Excited states reaction profiles

Figure 2.4 Excited-state reaction profiles for (a) sloped and (b) peaked intersections.

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. 

Figure 2.5 Reaction profiles for ultrafast excited-state reactions sloped (a) and peaked (b) crossings.

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 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.

In summary, although the computed structural details of the reaction profile depend on the method used for calculations, the general salient mechanistic conclusion is that the dioxetane thermolysis starts with the 0—0 bond rupture to generate the 0C(H2)—C(H2)0 triplet diradical, which is followed by C—C bond cleavage to afford the final ketone products one of them is formed preferentially in its triplet excited state. Since even simple 1,2-dioxetanes still present a computational challenge to resolve the controversial thermolysis mechanism, the theoretical elucidation of complex dioxetanes constitutes to date a formidable task. 

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. 

Excitations of molecules with femtosecond laser pulses lead to excited-state matter wave packets coherently, launching them with such well-defined spatial resolution and coherence in nuclear motions that they evolve like single-molecule trajectories. Both electronically excited and vibrationally excited ground-state species may be studied. The structural change versus time profile of a reaction turns out to be compatible with classical modes of thinking. 

The effect of the excited-state lifetime on the temporal profile of the MV + concentration is illustrated in Figure 6.12, where the reaction with MV2 + (Equation 6.58) causes the decrease of the excited-state lifetime. The increase in the overlap between... 

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