摘要:

AbstractThe measurement of pH, redox potentials, and corrosion potentials at high temperatures and pressures is often desirable for the control and monitoring of industrial processes; e.g., for controlling water chemistry in conventional and nuclear power generators, for process control in the chemical industry, and for monitoring performance in petroleum extraction and refining. Such measurements are also needed for research on the thermodynamics of high‐temperature aqueous solutions including those associated with geothermal and hydrothermal reactions. Zirconia electrodes for these purposes are described, and their application is illustrated with examples. While not yet routinely available on a commercial basis, useful versions can be readily fabricated in the laboratory. Some of the pitfalls encountered in making such measurements at elevated temperatures are also discussed because great care is often required to assure that valid and useful data are being obtaine

ISSN:0570-0833    

DOI:10.1002/anie.198701613

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

摘要:

AbstractA knowledge of the geometries at which excited molecules return to the electronic ground state (S0) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S1(lowest excited singlet) and T1(lowest triplet) surfaces, as well as S0–S1conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the S0and S1surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two‐electron two‐orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the S0and S1surfaces touch. The results obtained for the simple model are supported by ab initio large‐scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and π‐donor‐to‐π‐acceptor single bonds, and by similar calculations for “push‐pull” perturbed cyclobutadienes, some of which are predicted to have nearly degenerate S0, S1, and T1states. The likely consequences of these results for the detailed description of the mechanisms ofcis‐transisomerization, the formation oftwistedinternalcharge‐transfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocy

ISSN:0570-0833    

DOI:10.1002/anie.198701701

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

摘要:

AbstractStereochemical control is a major concern in the application of homogeneous catalysis to organic chemistry. In this context, the directed hydrogenation of olefins employing cationic rhodium or iridium catalysts has considerable potential, for very high selectivity can be attained under mild reaction conditions. The only requirement is a polar functional group in proximity to the double‐bond which remains bound to the metal during the catalytic cycle and thereby controls the Stereochemical course of hydrogen delivery through the constraints of chelation. The substituent is most frequently a hydroxy group OH but can also be an ester, amide or carbamate group; other groups remain to be scrutinized. In cyclic compounds, directed hydrogenation can lead to face‐selectivity, and the polar substituent may be in the β‐, γ‐, or δ‐position to the double‐bond. Acyclic stereoselection ensues with β‐ or γ‐substituents in appropriate compounds, and the configuration of reduced product is predictable on the basis of simple rules. The application of optically active rhodium complexes leads to useful kinetic r

ISSN:0570-0833    

DOI:10.1002/anie.198701901

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

摘要:

AbstractThe successive reduction of fully conjugated cyclic hydrocarbons leads to singly and multiply charged ions with unusual bonding. The charge distribution in these ions can be determined spectroscopically, and the information so obtained is then used in kinetically controlled trapping reactions for the regioselective introduction of electrophilic groups. When non‐benzenoid substrates are used, syntheses become possible which can either not be carried out or can only be carried out with great difficulty in other ways. Examples of new preparative applications are cycloannelation and bridging reactions as well as polymerization reactions. The ion pair structure of the intermediate and the type of electrophile used are of paramount importance in controlling the mechanism of these reductive transformation

ISSN:0570-0833    

DOI:10.1002/anie.198702041

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

摘要:

AbstractThe fact that DNA is transcribed into RNA has long been known and has become part of the “central dogma” of molecular genetics. The mechanisms responsible for controlling this process at the individual genes of higher cells (eukaryotes), however, are still not completely understood. RNA polymerases apparently require a number of auxiliary factors (transcription factors) for gene recognition. These factors combine with the enzyme at the gene to form a transcription complex. The structures of these complexes are starting to become clearer; most is known about the control of RNA polymerase III, the enzyme responsible for the synthesis of certain small RNA molecules. The model character of the results obtained with this system is clearly underlined by a series of recent publications. TF III A is an especially intensively studied protein, which is a positive regulator for the expression of ribosomal 5S RNA and possesses structural properties that were previously unknown in DNA‐binding proteins. It is becoming increasingly evident that the “architecture” of TF III A is not an exotic curiosity but probably exemplifies a general struct

ISSN:0570-0833    

DOI:10.1002/anie.198702181

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198702281

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198702291

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198702301

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198702321

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198702331

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY