energy of So H3 plotted at fixed distances from an origin corresponding to the conical intersection (at ri2 = r2j = rn). The ground state reaction paths correspond to minima (blue) on this surface...

The outline of the remainder of this contribution is as follows. In Section 3.7.2, we discuss radical anion dissociation in solution, in which a conical intersection has an important impact on the ground state reaction barrier, rate constant and reaction path, all of which are also influenced by nonequilibrium solvation. The excited electronic state conical intersection problem for the cis-trans isomerization of a model protonated Schiff base in solution is discussed in Section 3.7.3, focusing on the approach to, and passage through, the conical intersection, and the influence of nonequilibrium solvation thereupon. Some concluding remarks are offered in Section 3.7.4. We make no attempt to give a complete discussion for these topics, but rather focus solely on several highlights. Similarly, the references herein are certainly incomplete. We refer the interested reader to refs [1-9] for much more extensive discussions and references. 

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).

Benzene displays a symmetric Cs symmetry) S /So conical intersection between states that correlate with the E2g state 2 in the FC region) and the ground state of the planar equilibrium structure. hS9-92 Scheme 12, we show that the electronic character of the anti-aromatic (i.e. difference of the two Kekule structures indicated with a dashed circle) Si surface B2u) changes along the reaction path due to an avoided crossing between the S2 and Si states. The S2 CE2g) state can be described by a combination of quinoid (Dewar type) and antiquinoid spin couplings. As seen in Fig. 8(a)... 

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... 

Figure 11-9. CASSCF potential-energy profiles of the ground-state So (circles), the lnjr state (triangles), the Lb state (squares), and the La state (filled squares) of the 9H-adenine along the linear interpolation reaction path from the equilibrium geometry of the nit state to the CI32 (a) and CI16 (b) conical intersections. The diabatic correlation of the states is shown in (a). (From Ref. [138])...

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