Oxidation-reduction photochemistry

Oxidation-Reduction Photochemistry of Polynuclear Complexes in Solution... 

One problem we have had to overcome in developing metal-cluster oxidation-reduction photochemistry is the tendency of excited clusters to dissociate into radical fragments (for... 

H. B. Gray We must remember that we are still in the initial stages of systematic study of inorganic oxidation-reduction photochemistry. Nature has indeed some slick ways to optimize photochemical energy conversion. I am confident that inorganic chemists will do as well or better, perhaps even before the turn of the century ... 

The high concentration of oxygen in the atmosphere plays a central role in the photochemistry and chemical reactivity of the atmosphere. Atmospheric oxygen also defines the oxidation reduction potential of surface waters saturated with oxygen. The presence of oxygen defines the speciation of many other aquatic species in surface waters. 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Photoexcitation of complex ions need not necessarily lead to electron transfer even as a secondary stage. In a recent study of the photochemistry of various cobaltic and chromic complex ions by Adamson and Sporer (I) it was shown that oxidation-reduction, apart from aquation, racemization, or exchange reactions, occurred not only by photoexcitation in the electron transfer band region but also in ligand field bands where electron transfer is out of the question. Very similar observations had been made by Copestake and Uri (7) in a detailed study of the photochemical and thermal decomposition of the trioxalatocobaltate(III) complex. In this case electron transfer was not postulated as the primary reaction. The suggested scheme was... 

Oxidation, reduction, or excitation of many molecules in general, and chlorophyll a in particular, results in the bleaching of the red-most absorption band. When particles isolated from green plants, which have been enriched in photosystem I are excited with a pulse of light, photochemistry is initiated. In a very short time, perhaps in only a few picoseconds, the special photoactive chlorophyll a within the reaction center is oxidized. 

Basic Physical Chemistry for the Atmospheric Sciences covers the fundamental concepts of chemical equilibria, chemical thermodynamics, chemical kineHcs, solution chemistry, acid and base chemistry, oxidation-reduction reactions, and photochemistry. Over 160 exercises are contained within the text, including 50 numerical exercises solved in the text and 112 exercises for the reader to work on with hints and solutions provided in an appendix. 

Reviewed herein are some of the fundamental concepts associated with chemical equilibrium, chemical thermodynamics, chemical kinetics, aqueous solutions, acid-base chemistry, oxidation-reduction reactions and photochemistry, all of which are essential to an understanding of atmospheric chemistry. The approach is primarily from the macroscopic viewpoint, which provides the tools needed by the pragmatist. A deeper understanding requires extensive treatment of ihe electronic structure of matter and chemical bonding, topics that are beyond the scope of this introductory text. This book can be used for either self-instruction, or as the basis for a short introductory class... 

The second direction in which redox properties of sulfones and sulfoxides could manifest themselves in photochemistry is redox photosensitization108,110-114. In such a photosensitization the photosensitizer is transformed by light into a short-lived oxidant or reductant able to react with the substrate to be activated. Tazuke and Kitamura115 have discussed the parameters to play with when one... 

It is possible that colloidal photochemistry will provide a new approach to prebiotic syntheses. The work described previously on redox reactions at colloidal ZnS semiconductor particles has been carried on successfully by S. T. Martin and co-workers, who studied reduction of CO2 to formate under UV irradiation in the aqueous phase. ZnS acts as a photocatalyst in the presence of a sulphur hole scavenger oxidation of formate to CO2 occurs in the absence of a hole scavenger. The quantum efficiency for the formate synthesis is 10% at pH 6.3 acetate and propionate were also formed. The authors assume that the primeval ocean contained semiconducting particles, at the surface of which photochemical syntheses could take place (Zhang et al 2007). 

Photochemical reactions, like any chemical reaction, can be classified into various groups, depending on the reactants and products, for example, elimination, isomerization, dimerization, reduction, oxidation, or chain reaction. One important practical field of photochemistry is organic photochemistry. In solution photochemical reactions, the nature of the solvent can markedly influence the reaction. The absorbtion of the solvent and of the reaction products is an important parameter for the choice of the reaction conditions. It is useful to have a solvent with a relatively low absorption in the desired wavelength. Sometimes photosensitizers are used these are substances that absorb light to further activate another substance, which decomposes. 

The present volume contains 13 chapters written by experts from 11 countries, and treats topics that were not covered, or that are complementary to topics covered in Volume 1. They include chapters on mass spectra and NMR, two chapters on photochemistry complementing an earlier chapter on synthetic application of the photochemistry of dienes and polyenes. Two chapters deal with intermolecular cyclization and with cycloadditions, and complement a chapter in Volume 1 on intramolecular cyclization, while the chapter on reactions of dienes in water and hydrogen-bonding environments deals partially with cycloaddition in unusual media and complements the earlier chapter on reactions under pressure. The chapters on nucleophiliic and electrophilic additions complements the earlier chapter on radical addition. The chapter on reduction complements the earlier ones on oxidation. Chapters on organometallic complexes, synthetic applications and rearrangement of dienes and polyenes are additional topics discussed. 

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