Photochemical reactions, conceptual

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

In the absence of interaction with another chemical species an electronically excited state can do one of two things—it can change into a different electronic state of the same compound, or it can change into a different compound. Conceptually the two are not very different. The first is a photophysical process, with which we are concerned in this section, and the second is a photochemical reaction, which is the subject of the remaining chapters of the book. 

A conceptual model which is the centerpiece of this chapter is developed in Section III. This is preceded (Section II) by a brief introduction to various organized media. The validity and generality of the model is examined by two approaches. In the first (Sections IV-VI), selected photochemical reactions belonging to various classes and chromophores are presented as supporting examples. In the second (Sections VII and VIII), a critical reevaluation of the results reported on Norrish II reactions in a number of organized media is made on the basis of the model. However, although we examined the literature examples on the basis of our model, we often have deviated from the initial explanations offered by the authors. 

The first theoretical section is conceptually oriented. We shall discuss the special features occurring when more than one energy surface is involved in the chemical reaction (e.g., avoided and real crossings) as well as the general structure of the photochemical reaction path that connects the excited state reactant and the photoproducts through the funnel. 

We hope the reader has been convinced that it is technically feasible to describe a photochemical reaction coordinate, from energy absorption to photoproduct formation, by means of methods that are available in standard quantum chemistry packages such as Gaussian (e.g., OPT = Conical). The conceptual problems that need to be understood in order to apply quantum chemistry to photochemistry problems relate mainly to the characterization of the conical intersection funnel. We hope that the theoretical discussion of these problems and the examples given in the last section can provide the information necessary for the reader to attempt such computations. 

This chapter showed how to conceptualize reactivity in a variety of different fields and problems using just two archetypal diagrams shown in Fig. 6.1. Section 6.10 on electronic delocalization and Section 6.11 dealing with photochemical reactions, both prepare us for Chapter 7 which deals with YB descriptions of excited states for a variety of molecules. 

The last two chapters by Hoffmann and Ramamurthy deal with a collection of photochemical reactions with arenes, the ortho-, meta- and para photocycloadditions and with a conceptually exciting concept in organic photochemistry, the use of contrained media. Retigeranic acid (16, by formal asymmetric synthesis) was synthesized via a fabulous reaction sequence involving an intramolecular meta photocycloaddition as key step [16]. 

In summary, the control of photochemical reactions by molecular recogifition becomes one of the most active topics in supramolecular chemistry in recent years. However, supramolecular photochemistry is still conceptually less established, mechanistically less understood, and experimentally less explored compared to the conventional supramolecular chemistry in the ground state. We believe that further comprehensive studies in this area will reveal more intriguing features of supramolecular photochemistry and strength our capability to control photochemical reactions. 

Eyring s TST has provided the basic conceptual framework for the interpretation of the rates of nearly all chemical reactions on a bulk scale. He quickly applied his new theory to homogeneous gas-phase thermochemical reactions, photochemical reactions, heterogeneous catalysis, and reactions in solution [19]. He even considered such topics as viscosity and diffusion [19]. 

The excited state Si can also return to So without the emission of a photon. The excess energy is usually given off as heat to the medium. This energy wasting process is termed internal conversion (IC). Conceptually, it is similar to a productive photochemical reaction, and so we delay further discussion of IC until we are ready to discuss reactions. 

Similar photo-induced reductive dissolution to that reported for lepidocrocite in the presence of citric acid has been observed for hematite (a-Fe203) in the presence of S(IV) oxyanions (42) (see Figure 3). As shown in the conceptual model of Faust and Hoffmann (42) in Figure 4, two major pathways may lead to the production of Fe(II)ag i) surface redox reactions, both photochemical and thermal (dark), involving Fe(III)-S(IV) surface complexes (reactions 3 and 4 in Figure 4), and ii) aqueous phase photochemical and thermal redox reactions (reactions 11 and 12 in Figure 4). However, the rate of hematite dissolution (reaction 5) limits the rate at which Fe(II)aq may be produced by aqueous phase pathways (reactions 11 and 12) by limiting the availability of Fe(III)aq for such reactions. The rate of total aqueous iron production (d[Fe(aq)]T/dt = d [Fe(III)aq] +... 

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