Photochemical classification

Knowledge of the underlying nuclear dynamics is essential for the classification and description of photochemical processes. For the study of complicated systems, molecular dynamics (MD) simulations are an essential tool, providing information on the channels open for decay or relaxation, the relative populations of these channels, and the timescales of system evolution. Simulations are particularly important in cases where the Bom-Oppenheimer (BO) approximation breaks down, and a system is able to evolve non-adiabatically, that is, in more than one electronic state. 

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... 

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... 

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. 

A. TROPOSPHERIC OZONE AND ASSOCIATED PHOTOCHEMICAL OXIDANTS TABLE 16.1 Classification of Organics in the RCAM and RADM2 Mechanism0 889... 

Farley FF. 1977. Photochemical reactivity classification of hydrocarbons and other organic compounds. In EPA-600/3-77-001B. Inter Conf Photochemical Oxidant Pollut Control. Dimitriades B, ed. Research Triangle Park, NC, 713-726. 

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.

L. Mattes and S. Farid, in A. Padwa (ed.). Organic Photochemistry, vol. 6, Dekker (1983). The photochemical electron transfer reactions of olefins are described in this account, which provides an interesting perspective that cuts across classifications based on an overall reaction type. 

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... 

The classification ofthese reactions is difficult as the panorama is variable. Here, we will present only selected examples that indicate the potential of photochemical methods. Other than some bromination reactions, most attention will be paid to functionalization with the formation of a C—C bond. 

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. 

Even so, the distinction between the two is sometimes a more subtle matter. Thus, in a photoisomerization a common excited state intermediate may undergo a transformation to either of the two isomeric cis-trans species of a planar ground-state molecule. These two transformations are virtually identical in nature, yet one leads back to the original species and is therefore a photophysical primary step, e.g., internal conversion or intersystem crossing, while the other leads to the chemically distinct isomer and should be called a photochemical primary step. As another example, the distinction between the formation of an excimer and of a photodimer lies in the instability and stability, respectively, of the dimeric species in the ground state. Excimer formation is usually considered as photophysical and photodimer formation as photochemical. These examples show that the classification of steps as photochemical and photophysical is in some cases arbitrary. 

---