Photochemical reactions, energy profiles

Pross and Shaik [23] have proposed a qualitative valence-bond (VB) configuration model to describe how reaction energy profiles can be built from VB configurations. Structures for the transition states such as the ones previously presented can be considered within that VB-model. Yates [24] has recently made a comparative study of several intersecting state models, with respect to the problem of photochemical proton transfers, and has concluded that ISM is one of the most general. 

A photochemical reaction coordinate has two branches an excited state branch and a ground state branch that is reached after decay at a conical intersection. Thus a conical intersection between ground and excited states of a molecule is a precursor to ground state reactivity, and conforms to the above definition of a reactive intermediate. The main focus of our article will be to develop this idea. In Figure 9.1b, we show the energy profile for a photochemical reaction with a conical intersection... 

Figure 9.1. Energy profile for (a) a thermal reaction showing a high-energy intermediate, and (b) a photochemical reaction showing a conical intersection.

F ure 9.26. Energy profile along (a) the reaction coordinate at an avoided crossing for a photochemical reaction and (h) an electron transfer process. 

Figure 3.1 Energy profiles for photochemical and thermal reactions.

Figure 1. Energy profile for a general endergonic photochemical reaction Ef, is the minimum energy gap between the lowest vibrational levels of the excited state R and the ground state R of the absorber. Er is the activation energy for the back reaction P R.

FIGURE 3.1 Comparison of the energy profile of thermal and photochemical reaction courses. R and R are reactants, TS and TS are transition states, E, and E,, activation energies for the ground and excited states, respectively IP, intermediate photochemical product P and Pph, products of thermal and photochemical reactions, respectively. 

Fig. 12.11. CASPT2//CASSCF/AMBER energy profile for the Rh—> bathoRh photochemical reaction compared to the experimental values (in bold) (redrawn with permission from Ref. [50] 2004 by The National Academy of Sciences of the USA).

FIGURE 5.1 Energy profiles for competing a-deprotonation and a-desilylation reactions of zwitterionic biradicals 44 and 45 serving as intermediates in SET-promoted photochemical reactions of bis-donor substituted phthalimides. See Scheme 5.17 for the structures of the zwitterionic biradicals 44A—D and 45 and biradicals 46-47. 

The potential energy snrfaces for excited-state reactions are extensions of the reaction profiles nsually fonnd in the ground state (two different minima connected by a transition structure). In photochemistry, several reaction profiles are connected by a state crossing. In Fignre 2.4 we ontline two reaction profiles to introduce some important concepts in the analysis of photochemical reactivity. We also give an overview of the conclusions that can be drawn from these calculations, together with the more difficult problems that must be addressed with dynamics. 

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