bluuebunny/crossref_metadata_2025_split huggingface dataset:
Benzene and Its Derivatives as Bridging Ligands in Transition‐Metal Complexes
Authors: Wadepohl Hubert 
Published: December 2003

AbstractTransition‐metal complexes in which two or more metal atoms are bridged by one or more arene ligands led a shadowy existence in comparison to the extensive class of mononuclear arene complexes. Arene bridges can occur in a variety of coordination modes and with almost all of the transition–metal elements of the periodic table. Nowhere else are found so many forms of distorted and bent arene rings. The binuclear compounds can be divided into two classes: adducts which show relatively weak metal–arene bonding and complexes which show strong arene–metal interaction. Most of the adducts are in equilibrium with mononuclear complexes in solution or are only stable in the solid state (often as polymers). In both classes syn and anti coordination occurs; their geometries show a wide variation between the extreme cases of η1 : η1‐bridge and η6 : η6‐triple‐decker structure. Metal surfaces with chemisorbed arenes can be seen as a form of multinuclear arene–metal complexes. On transition‐metal surfaces, benzene can be bonded to one, two, or four surface atoms. Molecular clusters with face‐capping arene ligands that are bonded to three metal atoms have until now mainly been limited to two classes. The arenes bound to {(CO)3M}3 (M = Ru, Os) or (CpCo)3 clusters as μ3‐η2 : η2 : η2 ligands show only a weak trigonal distortion towards a Kekulé structure. Detailed investigations of the molecular structure and ligand dynamics of [(CpCo)3(μ3‐arene)] complexes considerably help the understanding of the bonding of arenes to metal clusters and to metal surfaces.
High Resolution Spectroscopy below the Doppler Width
Authors: Neusser Hans Jürgen , Schlag Edward William 
Published: December 2003

AbstractMicroscopic insights into elementary molecular processes are only feasible through the observation of unique quantum states and the study of their couplings and time‐dependent behavior. In the case of larger chemically relevant molecules, the corresponding transitions lie so close that they mostly overlap and lead to band structures. For example, electronic band spectra of large molecules cannot be resolved into single lines even by high resolution UV‐spectrometers. The reason for this is the Doppler broadening, arising from the random velocity of the molecules in the gas phase. In this article, we want to demonstrate that modern laser spectroscopic methods make it possible to overcome this “Doppler barrier” and to resolve the line structure in band spectra. The selective excitation of unique quantum states yields precise information on the mechanisms responsible for energy redistribution in molecules. Energy redistribution processes represent a crucial first step preceding chemical reactions, which they influence decisively. Very high resolution spectroscopy elucidates the influence of molecular rotation on the energy redistribution. Through Coriolis forces in rotating molecules, vibrations can couple, causing a flow of energy from the selectively excited vibrations to the many other vibrational degrees of freedom. Weak interactions, namely van der Waals interactions and hydrogen bonding, determine the structure of large molecules (biomolecules) and supermolecular systems. Weak intermolecular interactions, given their selectively resolved quantum states, can be analyzed and understood at the microscopic level. “Doppler‐free spectroscopy” with its resulting line spectra furnishes information on the structure, including bond lengths, of van der Waals complexes. With this information, Doppler‐free spectroscopy provides the model parameters indispensable for the quantitative description of complex systems.
Multiple Peptide Synthesis Methods and Their Applications. New Synthetic Methods (87)
Authors: Jung Günther , Beck‐Sickinger Annette G. 
Published: December 2003

AbstractRapid developments in the biotechnology of new proteins, as well as advances in immunology and the development of pharmaceuticals based on inhibitors and antagonists, have led to immense demands for synthetic peptides. Simultaneous preparation of 100–150 completely different peptides, having chain lengths of up to 20 amino acids can nowadays be achieved using multiple synthesis methods. The yields and qualities of the peptides so obtained are high enough to permit reliable in vivo and in vitro screening for biological activities. Moreover, it is possible to optimize synthetic conditions and to carry out comparative studies on the secondary structures and conformational mapping of proteins. Special multiple synthesis methods facilitate the epitope mapping of larger peptides for diagnostic purposes and for the development of vaccines based on a few hundreds of free or rod‐bound peptides that are useful for immunoassays. Multiple methods of peptide synthesis also enable the preparation of so‐called peptide libraries which could comprise hundreds of thousands of peptides, and by which new perspectives for the screening of lead structures will be opened up. Peptide synthesis using a combination of photolabile protecting groups and photolithographic procedures enables the assembling of peptide libraries on small plates for use in miniature immunoassays. Furthermore, lipopeptide‐antigen conjugates allow both the preparation of peptide‐specific and monoclonal antibodies as well as a complete screening of epitopes of B‐, T‐helper and T‐killer cells. Applications in the areas of AIDS diagnosis, the development of vaccines, and screening for the hormone analogues, demonstrate just some of the possibilities that have been opened up by multiple peptide synthesis methods.
On the Influence of Heteroatoms in α‐ and β‐Functionalized Alkyl Transition‐Metal Compounds
Authors: Steinborn Dirk 
Published: December 2003

AbstractThe first homoleptic trimethylsilylmethyl transition‐metal complexes were synthesized about 20 years ago in the research groups of Wilkinson and Lappert, resulting in a significant advance in the understanding of the thermodynamic and kinetic stability of transition‐metal‐carbon σ bonds. The relatively high thermal stability of these bonds provides a dramatic contrast to the Köbrich lithium carbenoids XCH2Li, known for about 30 years and famous for their extraordinarily high reactivity. Both compound classes illustrate that introduction of a heteroatom into an alkyl ligand can have a profound influence on the structure, stability, and reactivity of an alkylmetal compound, opening the possibility of entirely new reactions. Because of their simple and unambiguous structure, α‐ and β‐heteroatom‐substituted transitionmetal compounds containing the unit [MCH2Y] or [MCH2CH2–Y] (where Y is an element from the 4th–7th main groups or groups 14–17, not including C) are particularly suitable for studying effects of the nature of a heteroatom center (neutral/cationic, coordinatively saturated/Lewis basic) on structure, stability, and reactivity as a function of the type of complex, the nature of M, and the number of methylene groups separating M and Y. The diversity of stabilizing and destabilizing influences emanating from the heteroatom center generally produces a broad spectrum of reactivity and stability. This is reflected in an impressive way in homogeneous catalysis, where functionalized alkylmetal compounds arise as intermediates, as well as in the electronic influences of functionalized alkyl ligands, which are determined largely by the nature of the heteroelement present.
Transport and Fate of Organic Compounds in the Global Environment
Authors: Ballschmiter Karlheinz 
Published: December 2003

AbstractThe role of chemistry in our soon‐to‐be global industrial society requires a global perspective for the discussion of the uptake, transport, and conversion of chemical compounds in the environment. The fate of organic compounds in the volume flow of the atmosphere and hydrosphere can be categorized into transport pathways and adjustments of equilibria in the multiphase system atmosphere‐oceans‐land surface. The global volume flow in the atmosphere (wind, areas of high and low pressure) and in the hydrosphere (rivers, circulation of water in lakes, ocean currents) alone would account for the transport of organic compounds if they were stable and if all these compounds were molecularly distributed in these phases. However, this particular case is observed only for volatile chlorofluorocarbons and tetrachloromethane. For most organic compounds, complex adsorption/desorption processes on finely dispersed particle phases in the air and water as well as abiotic and biotic transformations determine the transport behavior.The global fate of a compound includes its accumulation as well as its decomposition in defined environmental compartments. Both phenomenon may have long‐term consequences in the af‐fected areas; this is indicated by the accumulation of polychlorobiphenyls and analogous compounds in marine mammals and deep‐sea fauna, and is evident in the decomposition of chlorofluorocarbons in the stratosphere. The accepted definition of the level of a risk, the product of the probability of an event and the potential amount of damage, takes on a new dimension when viewed from a global perspective‐especially when one further connects the extent of the damage to the possible means of correction. It is the responsibility of scientists to point out the irreparable regional to global consequences directly and indirectly connected to the production of chemicals and to provide suggestions for prevention. Politicians can only react by applying their methods to limit the damage; however, they cannot generate any solutions based on science.
Reaction Dynamics of Aligned Molecules
Authors: Comes Franz Josef 
Published: December 2003

AbstractPhotofragmentation—also termed ‘half collision’—is a method to comprehend the complicated process of a chemical reaction (the ‘full collision’) better. The use of polarized light enabled the study of the spectroscopy of aligned molecules, yielding information about the photochemically produced intermediate state—the transition state. Even though stable reaction products were studied, the forces responsible for the motions of the fragments can be visualized. In this way a model can be constructed that represents the part of the potential hypersurface responsible for the observed motions. The application of this spectroscopic method to hydrogen peroxide H2O2 has become a classic example. The complex decomposition of hydrazoic acid, HN3, represented a challenge, and its successful solution proves the general applicability of this method.
Transition Structures of Hydrocarbon Pericyclic Reactions
Authors: Houk Kendall N. , Li Yi , Evanseck Jeffrey D. 
Published: December 2003

AbstractTwenty‐five years after the discovery of a vast class of organic reactions named “pericyclic reactions” by Woodward and Hoffmann, ab initio quantum mechanics provides a detailed analysis of the geometries, energies, and electronic characteristics of the transition structures of these reactions. Common features are found in all these reactions, and generalizations permit prediction of other transition‐structure geometries and energies. At the same time, great diversity is observed—from strongly bonded, rigid, closed‐shell entities to weakly interacting, flexible diradical structures.
Preparation and Properties, Reactions, and Applications of Radialenes
Authors: Hopf Henning , Maas Gerhard 
Published: December 2003

AbstractRadialenes are alicyclic compounds in which every ring atom is sp2‐hybridized and carries an exocyclic CC double bond. Because of their unusual topology and electronic structures these hydrocarbons have aroused the interest of both preparative chemists and theoreticians. Although considerable progress has been achieved recently, directed and efficient syntheses of these molecules remain a challenge. The study of their chemical behavior offers vast opportunities. Very recently materials scientists have become interested in these unusual compounds in their search for organic conductors and ferromagnets.
In Vitro Selection of Specific Ligand‐binding Nucleic Acids
Authors: Famulok Michael , Szostak Jack W. 
Published: December 2003

AbstractIn vitro selection is a method that allows the simultaneous screening of very large numbers of nucleic acid molecules for a wide range of properties from binding characteristics to catalytic properties; moreover, the isolation of the very rare functional molecules becomes possible. Binding sites between proteins and nucleic acids, for example, have been evaluated by this methodology in order to gain information about protein/nucleic acid interactions. Structure and function of catalytic RNA (“ribozymes”) has been studied by in vitro selection and has led to new ribozymes with improved catalytic function. Substrate specificity of catalytic RNA has been changed and has led to a ribozyme that cleaves DNA. Other applications include the isolation of nucleic acids that bind specifically to small organic molecules and of RNA molecules that form triple helices with double‐stranded DNA. In this article we discuss the background, design, and results of in vitro genetic experiments, which bridge biochemical/molecular biological and organic chemical approaches to molecular recognition.
Synthetic Approaches toward Molecular and Polymeric Carbon Allotropes
Authors: Diederich François , Rubin Yves 
Published: December 2003

AbstractLife on earth is based on compounds that have carbon frames and backbones. Today, chemists have added to the world of biomolecules and biopolymers approximately 107 different synthetic molecules and polymers, the structures of which also depend on the formation of strong, stable carbon–carbon bonds. Although the stability of carbon–carbon bonds has been recognized for more than a century, the two natural modifications graphite and diamond were, until recently, the only allotropic forms of carbon on earth that were available in macroscopic quantities and were structurally well characterized. With the synthesis of macroscopic quantities of buckminsterfullerene (C60) and the higher fullerenes (C70, C76, C78, etc.) and the exploration of the fascinating properties of these all‐carbon spheres, this situation has completely changed. In the coming decades, the design, preparation, and study of novel molecular and polymeric allotropic forms of carbon will be a central topic in chemistry. Research in this area will dramatically advance the fundamental knowledge on carbon‐based matter and, as already illustrated by the ongoing work on C60, generate unprecedented technological perspectives. This review surveys synthetic organic‐chemical approaches toward the preparation and study of all‐carbon molecules and polymers that differ from the familiar networks of graphite and diamond as well as from the fullerenes. We will also discuss the ongoing research on fullerenes with a particular focus on the synthetic approaches to these all‐carbon spheres and their transition metal complexes.
Flexible Molecules with Defined Shape—Conformational Design
Authors: Hoffmann Reinhard W. 
Published: December 2003

AbstractChemists have for a long time considered molecules simply in terms of their constitution. The importance of molecular shape was recognized perhaps for the first time by the semio‐chemists, who were interested in the interactions of fragrant substances with a receptor. Apart from the case of rigid molecules, our ideas and concepts of molecular shape simply reflect an instantaneous situation, because flexible molecules take advantage of the whole range of conformation that is available to them. Nature frequently utilizes flexible molecules whose conformational space is restricted, in other words, molecules that can adopt only a few preferred conformations. This review discusses some of the principles that nature employs in order to impart a defined shape to flexible molecules. The purposeful application of these principles then allows the conformational design of molecular skeletons.For corrigendum see DOI:10.1002/anie.199215401For corrigendum see DOI:10.1002/anie.199216571
The Organosulfur Chemistry of the Genus Allium – Implications for the Organic Chemistry of Sulfur
Authors: Block Eric 
Published: December 2003

AbstractA Cook's tour is presented of the organosulfur chemistry of the genus Allium, as represented, inter alia, by garlic (Allium sativum L.) and onion (Allium cepa L.). We report on the biosynthesis of the S‐alk(en)yl‐L‐cysteine S‐oxides (aroma and flavor precursors) in intact plants and on how upon cutting or crushing the plants these precursors are cleaved by allinase enzymes, giving sulfenic acids—highly reactive organosulfur intermediates. In garlic, 2‐propenesulfenic acid gives allicin, a thiosulfinate with antibiotic properties, while in onion 1‐propenesulfenic acid rearranges to the sulfine (Z)‐propanethial S‐oxide, the lachrymatory factor (LF) of onion. Highlights of onion chemistry include the assignment of stereochemistry to the LF and determination of the mechanism of its dimerization; the isolation, characterization, and synthesis of thiosulfinates which most closely duplicate the taste and aroma of the freshly cut bulb, and additional unusual compounds such as zwiebelanes (dithiabicyclo[2.1.1]hexanes), a bis‐sulfine (a 1,4‐butanedithial S,S′‐dioxide), antithrombotic and antiasthmatic cepaenes (α‐sulfinyl disulfides), and vic‐disulfoxides. Especially noteworthy in the chemistry of garlic are the discovery of ajoene, a potent antithrombotic agent from garlic, and the elucidation of the unique sequence of reactions that occur when diallyl disulfide, which is present in steam‐distilled garlic oil, is heated. Reaction mechanisms under discussion include [3, 3]‐ and [2, 3]‐sigma‐tropic rearrangements involving sulfur (e.g. sulfoxide‐accelerated thio‐ and dithio‐Claisen rearrangements) and cycloadditions involving thiocarbonyl systems. In view of the culinary importance of alliaceous plants as well as the unique history of their use in folk medicine, this survey concludes with a discussion of the physiological activity of the components of these plants: cancer prevention, antimicrobial activity, insect and animal attractive/repulsive activity, olfactory–gustatory–lachrymatory properties, effect on lipid metabolism, platelet aggregation inhibitory activity and properties associated with ajoene. And naturally, comments about onion and garlic induced bad breath and heartburn may not be overlooked.
August Wilhelm Hofmann—“Reigning Chemist‐in‐Chief”
Authors: Meinel Christoph 
Published: December 2003

AbstractOne hundred and twenty‐five years ago, on November 11th, 1867, the German Chemical Society of Berlin held its inaugural meeting. The main purpose of the Society was to unite pure and applied chemistry and to foster cooperation between academic research and the chemical industry. And, indeed, it soon became the major forum of German and even European chemistry. Its program clearly bears the hallmark of a single individual: August Wilhelm Hofmann, the Society's first president, who died 100 years ago. For his contemporaries, Hofmann represented a new type of chemistry professor. At no time since have professional chemists felt as abundantly endowed with potential for the future and with public esteem. Hofmann's portrait was monumental even then, and still today it would belong in any gallery devoted to our distinguished forebears.Anniversaries provide an opportunity to direct our attention toward the past—and thus to ourselves as well. We are, after all, heirs to that period from which the modern world derives its profile. Questions from our own time lead us to reacquaint ourselves with one of the founders of modern chemistry, but we may also benefit from a fresh look at an epoch which, beneath the surface of prosperity and progress, was as contradictory as our own, an epoch struggling to understand the role of science in the new industrial era.
The Chemistry of GALLEX—Measurement of Solar Neutrinos with a Radiochemical Gallium Detector
Authors: Henrich Edmund , Ebert Klaus H. 
Published: December 2003

AbstractIn the GALLium Experiment, GALLEX, low‐energy solar neutrinos produced in the main hydrogen fusion reaction inside the Sun are being counted for the first time in a radiochemical gallium detector. In this way, the theories about stellar structure may be verified and the question of the existence of a neutrino rest mass be brought closer to a solution. The experiment is being carried out with international collaboration at the Gran Sasso underground laboratory (LNGS) in the Abruzzi mountains in Italy. In this paper we survey the planning, construction and initial experimental operation of the gallium detector, paying special attention to the chemical aspects of the experiment. The chemical problem to be solved is the separation of very few 71Ge atoms from 100 metric tons of a highly concentrated GaCl3 solution acidified with hydrochloric acid. In addition, these Ge atoms must be isolated and converted into germane, GeH4, for low‐level counting of 71Ge in a gas proportional counter. After extensive preparatory work to meet the extreme requirements of the experiment, the first results have now become available.
Emil Fischer's Proof of the Configuration of Sugars: A Centennial Tribute
Authors: Lichtenthaler Frieder W. 
Published: December 2003

AbstractToday's textbooks convey the deceptive impression that chemistry developed in a rational and orderly process with discoveries following one upon another in a vertical progression—an impression that tends to classify Fischer's achievement as a matter‐of‐fact historical development. This misconception does not consider the instances of serendipity invariably involved, and entirely fails to appreciate the human endeavor, the intellectual struggle of the dedicated researcher, and the forging force of his personality that eventually led to this key insight. The hundredth anniversary of Fischer's classical piece of work provides a welcome opportunity not only to highlight its paramount importance for the development of carbohydrate chemistry, and of organic chemistry in general, but to trace the creative processes underlying this fundamental discovery, the thought patterns at a conceptual level, and the constructive reasoning that eventually led to it. Their understanding and appreciation is essential for emulating Fischer's achievement in a modern context.Die Geschichte einer Wissenschaft ist diese Wissenschaft selbst, J. W. von Goethe