Photochemical reactions diabatic

In addition to such minima the lowest excited states tend to contain numerous minima and funnels at biradicaloid geometries, through which return to the ground state occurs most frequently. Most photochemical reactions then proceed part way in the excited state and the rest of the way in the ground state, and the fraction of each can vary continuously from case to case. (Cf. Figure 6.3, path a.) It is common to label adiabatic only those reactions that produce a spectroscopic excited state of the product (cf. Figure 6.3, path h), so distinction between diabatic and adiabatic reactions would appear to be sharp rather than blurred. But this is only an apparent simplification, since it is hard to unambiguously define a spectroscopic excited state. 

Figure 1. Photochemical reaction process of 6-hydroxy-1, 3, 3 -trimethylspiro[2H-1 -benzopyran-2,2 -indoline] (6-OH-BIPS 7). The reaction proceeds along the diabatic surface (dashed line) i.e., direct dissociation of the Cipjro-0 bond takes place. B is one of the several possible merocyanine isomers, and X refers to the initial metastable product formed soon after the C,pjro—O bond cleavage. (Reprinted from Ref. 10 with permission of the American Chemical Society.)...

A crossed-beam experiment has been devoted to study the photochemical reaction at threshold Cs(7P)+H2 -> CsH+H. It is shovm that use of lasers (excitation of Cs atoms, detection of CsH products) and calculation of diabatic potential energy surfaces are able to provide a good understanding of this particular reactive process. 

The description of the course of photochemical processes outlined so far implies that there is a continuous spectrum of reactions from unequivocally diabatic ones that involve a nonradiative jump between surfaces to unequiv-... 

A second obvious problem with the ordinary definition of adiabatic reactions is the vagueness of the term product. If the product is what is actually isolated from a reaction flask at the end, few reactions are adiabatic. (Cf. Example 6.7.) If the product Is the first thermally equilibrated species that could in principle be isolated at sufficiently low temperature, many more can be considered adiabatic. A triplet Norrish II reaction is diabatic if an enol and an olefin are considered as products. It would have to be considered adiabatic, however, if the triplet 1,4-biradical, which might easily be observed, were considered the primary photochemical product. (See Section 7.3.2.)... 

There are variations on the diabatic process. One important feature is that some reactions will have a small barrier on S that separates the initial excited state geometry from the funnel geometry. This can adversely affect photochemical efficiency and produce temperature dependent quantum yields. Still, the basic idea of finding geometries in which the excited state and ground state are close in energy is central to photochemistry. 

While the diabatic mechanism we just discussed is typical of photochemical processes, there are some other less common yet interesting paths, summarized in Figure 16.16. In an adiabatic reaction (Figure 16.16 A), the conversion from reactant geometry to product geometry occurs on just one surface, the excited state surface. This is then followed by relaxation back down to the ground state. This relaxation could in principle be emissive, such that we would excite the reactant and see fluorescence from the product. 

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