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.

SCHEME 2. Excited state reaction path for butadiene... 

Standard wavefunction methods (i.e., other than DFT), which have been extensively applied both to the computation of vertical (i.e., at ground state equilibrium geometry) excitation energies and excited state reaction paths are the current preferred method for applications in this field. Wavefunction methods that are used in studying photochemical mechanisms are limited to those that can describe excited states correctly. Unfortunately, standard methods for the evaluation of the ground state PES such as SCF and DFT cannot describe excited states because they are restricted to the aufbau principle. 

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. 

Density functional methods are competitive with the above traditional wave function methods for numerous applications such as the computation of ground-state PES. A few applications of transition metal photochemistry have been proposed on the basis of the A-SCF approach implying several approximations on the excited-state reaction-path definition by symmetry constraints not always appropriate in a coordinate driving scheme. Excited-state gradients have been recently implemented in DFT for various functionals, the feasibility of the approach having been tested for small molecules... 

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. 

Fig. 12.12. Geometry evolution along the excited state reaction path connecting Rh to CI-Rh (adapted from Ref. [8]).

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.

A photochemical reaction path begins, after photoexcitation, in an excited state and ends at ground-state products. Thus the photochemical reaction path has two branches, one on the excited-state potential surface and one on the ground-state potential surface. (Henceforth, we will use excited state, to mean excited-state potential surface. ) It has now been established, by both theoretical computations and complementary experiments, that the point where the excited-state reaction path and ground-state... 

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... 

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