Photochemical reactions, dissociation rearrangement

Photochemical reactions, dissociation of trihalometh-anes, 52, 132 rearrangement, 52, 53 Phthalic acid, reduction, 50,... 

Another possible process is internal conversion, radiationless transition to an electronic state of the same spin multiplicity, for example, S - 82, involving excited singlet states. This can be followed by delayed fluorescence. Chemical bonds in electronically excited molecules can also dissociate or rearrange themselves, thereby taking part in photochemical reactions. 

Yang, L., Zhang, M., Liu, Y, Liu, Z., and Chow, Y.L., Triplex promoted intersystem crossing of ion-radical pairs in the photosensitized valence isomerization of quadricyclane chemically induced dynamic nuclear polarization (CIDNP) evidence,/. Chem. Soc., Chem. Commun., 1055-1056,1995. Kajitani, M., Fujita, X, Hisamatsu, N., Hatano, H., Akiyama, X, and Sugimori, A., Photochemical reactions of several adducts of metalladithiolenes and metalladithiazoles. Dissociation and rearrangement, Coord. Chem. Rev., 135, 175-180, 1994. 

Secondary rearrangements apparent isomerizations through radical recombination reactions. In the rearrangement reactions considered so far, the isomerization step is the primary photochemical process, except when a biradical is formed as an intermediate for in that case the primary photochemical process is really a dissociation, even though the fragments cannot separate. There are however cases of overall isomerizations which result from the recombinations of separated free radicals formed through a process of photodissociation. The photo-Fries reaction is an important example of this mechanism, and is illustrated in Figure 4.43. 

In valence isomerization reactions the most common case is the rearrangement of a ligand, as in the example in Figure 4.79 in which a nitro group, N02, isomerizes to a nitrito group, O—N=0. The primary photochemical process is in fact a dissociation of the Co-N bond, with formation of free radicals. The nitro radical isomerized to its nitrito form, then binds again to make the new complex. 

Chapter 13 gave a brief introduction to carbonyl dissociation reactions, in which CO may be lost thermally or photochemically. Such a reaction may result in rearrangement of the remaining molecule or replacement of CO by another ligand ... 

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