Photochemical reactions classification

We will show here the classification procedure with a specific dataset [28]. A reaction center, the addition of a C-H bond to a C=C double bond, was chosen that comprised a variety of different reaction types such as Michael additions, Friedel-Crafts alkylation of aromatic compounds by alkenes, or photochemical reactions. We wanted to see whether these different reaction types can be discerned by this... 

Phosphine(s), chirality of, 314 Phosphite, DNA synthesis and, 1115 oxidation of, 1116 Phospholipid, 1066-1067 classification of, 1066 Phosphopantetheine, coenzyme A from. 817 structure of, 1127 Phosphoramidite, DNA synthesis and, 1115 Phosphoranc, 720 Phosphoric acid, pKa of, 51 Phosphoric acid anhydride, 1127 Phosphorus, hybridization of, 20 Phosphorus oxychloride, alcohol dehydration with. 620-622 Phosphorus tribromide, reaction with alcohols. 344. 618 Photochemical reaction, 1181 Photolithography, 505-506 resists for, 505-506 Photon, 419 energy- of. 420 Photosynthesis, 973-974 Phthalic acid, structure of, 753 Phthalimide, Gabriel amine synthesis and, 929... 

Photochemical reactions of organic substrates with molecular oxygen have been extensively studied, with respect to both their preparative and mechanistic aspects. This article will be restricted to a certain type of these reactions which we may call type II (direct and indirect) photooxygenation reactions in solution. This classification is based on the following definitions. 

JIFor a different kind of classification of photochemical reactions, sec Dauben Salem Turro Acc. Chem. Res. 1975,. 41. For reviews of photochemical reactions where the molecules are geometrically constrained, see Rama-murlhy Tetrahedron 1986, 42. 5753-5839 Ramamurlhy Eaton Acc. Chem. Res. 1988, 21. 300-306 Turro Cox Paczkowski Top. Curr. Chem. 1985, 129. 57-97. 

Figure 7.1 Classification of photochemical reactions according to the nature of potential energy surfaces.

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

Turning back to the definition of photochemistry and anticipating the classification of photochemical reactions of metallotetrapyrroles, it should be kept in mind that a true photochemical process is only that involving an electronically excited particle (in this review it means an excited tetrapyrrole complex). All subsequent reactions are spontaneous (in photochemistry they are familiarly called dark reactions). Exactly speaking, each classification of photochemical reactions should start with an answer to the question what is the nature of the primary photochemical step involving a complex in its photochemically reactive excited state It must be admitted that for the... 

In summary, chiral solvents have only induced limited enantioselectivity into different types of photochemical reactions as pinacolization, cyclization, and isomerization reactions. These studies are nevertheless very important, because they are among the early examples of chiral induction by an asymmetric environ ment. Based on our classification of chiral solvents as chiral inductors that only act as passive reaction matrices, effective asymmetric induction by this means seems to be intrinsically difficult. From the observed enantioselectivities it can be postulated that defined interactions with the prochiral substrate during the conversion to the product are a prerequisite for effective template induced enantioselectivity. 

This chapter is intended to focus on catalysis in both thermal and photoinduced electron transfer reactions between electron donors and acceptors by investigating the effects of an appropriate substance that can reduce the activation barrier of electron transfer reactions. It is commonly believed that a catalyst affects the rate of reaction but not the point of equilibrium of the reaction. Thus, a substance is said to act as a catalyst in a reaction when it appears in the rate equation but not in the stoichiometric equation. However, autocatalysis involves a product acting as a catalyst. In this chapter, a catalyst is simply defined as a substance which affects the rate of reaction. This is an unambiguous classification, albeit not universally accepted, including a variety of terms such as catalyzed, sensitized, promoted, accelerated, enhanced, stimulated, induced, and assisted. Both thermal and photochemical redox reactions which would otherwise be unlikely to occur are made possible to proceed efficiently by the catalysis in the electron transfer steps. First, factors that accelerate rates of electron transfer are summarized and then each mechanistic viability is described by showing a number of examples of both thermal and photochemical reactions that involve catalyzed electron transfer processes as the rate-determining steps. Catalytic reactions which involve uncatalyzed electron transfer steps are described in other chapters in this section [66-68]. 

If all atoms involved in the reaction lie in the same plane, the unpaired electron of the acyl radical may be either in an orbital that is symmetric with respect to this plane or in an orbital that is antisymmetric—that is, either in a 0 or in a r orbital, whereas only a o orbital is available for the unpaired electron of radical R. Instead of just one singlet and one triplet covalent biradicaloid structure (Figure 4.5), there are now two of each, which may be denoted as B and B respectively. Similarly, there are also different zwitterionic structures to be expected. The increase in complexity and the number of states that results from the presence of more than two active orbitals on the atoms of a dissociating bond has been formalized and used for the development of a classification scheme for photochemical reactions ( topicity ), as is outlined in more detail in Section 6.3.3. 

Since it is not the objective of this chapter to give either a complete review of all organic photoreactions or an exhaustive account of the applicability of the various theoretical models, neither of these classifications is followed strictly. Instead, the interpretation of experimental data by means of theoretical models will be discussed for selected examples from different classes of compounds or reaction types in order to elucidate the influence of molecular structure and reaction variables on the course of a photochemical reaction. 

The same classification is found in presence of photosensitizers. Poly-1 insolubilization above 290 nm is accelerated by nitro-2 fluorene, Michler s ketone and especially by xanthone. On the other hand when irradiation is carried out on films with additional 253.7 nm ray, nitrofluorene only gave an effect. The results obtained with the same sensitizers were similar in the case of Poly-3. The observations are quite different with Poly-2, the photochemical reaction of which is very sligthly improved by the presence of xanthone and greatly reduced by the two other compounds perhaps because of quenching effect. 

The classification of photosensitized reactions according to the polymer structure seems to be a resonable approach,but unfortunately the qxuuititative evidence necesary for such classification is still inadequate. In practice it is convenient to group the sensitized reactions according to the photochemical reactions of the initiators and sensitizers,e.g. 

The mechanism of a photoreaction should ideally include a detailed characterization of the primary events as outlined by the classification of photochemical reaction pathways in Section 2.3 the lifetimes of the excited states that are involved in the reaction path, the quantum yields and hence the rate constants of all relevant photophysical and photochemical processes, in addition to the information about the structure and fate of any reactive intermediates, their lifetimes and reactivities. 

A topochemical classification of photochemistry at interface between solid and gas or solid and liquid illustrated schematically in Fig.1. In Fig.1, the signs etc. stand for the types of photochemical reaction at interfaces. The S l reaction implies that the electrons and/or positive holes produced in the inside of solid by light... 

The classification of asymmetric photochemical reactions is generalized in Table... 

Dauben WG, Salem L, Turro NJ. A classification of photochemical reactions. Acc Chem Res 1975 8 41-54. 

Table 1.13 Classification of Photochemical Reactions Based on Types of Chemical Bonds... 

Salem5 has reported detailed calculations analysing several photochemical reactions with special reference to the way by which the excited state of the molecule decays back to the ground state. Another publication has dealt with the classification of photochemical reactions and is in part an elaboration of an earlier paper.7 Further attention has been directed at the Stern-Volmer analysis of photochemical reactions dealing with non-linearity when two excited states are reactive and one or both are quenched.8 9 A generalized treatment of the equation has resulted.10... 

The classification described above is an approximate subdivision of aU reactions known in accordance with their mechanisms. One example was given in eq. (1.6). Such process can proceed with participation of ligands of metal complex. Photochemical reaction between, for example, alkane, RH, and PtC [5], depicted by eq. (1.7) and initiated via mechanism of the third type can lead to the formation of an cr-organyl derivative of the metal and the entire process then belongs to the first type. 

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