Photochemical processes, in solution

C. Teixeira, I. Wadso. A Microcalorimetric System for Photochemical Processes in Solution. J. Chem. Thermodynamics 1990, 22, 703-713. 

When a photochemical process in solution gives a photoresponse at the electrode, the system can form a photogalvanic cell. The photoinduced redox reaction is typical for photogalvanic cells. The most well known photoredox system is thionine (TH+ 23) and ferrous ion88). The excited TH+ is reduced by Fe2+ to give TH and Fe3+ (Eq. (25)). 

Nov. 21, 1931, Tbilisi, Georgia, USSR - May 13, 1985) Dogonadze was one of the founders of the new science - electrochemical physics [i]. The main scientific interests of Dogonadze were focused on condensed-phase reactions. His pioneering works of 1958-59 have laid the foundations of the modern quantum-mechanical theory of elementary chemical processes in electrolyte solutions. He developed a comprehensive quantum-mechanical theory of the elementary act of electrochemical reactions of -> electron and -> proton transfer at metal and - semiconductor electrodes [ii—v]. He was the first to obtain, by a quantum-mechanical calculation, the expression for the electron transfer probability, which was published in 1959 in his work with -> Levich. He conducted a number of studies on the theory of low-velocity electrons in disordered systems, theory of solvated electrons, and theory of photochemical processes in solutions. He made an impressive contribution to the theory of elementary biochemical processes [vi]. His work in this area has led to the foundation of the theory of low-temperature -> charge-transfer processes cov-... 

The apparent similarity between the McLaiferty rearrangement and the Norrish type II photochemical process, in solution and in the gas phase, was noted by Nicholson (1954) and by Martin and Pitts (1955). The photochemical process follows the reaction path shown for 2-pentanone (122 for leading references see Nicol and Calvert, 1967 Wagner, 1967). 

It is clear that [2-1-2] photocycloaddition had dominated the recent literature of the asymmetric photochemical process in solution, and we have highlighted some of the more interesting examples that have appeared over the last 6 years. However, this chapter does not seek to describe this area in depth, and the reader s attention should be directed to the reviews mentioned to obtain a more specialized view of this area of photochemistry. 

E. Rabinowitch and W. C. Wood, Trans. Faraday Soc., 32, 1381 (1936). The Collision Mechanism and the Primary Photochemical Process in Solutions. 

In order to learn more about the photocrosslinking process, we synthesized 1,1-bis(trimethylsiloxy)-1-phenyl(trimethyl)disilane (5) as a model compound and examined its photochemical behavior in solutions. Compound 5 could readily be prepared by cohydrolysis of 1,1-dichloro-1-phenyl(trimethyl)disilane with a large excess of chloro-trimethylsilane in high yield. 

The reactions depicted in Fig. 32 are most often carried out at low temperatures. The incursion of a thermal process at elevated temperatures has occasionally been observed. In some cases the thermal oxygenation products are identical to the photochemical products and in other cases are different. For example, when 2,3-dimethyl-2-butene/02 NaY is warmed above — 20 °C a reaction was observed which led to pinacolone (3,3-dimethyl-2-butanone) as the major product.98,110 Pin-acolone is not formed in the photochemical reaction at the same temperature. On the other hand, identical products were observed in the thermal and photochemical intrazeolite oxygenations of cyclohexane.114,133 135 These intrazeolite thermal processes occur at temperatures well below that necessary to induce a classical autooxidation process in solution. Consequently, the strong electrostatic stabilization of oxygen CT complexes may also play a role in the thermal oxygenations. Indeed, the increase in reactivity of the thermal oxygenation of cyclohexane with increasing intrazeolite electrostatic field led to the conclusion that initiation of both the thermal and photochemically activated processes occur by the same CT mechanism.134 Identical kinetic isotope effects (kH/kD — 5.5+0.2) for the thermal and photochemical processes appears to support this conclusion.133... 

Photodynamic studies on excited state charge transfer has advanced to the point where some of the simpler prototype molecules are now well understood. The charge transfer dynamic measurements have also been centrally important in the great progress being made on understanding of the solvent s role in photochemical and nonphotoinduced fast charge transfer processes in solution. 

Reisz E, Schmidt W, Schuchmann H-P, von Sonntag C (2003) Photolysis of ozone in aqueous solution in the presence of tertiary butanol. Environ Sci Technol 37 1941-1948 Reuther A, Labereau A, Nikogosyan DN (1996) Primary photochemical processes in water. J Phys Chem 100 16794-16800... 

The definition of environmental chemistry given above is illustrated for a typical environmental pollutant in Figure 2.3. Pollutant sulfur dioxide is generated in the anthrosphere by combustion of sulfur in coal, which has been extracted from the geosphere. The S02 is transported to the atmosphere with flue gas and oxidized by chemical and photochemical processes in the atmosphere to sulfuric acid. The sulfuric acid, in turn, falls as acidic precipitation, where it may have detrimental effects, such as toxic effects, on trees and other plants in the biosphere. Eventually the sulfuric acid is carried by stream runoff in the hydrosphere to a lake or ocean, where its ultimate fate is to be stored in solution in the water or precipitated as solid sulfates and returned to the geosphere. 

Sodium sulfide was used as the electron donor in the reaction under study. In aqueous solution with pH 10-12, the sulfide ion occurs mainly as HS anions [48]. Photochemical processes in the presence of CdS colloids are caused by the generation of nonequilibrium holes (h) and electrons (e) in CdS ... 

For years, the capabilities of TDDFT to describe excited states have been limited to isolated molecules, despite the fact that a large part of the spectroscopic experiments probe molecules in liquids. However, in the last few years there have been several extensions of the TDDFT to describe excited state of molecules in solution. These extensions are of particular interest as they will allow to expand the areas of application of the TDDFT to several photophysical and photochemical processes in condensed phase. 

Photochemical processes in aquatic systems are not limited to substances in solution. Photochemical reactions may involve particles. One can distinguish two mechanisms ... 

Photochemical processes in heterogeneous systems, and across micelle boundaries in particular, clearly has great potential. The photolysis of amphipathic alkylcobaloximes in mixed micelles shows a co-operative effect owing to structure.The photoreduction of anthraquinone in aqueous micellar solution has been compared with that in non-aqueous solution.The dimerization of 3-(n-butyl)cyclopentenone is solvent-dependent and the ratio of isomeric products depends on surfactant concentration. It is suggested that this can be used as a means of critical micellar concentration determination. 

Aqueous polysaccharide gels have been investigated by Phillips and co-workers. These workers found that the photochemical processes in gels were the same as in solution suggesting that the gel structure is fluid. 

Up to now, heterolytic photochemical processes in aqueous solution have been Identified with a limited number of simple molecules. However, these molecules are good models for leurge classes of organic compounds appearing as xenobiotics in the aquatic media. It is again em asized that ionic photochemical processes are characterized by... 

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