Photochemical reaction radicals

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

Heterogeneous photochemical reactions fall in the general category of photochemistry—often specific adsorbate excited states are involved (see, e.g.. Ref. 318.) Photodissociation processes may lead to reactive radical or other species electronic excited states may be produced that have their own chemistry so that there is specificity of reaction. The term photocatalysis has been used but can be stigmatized as an oxymoron light cannot be a catalyst—it is not recovered unchanged. 

Wong S K, Hutchinson D A and Wan J K S 1973 Chemically induced dynamic electron polarization. II. A general theory for radicals produced by photochemical reactions of excited triplet carbonyl compounded. Chem. Phys. 58 985-9... 

Oxygen Difluoride as a Source of the OF Radical. The existence of the OF radical [12061 -70-0] was first reported in 1934 (27). This work was later refuted (28). The OF radical was produced by photolysis of OF2 in a nitrogen or argon matrix at 4 K. The existence of the OF species was deduced from a study of the kinetics of decomposition of OF2 and the kinetics of the photochemical reaction (25,26) ... 

In the case of photochemical reactions, light energy must be absorbed by the system so that excited states of the molecule can form and subsequendy produce free-radical intermediates (24,25) (see Photochemicaltbchnology). 

Electrophilic attack Nucleophilic attack Free radical attack Photochemical reactions Oxidative and reductive reactions... 

The dominant photochemical reaction of ketones in the gas phase is cleavage of one of the carbonyl substituents, which is followed by decaibonylation and sidsoetprait reactions of the alkyl free radicals that result ... 

A simple aliphatic ketone such as acetone, when promoted to its n,n excited state, undergoes a single unimolecular photochemical reaction in high quantum yield namely a-cleavage giving a methyl and acetyl radical which react further in secondary dark processes. In general, competition... 

The reaction of (fluoroalkenyl)carboranes with potassium permanganate in acetone leads to formation of a-diketones [119. These compounds react by photochemical reaction in which the radical formation at boron is followed by addition to the double bond [120] (equation 92). 

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

Bamford and coworkers [19] have shown that a prolonged aftereffect can be obtained with Mn-carbonyl in the presence of certain additives, notably cyclohexane and acetylacetone (S), It was suggested that the photochemical reaction between Mn2(CO)io and (S) produces the active species (Z), which generates free radicals by interaction with halide and Z probably formed from Mn (CO)6 species ... 

The transfer of an electron from a photoexcited donor molecule (D) to an acceptor molecule (A) to generate a highly reactive radical ion pair is the most fundamental photochemical reaction, and it can be generally expressed as... 

Reactions. It exists in equil with the N difluoride free radical N2F4 4 2.NF , which is shifted to the right as the temp is raised (Ref 4) Qdissociation 20 l.Okcal/mole (Ref 5). It decomps at 350— 480° and 150-1200mm into N2 and N tri-fluoride (Ref 8). It undergoes a large number of thermal and photochem reactions with org compds (for individual reactions see in the Encycl under the parent org compd)... 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

Sulfoxidation is a photochemical reaction. The radical chain reaction is initiated by triplet sulfur dioxide (3 S02), excited by ultraviolet (UV) light of wavelength longer than 320 nm ... 

Figure 7-11 and its caption (Crutzen, 1983) depict the most important of the gas phase and photochemical reactions in the atmosphere. Perhaps the single most important interaction involves the hydroxyl free radical, OH-. This extremely reactive radical is produced principally from the reactions of electronically excited atomic oxygen, 0( D), with water vapor. Photo-... 

Interestingly, many of these free radicals are produced from photochemical reactions in the atmosphere of O2 and O3, for example... 

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