Showing papers in "Accounts of Chemical Research in 1981"

The semiclassical way to molecular spectroscopy

Abstract: This Account deals with a new perspective on the interaction of light with molecules. Although much of chemistry may take place in the dark (literally and figuratively), our knowledge of fundamental molecular and chemical processes is often greatly enhanced when we shed light on the subject! Light of various wavelengths and intensites can be used as a probe of molecular structure and dynamics, and sometimes light acts as a reagent or product in a chemical reaction. Very often, we can profit from a classical or semiclassical picture of the interaction of light with molecules. It is easy, for example, to imagine a heteronuclear

Conversion of Sunlight into Electrical Power and Photoassisted Electrolysis of Water in Photoelectrochemical Cells

Abstract: The development of photoelectrochemical solar cells based on semiconductor liquid junctions is discussed. Over the past 6 years, the efficiency of solar energy conversion has increased from < 1% to 12%, and the operational life of these cells has increased from hours to months. Understanding the chemistry of surfaces and grain boundaries has made it possible to manipulate the position of the surface and grain boundary states and thus reduce losses due to electron-hole recombination. Cells exhibiting approx. 12% efficiency of conversion are: (1) chemisorbed Ru/sup 3 +/ ions on n-GaAs photoanodes; (2) diffused Ru/sup 3 +/ and Pb/sup 2 +/ into grain boundaries of polycrystalline n-GaAs films on graphite; (3) p-InP/VCl/sub 3/-VCl/sub 2/-HCl/C cell; and (4) p-InP(Ru)/HCl-KCl/Pt cell. Photoassisted electrolysis of water for conversion of solar energy to hydrogen using the last mentioned photoelectrochemical cell represents the most efficient system for solar energy conversion. 61 references are cited. (BLM)tic stainless steel is required to prevent stress corrosion cracking byd stuffs contamination in the USSR takes place in the framework of variations observed in separate countries of northern hemisphere.

Alamethicin, a Transmembrane Channel

Abstract: Peptides possessing antibiotic activity, isolated from microbial sources, have been the subject of intensive structural and biological investigation over the past two decades.

The water gas shift reaction: homogeneous catalysis by ruthenium and other metal carbonyls

Abstract: A number of chemical systems are active for homogeneous, solution phase catalysis of the water gas shift reaction (WGSR). Catalysis of the shift reaction appears to be a quite general property of carbonyl complexes in alkaline solutions. The key mechanistic steps for which some kinetics information is available are the activation of coordinated CO by reaction with hydroxide and the reductive elimination of dihydrogen from a resulting metal hydride. Which of these is rate limiting in a cycle is a function of the specific metal system and the specific reaction conditions. A basic solution is not a necessary condition for WGSR catalysis. Although WGSR catalysis is not as general a phenomenon in acidic media, several such systems have been characterized with activities which compare very favorably to those seen in basic solutions. While logical mechanisms have been proposed for several of the better characterized catalysts, it is clear that a more complete mechanistic understanding of the key steps in potential cycles is greatly needed. Recognition of such needs has led to fundamental studies into the acid/base natures of metal carbonyl hydride clusters, of CO activation on cluster and mononuclear metal carbonyls by bases, and of dihydrogen elimination from and declusterificationmore » and clusterification reactions of metal carbonyl hydrides. While none of these processes is in itself catalytic, the sum of several represents possible cycles. With regard to the future of homogeneous WGSR catalysts, it is obvious that under the conditions which these have been tested, the known systems are not commercially viable. Certainly, for a reaction such as the WGSR for which effective heterogeneous catalysts are known, it will take substantial advantages for a new system to be an attractive alternative to established technology.« less

Quantum-chemistry and catalysis in oxidation of hydrocarbons

Abstract: Results presented in this account show that the quantum-chemical description of intermediate complexes formed in the course of the catalytic reactions may supply valuable information concerning catalysis. Modification of the reactivity of different bonds of the reacting molecule as the result of its interaction with the active center of the catalyst, have been found. This may contribute to an important degree to the understanding of the mechanism of the catalytic reaction and to identification of factors influencing its selectivity. At present, exact discussion of the interaction of an adsorbed molecule with the group of atoms of the solid forming the active center is not yet possible, and many approximations must be introduced. The model choosen and the quantum-chemical method used depends on the particular problem to be solved. Advantages and limitations of various quantum chemical methods usually used to solve catalytic problems are summarized. It may be hoped that with the development of quantum chemistry on one hand, and with further improvement in the knowledge of the mechanism of elementary acts in catalysis on the other hand, it will become possible to give a full description of the interactions between the reacting molecules and the catalyst as a basis formore » the development of the theory of catalysis.« less

Solvated electron structure in glassy matrixes

Abstract: Current knowledge of the detailed geometrical structure of solvated electrons in aqueous and organic media is summarized. The geometry of solvated electrons in glassy methanol, ethanol, and 2-methyltetrahydrofuran is discussed. Advanced electron magnetic resonance methods and development of new methods of analysis of electron spin echo modulation patterns, second moment line shapes, and forbidden photon spin-flip transitions for paramagnetic species in these disordered systems are discussed. 66 references are cited. (BLM)

Development of alkali-promoted ruthenium as a novel catalyst for ammonia synthesis

Abstract: It has been demonstrated that ruthenium is remarkably promoted by addition of alkali (metallic or oxide) for the activation of dinitrogen, although it has the drawback of suffering from inhibition by hydrogen. The high activity thus attained makes it possible to reduce the reaction temperature so that the total pressure may be lowered to attain a desired conversion. Thus there is a possibility to get out of the energy-consuming compression process. This would be an advantage of a ruthenium catalyst to be considered in the near future in connection with energy prices. Another advantage of ruthenium is that it gives rise to less susceptibility to poisons such as H/sub 2/O and CO than iron, when promoted with basic oxides instead of metallic alkali. In fact, it has been demonstrated that ammonia synthesis can be carried out in the presence of water and carbon monoxide on ruthenium catalysts. Thus the direct synthesis of ammonia from dinitrogen, carbon monoxide, and water is possible on ruthenium.

Tetracarbonylhydrocobalt, the quintessential catalyst

Abstract: Despite the more than 30 years of work on the mechanism of the hydroformylation, much remains to be investigated. HCo(CO)/sub 4/ has unquestionably turned out to be a most unusual compound. It is considered a metal hydride that functions as an acid catalyst; its reactions are influenced by the coordination state of the cobalt; it forms ..pi.. (eta/sup 2/), ..pi..-allyl (eta/sup 3/), and sigma (eta/sup 1/) complexes; the sigma complexes undergo migratory insertion, oxidative addition, and reductive elimination reactions as well as skeletal rearrangements; it is a potent isomerization catalyst, and its reactions with olefins can lead to aldehydes, alcohols, and/or hydrocarbons. The partitioning among these products can be influenced by temperature, pressure, and ligand structure. It can show both normal and inverse isotope effects and can be a remarkably effective hydrogenation agent. Its reactions encompass the whole gamut of the known reactions of organometal catalysts entitling it to be called the quintessential catalyst.