摘要:

AbstractWhenArchimedeswas asked by King Hiero of Syracuse to prove whether a handcrafted crown really consisted of pure gold and not of gold with a proportion silver, he immersed the crown and an equal weight of pure gold, both attached to a beam balance, into a vessel of water. We all know that according to Archimedes's principle the scale beam rose on the side the crown was attached proving that the goldsmith had intermingled some silver. In spite of the simplicity of this experiment, the result convincingly demonstrates the physical integrity of this classical approach which is based on a weighing procedure. The mass sensitive technique we shall report on in this article is based on the principle that a shift in the resonance frequency of an oscillating AT‐cut quartz disk can be correlated quantitatively with a change in mass in the nanogram range. One side of the quartz disk is in contact with a liquid, while the other side remains exposed to the atmosphere. The changes in weight involved in testing Hiero's crown were probably several hundred grams which corresponds to about 1025water molecules. By applying the quartz microbalance, e.g., to solvent migration through thin films, one can sense entrapment of water with a resolution down to 1014molecules per unit surface area. If adsorbate layers are in question, fractional coverages down to 0.02 monolayers can be monitored, which only goes to prove the capability of this instrument for investigating interfacial processes on a microscopic level. The option of recording charge flux and interfacial capacitance along with the mass change makes this instrument a powerful in‐situ tool for scientific and technological applications in many fie

ISSN:0570-0833    

DOI:10.1002/anie.199003293

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractOver the last few years new experimental and theoretical methods have made it possible to gain a more detailed insight into the chemistry of short‐lived reaction intermediates. In 1949Criegeepostulated the intermediacy of carbonyl oxides in the mechanism of ozonolysis, and since then these species have become the goal of much research effort. Even though the formation of “Criegee zwitterions” during ozonolysis and carbene oxidations was proven by scavenger experiments, the electronic structure—zwitterion or diradical—of this short‐lived species is still a subject of debate. To date no stable carbonyl oxide has been found to exist under “normal” laboratory conditions, although by using matrix isolation and laser spectroscopy, it has been possible to obtain highly resolved IR and UV/VIS spectra of carbonyl oxides as well as to determine their dipole moments experimentally. The influence of substituents, exact kinetic data on modes of formation, and the subsequent reactions of carbonyl oxides as well as their photochemistry complete the picture. In accordance with ab initio calculations carbonyl oxides, are best viewed as polar diradicals. The zwitterionic state lies at higher energies and should be stabili

ISSN:0570-0833    

DOI:10.1002/anie.199003441

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractOur current understanding of enzyme catalysis is dominated by the hypothesis of transition‐state binding launched byLinus Paulingin 1946. Transition‐state binding and the interplay of properly positioned, catalytically active functional groups can largely explain both the acceleration and specificities of many enzymic reactions for which chemical analogy exists. Here it is postulated that, for chemically “difficult” or “improbable” reactions, enzymes may resort to an additional device. Using cofactors of potentially high energy (such as coenzyme B12), stabilized radicals, or light, they can transform the bound substrate into a highly reactive (i.e., unstable) intermediate. Selectivity is now achieved bynegative catalysis, that is, by preventing the “hot” intermediate from undergoing reactions that would occur in solution or in the gas phase and thus prolonging its lifetime. The highly reactive intermediate may then undergo reactions whose activation energy is relatively high and which would therefore be suppressed without negative catalysis. Reaction selectivity is thus achieved by preventing undesired reactions rather than by facilitating the target ones. The high reactivity may be transferred reversibly if its source is, for instance, coenzyme B12or irreversibly if it is generated by light or ATP. Highly reactive intermediates are often radicals but can also be other unstable species. In this article a number of enzymic reactions are discussed that seem to support the

ISSN:0570-0833    

DOI:10.1002/anie.199003551

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractPhotons are emitted at low levels from a wide variety of organic materials at wavelengths ranging from the visible to infrared. This light emission accompanies free‐radical autoxidation and charge‐recombination processes and is particulary useful in assessing thermal oxidation and radiation damage as well as polymer curing reactions. IR chemiluminescence, which offers advantages because of reduced self‐absorption, has been shown to mark the end of induction periods in inhibited systems. With the introduction of sensitive solid‐state devices, the chemiluminescence technique has been extended to two‐dimensional imaging and pre

ISSN:0570-0833    

DOI:10.1002/anie.199003621

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractMany modern methods designed to enhance the power of NMR spectroscopy have been widely adopted by the scientific community. Some of the best known among these techniques are one‐dimensional methods for polarization enhancement and spectral editing (“INEPT” and “DEPT”) and various two‐dimensional experiments such as correlation spectroscopy (“COSY”) and nuclear Overhauser spectroscopy (“NOESY”). Most of these methods cannot be described in terms of classical magnetization vectors, and although they can be understood with the help of the density operator formalism, this approach remains difficult for many users, in part because of its cumbersome notation, which prevents one from recognizing the important features at a glance. In this paper, a set of graphical conventions is described which allows one to represent the effect of sequences of radio‐frequency pulses on spin systems in a variety of experiments. The graphs provide a concise description of the network of spins that are coupled through scalar and dipolar interactions, of the state of a spin system expressed in terms of products of operators, and, most importantly, of the transformations from one state to another due to the effect of radio‐frequency pulses, chemical shifts, and scalar couplings. The graphical approach also allows one to represent the effect of phase‐cycling and multiple‐quantum filtration, cross‐relaxation (Overhauser effects) an

ISSN:0570-0833    

DOI:10.1002/anie.199003741

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199003841

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199003851

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199003861

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199003881

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199003901

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY