摘要:

AbstractOptical spectroscopy of a single impurity molecule provides the first truly local probe of host–guest interactions in doped solids. In conventional optical high‐resolution experiments with many molecules only ensemble averages of the microscopic parameters can be obtained. In the single‐molecule regime, the exquisite sensitivity of an individual dopant molecule to both the local environment and to external perturbations has been exploited in recent experiments to reveal a wealth of fascinating novel phenomena such as spectral diffusion in crystals and polymers, optical modification of a single impurity molecule (spectral hole‐burning), local field measurements at the site of a single impurity molecule, and quantum optics in

ISSN:0570-0833    

DOI:10.1002/anie.199304573

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractAlong with polyisoprenoids, polypeptides, polysaccharides, and polynucleotides, Nature contains a further group of biopolymers, the poly(hydroxyalkanoates). The commonest member of this group, poly[(R)‐3‐hydroxybutyrate] P(3‐HB), had been identified by Lemoigne as early as the 1920s, as a storage substance in the microorganismBacillus megateriummade up of more than 12000 (3‐HB) units. However, the widespread distribution and significance of these biopolymers has only become clear recently. The work of Reusch, in particular, has shown that low molecular weight P(3‐HB) (100–200 3‐HB units) occurs in the cell membranes of prokaryotic and eukaryotic organisms. The function of P(3‐HB) in the latter sources is largely unknown; it has been proposed that a complex of P(3‐HB) and calcium polyphosphate acts as an ion channel through the membrane. Indeed, it has even been speculated that P(3‐HB) plays a role in transport of DNA through the cell wall. In the present article, the following subjects will be discussed: metabolism of P(3‐HB) and analogous polyesters in the synthesis and degradation of storage materials; P(3‐HB) as a starting material for chiral synthetic building blocks; synthesis of cyclic oligomers (oligolides) of up to ten 3‐HB units, and their crystal structure; high molecular weight bio‐copolymers of hydroxybutyrate and hydroxyvalerate (BIOPOL) as biologically degradable plastics; nonbiological production of polyhydroxyalkanoates from 3‐hydroxy carboxylic acids and the corresponding β‐lactones; specific synthesis of linear oligomers with a narrow molecular weight distribution, consisting of about 100 (R)‐3‐hydroxybutyrate units, by using an exponential coupling procedure; structure of the polyesters, and a comparison with other polymers; the experimental results which led to the postulation of a P(3‐HB) ion channel through the cell wall; modeling of P(3‐HB) helices of various diameters, by using the parameters obtained from the crystal structures of oligolides; formation of a crown ester complex and ion transport experiments with the triolide of 3‐HB. The article describes one example of the contributions that synthetic organic chemists can make to important biological

ISSN:0570-0833    

DOI:10.1002/anie.199304771

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe capabilities of the human brain have always fascinated scientists and led them to investigate its inner workings. Over the past 50 years a number of models have been developed which have attempted to replicate the brain's various functions. At the same time the development of computers was taking a totally different direction. As a result, today's computer architectures, operating systems, and programming have very little in common with information processing as performed by the brain. Currently we are experiencing a reevaluation of the brain's abilities, and models of information processing in the brain have been translated into algorithms and made widely available. The basic building‐block of these brain models (neural networks) is an information processing unit that is a model of a neuron. An artificial neuron of this kind performs only rather simple mathematical operations; its effectiveness is derived solely from the way in which large numbers of neurons may be connected to form a network. Just as the various neural models replicate different abilities of the brain, they can be used to solve different types of problem: the classification of objects, the modeling of functional relationships, the storage and retrieval of information, and the representation of large amounts of data. This potential suggests many possibilities for the processing of chemical data, and already applications cover a wide area: spectroscopic analysis, prediction of reactions, chemical process control, and the analysis of electrostatic potentials. All these are just a small sample of the great many possibilitie

ISSN:0570-0833    

DOI:10.1002/anie.199305031

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractWithin the last few decades—though with its beginnings traceable to as early as 60 years ago—a direction of research has developed almost unnoticed by the classical chemical disciplines, such that one can now recognize a chemistrywithinthe solid state that is analogous to the long‐familiar chemistry in the liquid state. It arises from those departures from the ideal structure that are thermodynamically unavoidable, the point defects, and is referred to as defect chemistry. It includes the description of ionic and electronic effects, and it considers diffusion as a special step of the overall reaction. This area of chemistry enables one to describe and treat in a unified way many apparently widely different phenomena such as ionic conduction in crystals, doping effects and p–n junctions in semiconductors, color centers in alkali metal halides, image development in photography, passivation and corrosion of metals, the kinetics of the synthesis and sintering of solid materials, the problems of rock formation during the earth's evolution, the mechanisms of gas sensors and high temperature fuel cells, the performance of photosentive electrodes, variations of the electron balance in high temperature superconductors, elementary processes of heterogeneous catalysis, nonequilibrium transitions and oscillations in semiconductors in electric fields, and many more. In such phenomena the equilibrium concentration of defects has an important double role: it not only determines the disorder and the departures from the stoichiometric composition in the equilibrium state, but also, together with the mobility as the kinetic parameter, is the key parameter concerning the rates of physical processes, which are the main subject of the present discussion. Following the account of the thermodynamic fundamentals in Part I, this second part of the review will be concerned with kinetic processes. These are essentially the equilibration processes (physical diffusion, electrical conduction, chemical diffusion, and reactions) that take place under the influence of chemical and electric driving forces. As in Part I, here again the general characteristics of defect chemistry will clearly emerge, including the coexistence of ionic and electronic charge carriers, and the importance of the spatial coordinates. The concluding section will discuss some special characteristics of nonlinear processes that are too often overlooked in solid‐stat

ISSN:0570-0833    

DOI:10.1002/anie.199305281

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199305431

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199305451

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199305471

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199305501

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199305511

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199305521

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY