bluuebunny/crossref_metadata_2025 huggingface dataset:
Tetrasilylmethane, C(SiH3)4, the Si/C‐Inverse of Tetramethylsilane, Si(CH3)4
Authors: Hager Rudolf , Steigelmann Oliver , Müller Gerhard , Schmidbaur Hubert , Robertson Heather E. , Rankin David W. H.
Published: December 2003

Better precursors are required for plasma enhanced chemical vapor deposition of amorphous silicon and silicon carbide in order to prepare solar cells and optoelectronic materials. Silylmethanes, in which the central C atom is bonded exclusively to Si atoms, are particularly promising. Tetrasilylmethane (4) has now been synthesized from phenylchlorosilane (1) via intermediates 2 and 3. SiC bond cleavage in the last step affords, in addition, trisilylmethane.
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[nBuLi · LiOtBu]4, Solid‐State Structure of an n‐Butyllithium–Lithium tert‐Butoxide Complex
Authors: Marsch Michael , Harms Klaus , Lochmann Lubomir , Boche Gernot
Published: December 2003

Strikingly short Li–β‐C(Bu) and Li–β‐H distances (ca. 235 and 205 pm) characterize the crystal structure of the tetramer [nBuLiLiOtBu]4. Half of the Li atoms are threefold coordinated (to two O atoms and one α‐C atom), the other half fourfold coordinated (to one O atom, two α‐C atoms, and one α‐C atom). On the basis of the structure of this tetramer one can speculate on the structure of the complex superbases (“Lochmann bases”) [RLi · MOR′] (M = Na, K, Rb, Cs).
The Quartz Microbalance: A Novel Approach to the In‐Situ Investigation of Interfacial Phenomena at the Solid/Liquid Junction [New Analytical Methods (40)]
Authors: Schumacher Rolf
Published: December 2003

WhenArchimedeswas 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 fields.
Enzymic Reaction Selectivity by Negative Catalysis or How Do Enzymes Deal with Highly Reactive Intermediates?
Authors: Rétey János
Published: December 2003

Our current understanding of enzyme catalysis is dominated by the hypothesis of transition‐state binding launched by Linus Pauling in 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 by negative 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 B12 or 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 above postulate.
Isolation of an L‐Stereospecific N‐Acyl‐L‐proline‐Acylase and its Use as Catalyst in Organic Synthesis
Authors: Groeger Ulrich , Drauz Karlheinz , Klenk Herbert
Published: December 2003

Microbiological screening has resulted in the selection of the microorganism Comamonas testosteroni. Growth of this bacterium in the presence of N‐acetyl‐L‐proline leads to formation of the enzyme N‐acyl‐L‐proline acylase. The enzyme (molecular weight 380 ± 40 kDa, with eight identical subunits),is thermally stable to 70 °C. At pH values between 5 and 10 the enzyme causes stereospecific hydrolysis of N‐acetyl‐L‐proline, making it useful for the resolution of racemic D,L‐proline mixtures.
What Are the Structures of Fenestin A and Fenestin B?
Authors: Schmidt Ulrich , Lieberknecht Albrecht , Kazmaier Uli , Haslinger Ernst
Published: December 2003

1H NMR signals at unusually high field (δ = 3.9–4.1 rather than 4.0–4.6 as in known cyclopeptides) render the proposed structures 1 and 2 unlikely for fenestin A and fenestin B, two cyclopeptides isolated from Pacific sponges. Synthesis and analysis of the postulated compound 2 as well as the dimer of 1 have made it possible to show that the suggested structure for fenestin B is wrong, while that for fenestin A must at least be regarded as questionable.
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The Chemistry of Interstellar Space
Authors: Herbst Eric
Published: December 2003

Within the last ten to twenty years, radioastronomers have discovered the existence of almost 100 different molecules in interstellar space. Ranging in complexity from two to thirteen atoms, these molecules are found in cold, rarefied regions called interstellar clouds, which are giant accumulations of gas and dust located in our galaxy as well as many others. Interstellar clouds are also the birthplaces of future generations of stars and are of great interest to astronomers. The observation of the large sample of gaseous molecules, detected mainly via their rotational spectral patterns, tells astronomers about the detailed physical conditions in interstellar clouds, and tells chemists about the extent of molecular synthesis possible under the seemingly harsh conditions of low temperature and density. The molecules are mainly organic in nature and comprise species known to be both stable and common in the laboratory as well as those both unstable and uncommon under terrestrial conditions, including radicals and molecular ions. Although the gas phase of interstellar clouds is well studied via spectroscopic techniques, the dust particles are much more poorly characterized via their scattering and absorption of visible radiation as well as some broad resonances in the ultraviolet and infrared regions of the spectrum. It is normally thought that these submicron‐sized particles consist of cores that are composites of silicate and carbonaceous materials with mantles that contain material deposited from the gas such as ices of water, ammonia, and methane. In addition to the dust particles and gaseous molecules, there is some evidence for very large aromatic molecules (the so‐called polycyclic aromatic hydrocarbons or PAH's) which occupy a nether region in between large gas‐phase species and small dust particles. As our understanding of the chemical processes in interstellar clouds increases, it may be possible to speculate how large interstellar molecules can come into existence and whether or not there is a clear connection between interstellar chemistry and the start of life on earth.
Lithium‐Ion‐Catalyzed Epoxidation by Triplet Dioxygen: An ab initio Study
Authors: Hofmann Heinz , Clark Timothy
Published: December 2003

Epoxidation is exothermic with −19.3 kcalmol−1 [Eq. (b)] instead of endothermic with +27.7 kcal mol−1 [Eq. (a)] when carried out in the presence of Li⊕. This emerged from demanding ab‐initio calculations for this model system. Since Li⊕‐catalysis is largely a result of an electrostatic effect, other metal ions should also be able to act as catalysts for epoxidation.
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Synergic Destabilization by Geminal Ester Groups
Authors: Verevkin Sergej , Dogan Barbara , Beckhaus Hans‐Dieter , Rüchardt Christosph
Published: December 2003

An inverse effect of geminal acceptor substituents should correspond to the anomeric effect of geminal donor substituents. This could be confirmed in the case of the esters 1 and 2 by derivation of group increments: the diesters are destabilized ca. 6 kcal mol−1 compared to monoesters, and the triester is destabilized ca. 6 kcal mol−1 compared to the diesters. The reactivity of these compounds (e.g. their acidity) is seen in a new light with this finding (R = H, Me).
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Nitrogen Triiodide
Authors: Tornieporth‐Oetting Inis , Klapötke Thomas
Published: December 2003

Deep‐red, adduct‐free NI3 can be isolated from the reaction of boron nitride with iodine monofluoride at −30°C by sublimation at low temperature. Its low‐temperature Raman spectrum is consistent with a C3v molecular symmetry.
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Retroviruses and Oncogenes I (Nobel Lecture)
Authors: Varmus Harold E.
Published: December 2003

The suggestion that genetic changes might be responsible for cancer was long controversial. The joint results of Harold E. Varmus and J. Michael Bishop, obtained over many years, lent much support to this idea and led to the award of the Nobel Prize in medicine and physiology in 1989. Their interest in oncogenes and retroviruses opened up some of the most interesting areas in modern biology: the genetic basis of cancer, the transposition of DNA through RNA intermediates, the control of gene expression in eukaryotes, and the molecular basis of evolution. Their work takes on additional importance in light of the worldwide spread of a human retrovirus, HIV, which causes AIDS. The two articles give an overview of the history of research in this area and lead the reader to topics of current interest.
Retroviruses and Oncogenes II (Nobel Lecture)
Authors: Bishop J. Michael
Published: December 2003

The suggestion that genetic changes might be responsible for cancer was long controversial. The joint results of Harold E. Varmus and J. Michael Bishop, obtained over many years, lent much support to this idea and led to the award of the Nobel Prize in medicine and physiology in 1989. Their interest in oncogenes and retroviruses opened up some of the most interesting areas in modern biology: the genetic basis of cancer, the transposition of DNA through RNA intermediates, the control of gene expression in eukaryotes, and the molecular basis of evolution. Their work takes on additional importance in light of the worldwide spread of a human retrovirus, HIV, which causes AIDS. The two articles give an overview of the history of research in this area and lead the reader to topics of current interest.
Experiments with Separated Oscillatory Fields and Hydrogen Masers (Nobel Lecture)
Authors: Ramsey Norman F.
Published: December 2003

Very precise values for certain atomic properties have one thing in common with atomic clocks: they are both based on molecular beam experiments using apparatuses with static and oscillatory magnetic fields. The basis of these experiments and their practical applications are described by Norman F. Ramsey in his Nobel lecture.
Experiments with an Isolated Subatomic Particle at Rest (Nobel Lecture)
Authors: Dehmelt Hans
Published: December 2003

Individual atomic and subatomic particles may be observed over long periods by confining them in “traps without material walls”. The Nobel Prize in physics was awarded in 1989 to Hans Dehmelt and Wolfgang Paul (together with Norman F. Ramsey) for first introducing this concept. Such traps are made possible by extending the principles of two‐dimensional focusing of particles to three dimensions. This led, in turn, to precision radiofrequency spectroscopy and quadrupole mass spectrometry. For example, side‐band cooling, in which electrons absorb photons with an energy just below the resonance energy and use their kinetic energy to supply the balance, allowed an electron to be held for ten months in the center of a magnetic trap. In this way, ½g was determined to be 1.001159652188(4).
Synthesis and Structure of Novel Cu‐Clusters: [Cu30−xSe15(PiPr3)12] (x = 0,1) and [Cu36Se18(PtBu3)12]
Authors: Fenske Dieter , Krautscheid Harald , Balter Silke
Published: December 2003

In contrast to the system phosphane/AgCl, the system phosphane/CuCl reacts with Se(SiMe3)2 below 20°C to give metal‐rich cluster complexes in yields of 60%. Above 100°C, binary copper selenides are formed. Compounds 1–3 were characterized by X‐ray structure analysis. They are the largest Cu cluster compounds so far known. The structures of 1 and 2 reveal a cluster framework whose polyhedral surfaces are covered with μ4‐Se, μ5‐Se, and μ6‐Se ligands.
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Nonmolecular Metal Chalcogenide/Halide Solids and Their Molecular Cluster Analogues
Authors: Lee Sonny C. , Holm Richard H.
Published: December 2003

A variety of transition‐metal cluster structures are found within the extended arrays of solidstate materials. Although molecular analogues of some of these clusters have been synthesized—either by the self‐assembly of smaller components in solution or by the excision of intact cluster units directly from the solid‐state phase—molecular chemists are often unaware of the rich structural variety expressed in solid‐state clusters. This article presents these diverse structural types as potential targets for synthetic molecular chemistry by describing fundamental solid‐state cluster topologies in transition‐metal chalcogenide/halide phases and detailing cases where molecular counterparts have been prepared. Particular emphasis is placed on cluster excision as a method of molecular cluster synthesis.
Interfaces between Molecular and Polymeric “Metals”: Electrically Conductive, Structure‐Enforced Assemblies of Metallomacrocycles
Authors: Marks Tobin J.
Published: December 2003

The design, synthesis, characterization, and understanding of new molecular and macro‐molecular substances with “metal‐like” electrical properties represents an active research area at the interface of chemistry, physics, and materials science. An important, long‐range goal in this field of “materials by design” is to construct supermolecular assemblies which exhibit preordained collective phenomena by virtue of “engineered” interactions between molecular building blocks. In this review, such a class of designed materials is discussed which, in addition, bridges the gap between molecular and polymeric conductors: assemblies of electrically conductive metallomacrocycles. It is seen that efforts to rationally construct stacked metal‐like molecular arrays lead logically to structure‐enforced macromolecular assemblies of covalently linked molecular subunits. Typical building blocks are robust, chemically versatile metallophthalocyanines. The electrical optical, and magnetic properties of these metallomacrocyclic assemblies and the fragments thereof, provide fundamental information on the connections between local atomic‐scale architecture, electronic structure, and the macroscopic collective properties of the bulk solid.
A Synthetic Allosteric System with High Cooperativity between Polar and Hydrophobia Binding Sites
Authors: Schneider Hans‐Jörg , Ruf Dieter
Published: December 2003

No Absract.
Advanced Materials: Trends and Possibilities in Liquid Crystalline Polymers
Authors: Economy James
Published: December 2003

During the past ten years there has been a sharp increase in interest in the opportunities afforded by R & D in the field of specialty polymers. Interest is mainly being shown in two distinct categories of polymers, namely, (a) polymers which are used in very small quantities to fulfill critical needs as a part of device system, and (b) high‐performance engineering polymers which significantly extend their mechanical and thermal properties for structural applications. The first category ranges from advanced resists and insulating layers for microelectronic devices to membranes for filtration systems. The second category encompasses improved matrices for advanced composites as well as liquid crystalline polymers. In the present paper an overview is first given of the emerging opportunities for advanced materials and particularly specialty polymers. The status of work on liquid crystalline copolyesters is then discussed with special emphasis on one of the major problems confronting this field, namely interpreting the microstructure of the copolyesters.
Lewis‐Base‐Dictated Structural Variations in Sodium Amide Chemistry: X‐Ray Crystal Structures of Phenyl(2‐pyridyl)amidosodium with Hexamethylphosphoric Triamide (HMPA) and with Pentamethyldiethylenetriamine (PMDETA)
Authors: Andrews Philip C. , Clegg William , Mulvey Robert E.
Published: December 2003

Two Crystalline adducts of sodium amide 1 and 2 adopt contrasting binuclear structures, as revealed by X‐ray diffraction. Thus, the bidentate anions are linked to the Na⊕ ions in a terminal manner in 1 but in a bridging manner in 2. Each structure also displays unusual donor‐acceptor coordinations, with PA bridging to Na⊕ in 1, while only two of the three available PMDETA nitrogen centers complex to Na⊕ in 2.
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Decameric and Dodecameric Sodium Amides and an Organosodium Amide with Stack Structures: [Na10 (NMe2)10(tmeda)4],|Na12(NMe2)12(tmeda)4|, and [Na12(NMe2)10(CH2C6H4Me)2(tmeda)4]
Authors: Lorenzen Nis Peter , Kopf Jürgen , Olbrich Falk , Schümann Uwe , Weiss Erwin
Published: December 2003

The oligomeric sodiumamides 1–3, which were formed during attempts to metalate hydrocarbons with n‐butylsodium in the presence of teteramethylethylenediamine, have stacked ring structures. Correspondingly structured lithium amides are not known; rather, in this case ladder structures should be preferred.
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Enantioselective Addition of Organometallic Reagents to Carbonyl Compounds: Chirality Transfer, Multiplication, and Amplification
Authors: Noyori Ryoji , Kitamura Masato
Published: December 2003

Nucleophilic addition of organometallic reagents to carbonyl substrates constitutes one of the most fundamental operations in organic synthesis. Modification of the organometallic compounds by chiral, nonracemic auxiliaries offers a general opportunity to create optically active alcohols, and the catalytic version in particular provides maximum synthetic efficiency. The use of organozinc chemistry, unlike conventional organolithium or ‐magnesium chemistry, has realized an ideal catalytic enantioselective alkylation of aldehydes leading to a diverse array of secondary alcohols of high optical purity. A combination of dialkylzinc compounds and certain sterically constrained β‐dialkylamino alcohols, such as (–)‐3‐exo‐dimethylaminoiso‐ borneol [(–)‐DAIB], as chiral inducers affords the best result (up to 99% ee). The alkyl transfer reaction occurs via a dinuclear Zn complex containing a chiral amino alkoxide, an aldehyde ligand, and three alkyl groups. The chiral multiplication method exhibits enormous chiral amplification: a high level of enantioselection (up to 98%) is attainable by use of DAIB in 14% ee. This unusual nonlinear effect is a result of a marked difference in chemical properties of the diastereomeric (homochiral and heterochiral) dinuclear complexes formed from the dialkylzinc and the DAIB auxiliary. This phenomenon may be the beginning of a new generation of enantioselective organic reactions.
Bryophytes, a Source of Biologically Active, Naturally Occurring Material?
Authors: Zinsmeister Hans Dietmar , Becker Hans , Eicher Theophil
Published: December 2003

Chemical studies of the plant category of mosses (bryophytes) were neglected for a long time. They have now been shown to be a storehouse of naturally occurring materials, including some with novel chemical structures. Many of these materials display considerable biological activity. Investigations are hampered frequently by too small amounts of plant material. The resulting low yields of components are then generally inadequate to permit testing for biological activity. In vitro culture and appropriate chemical synthesis on a preparative scale are being undertaken to overcome this difficulty.
Iminophosphanes: Unconventional Compounds of Main Group Elements
Authors: Niecke Edgar , Gudat Dietrich
Published: December 2003

The large number of known stable compounds in which phosphorus has a low coordination number makes it clear that such compounds can no longer be regarded as “exotic” in main group chemistry. While the rich chemistry of PC multiply bonded systems makes clear their affinity to their organic congeners, iminophosphanes in particular are also of increasing importance. The linkage of a phosphinidine fragment with an imine fragment via a multiple bond gives rise to a class of compounds with an unusually wide range of structural types. This in turn leads to a broad spectrum of chemical behavior which makes iminophosphanes extremely useful synthetic building blocks in organoelement chemistry.
Spontaneous Resolution by Stirred Crystallization
Authors: McBride J. Michael , Carter Randall L.
Published: December 2003

New insights into the mechanism of crystal nucleation emerge from a study by Kondepudi et al., on the crystallization of NaClO3 from a stirred solution. Rectangular prisms of the chiral space group P213 precipitate from aqueous solutions of the achiral ions. If the solution is not stirred, crystals of both enantiomorphic forms precipitate in equal amounts—exactly as was reported almost one hundred years ago. However, when the NaClO3 solution is stirred, then almost exclusively only right‐handed or only left‐handed crystals are formed. There is a simple explanation for this extremely surprising and obviously reproducible phenomenon.
The Rationality of Random Screening—Efficient Methods of Selection of Peptides and Oligonucleotide Ligands
Authors: Plückthun Andreas , Ge Liming
Published: December 2003

The rapid automatic synthesis of oligonucleotides enables the production of a large number of variants in a single step. The oligonucleotides with random base composition can then serve either as ligands or receptors or encode peptide ligands or protein receptors. After selection, in which readily binding ligands are separated from nonbinding or poorly binding ligands, the desired compounds can be multiplied from virtually single molecules, either in vivo (by transfection or transformation of bacteria with this DNA) or in vitro by a polymerase chain reaction. Such strategies will play an increasingly important role in the investigation of molecular interactions of biochemical relevance. Random screening may be the most rational approach for solving many problems.
Chloride‐Induced Conversion of [Mn4O2(OAc)6(py)2(dbm)2] to [Mn4O3Cl(OAc)3(dbm)3]: Potential Relevance to Photosynthetic Water Oxidation
Authors: Wang Sheyi , Folting Kirsten , Streib William E. , Schmitt Edward A. , McCusker James K. , Hendrickson David N. , Christou George
Published: December 2003

Further insights into the function of chloride in the water‐oxidizing enzyme system of photosynthesis are provided by the title reaction, in which an Mn‐complex is converted into an MnMnIV complex, as given formally in Equation (a). Thus, the redox process S1 → S2 taking place in the active state in nature is copied, and suggests that also the biological relevance of Cl⊖ could lie in the stabilization of a higher oxidized Mn aggregates.
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Ribonuclease T1: Structure, Function, and Stability
Authors: Pace C. Nick , Heinemann Udo , Hahn Ulrich , Saenger Wolfram
Published: December 2003

Proteins carry out the most important and difficult tasks in all living organisms. To do so, they must often interact specifically with other small and large molecules. This requires that they fold to a globular conformation with a unique active site that is used for the specific interaction. Consequently, protein folding can be regarded as the “secret of life”. Biochemists and chemists have a great interest in elucidating the mechanism by which proteins fold and in predicting the folded conformation and its stability given just the amino acid sequence. This challenge is sometimes called the “protein folding problem”. The ability to construct proteins differing in sequence by one or more amino acids and to analyze their three‐dimensional structures by X‐ray crystallography and NMR spectroscopy is a powerful tool for investigating the conformational stability and folding of proteins. Several proteins are now under intensive study by this approach. One of these is ribonuclease T1.
Fluorine‐Stabilized Sulfur–Carbon Multiple Bonds
Authors: Seppelt Konrad
Published: December 2003

Alkylidene and alkylidyne sulfur fluorides contain sulfur–carbon multiple bonds. In contrast to the sulfur ylides, alkylidene sulfur fluorides fulfill all the criteria for double bonds, i.e. they have short bond lengths, strong anisotropic distribution of electron density, and rotation about the CS bond is restricted. Alkylidyne sulfur fluorides have especially short bond distances and, due to a high amplitude bending motion, appear to be more or less linear, depending on the physical state. The advantage of the CS multiple bond systems in contrast with numerous others, e.g. those of phosphorus and silicon, is that they exist without steric stabilization. Moreover, the limits of the triple‐bond principle are outlined: the prognosis for triple bonds between two elements of higher periods is poor, because carbene‐like or fully bridged structures win in terms of stability.
Molecular Structures from Density Functional Calculations with Simulated Annealing
Authors: Jones Robert O.
Published: December 2003

The geometrical structure of any aggregate of atoms is one of its basic properties and, in principle, straightforward to predict. One chooses a structure, determines the total energy E of the system of electrons and ions, and repeats the calculation for all possible geometries. The ground state structure is that with the lowest energy. A quantum mechanical calculation of the exact wave function Ψ would lead to the total energy, but this is practicable only in very small molecules. Furthermore, the number of local minima in the energy surface increases dramatically with increasing molecular size. While traditional ab initio methods have had many impressive successes, these difficulties have meant that they have focused on systems with relatively few local minima, or have used experiment or experience to limit the range of geometries studied. On the other hand, calculations for much larger molecules and extended systems are often forced to use simplifying assumptions about the interatomic forces that limit their predictive capability. The approach described here avoids both of these extremes: Total energies of predictive value are calculated without using semi‐empirical force laws, and the problem of multiple minima in the energy surface is addressed. The density functional formalism, with a local density approximation for the exchange‐correlation energy, allows one to calculate the total energy for a given geometry in an efficient, if approximate, manner. Calculations for heavier elements are not significantly more difficult than for those in the first row and provide an ideal way to study bonding trends. When coupled with finite‐temperature molecular dynamics, this formalism can avoid many of the energetically unfavorable minima in the energy surface. We show here that the method leads to surprising and exciting results.
Chemical Reactions in the Manufacture of Waveguides for Long Distance Optical Data Transmission
Authors: Binnewies Michael , Jerzembeck Marion , Kornick Andreas
Published: December 2003

One of the modern high technologies which has advanced enormously in the last few years is glass fiber technology. This is used in the manufacture of glass fibers for lighting purposes and for the optical transfer of analog and digital data with a high transfer density. The technical demands made on the glass fibers required for data transfer, the optical waveguides, are extremely high and are already fulfilled to a large extent by industry. At present about four million kilometers of fiber, worth ca. 800 million DM are produced worldwide (10% in the Federal Republic of Germany). Numerous chemical processes take place during the manufacture of optical waveguides. However, in contrast both to the high and constantly growing demands on the quality and to the increasing production volume of such fibers, little is in fact known about the reactions involved. The present article will attempt to develop a picture of the multifarious reactions occurring in the course of this technical process on the basis of literature data and our own studies.
Activation of Methane
Authors: Schwarz Helmut
Published: December 2003

The attempts to functionalize the alkanes was once dubbed the “search for the chemist's Holy Grail” by D. H. R. Barton. The controlled transformation of methane into “more valuable” products is of extreme economic importance and a scientific challenge, since not even today are all aspects of the activation of methane understood. Trikura and Beauchamp recently observed that slow ligand condensation follows the dehydrogenation of methane by some metals, leading to the coupling of several CH2 units [Eq. (a)], that is, only one metal atom need be involved.
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Enzymatic Two‐Step Synthesis of N‐Acetyl‐neuraminic Acid in the Enzyme Membrane Reactor
Authors: Kragl Udo , Gygax Daniel , Ghisalba Oreste , Wandrey Christian
Published: December 2003

For corrigendum see DOI:10.1002/anie.199117131
The Structure of IF
Authors: Mahjoub Ali‐Reza , Seppelt Konrad
Published: December 2003

The square‐antiprismatic structure of IF corresponds to that of XeF. This shows that the extra lone pair of XeF is located in a stereochemically inactive orbital. The counterion NO⊕(NOF)2 is a polymer in which the charge‐carrying N atom is surrounded by one O and four F atoms in a square‐pyramidal arrangement.
Phytochrome, the Visual Pigment of Plants
Authors: Rüdiger Wolfhart , Thümmler Fritz
Published: December 2003

Plants require light for photosynthesis. In order to adapt to the light conditions in their particular habitat, they have developed various photoreceptor systems. Of these, phytochrome allows even two‐color vision in the red/far‐red region. The photoreceptor phytochrome is of interest not only to botanists, but also to natural product chemists, photochemists, biochemists, photobiologists, and recently molecular biologists. Despite numerous studies, there are still considerable gaps in our knowledge of this photoreceptor. This article first describes the basic structural studies of the tetrapyrrole chromophore and its photochemical cis–trans isomerization, which is the source of the chromoprotein's photochromism. In the section on the protein moiety, beside other topics, the domain structure of phytochrome and the conformational changes during phototransformation are discussed. Finally, the known phytochrome genes are used to derive phylogenetic relationships, and possible structure–function relationships are discussed.
Globe‐trotting Hydrogens on the Surface of the Fullerene Compound C60H6(N(CH2CH2)2O)6
Authors: Hirsch Andreas , Li Qiaoying , Wudl Fred
Published: December 2003

The [1,5] sigmatropic migration of hydrogen in the adduct between buckminsterfullerene C60 and morpholine (Hmorph) is fast at −25°C already. Elemental analysis, thermogravimetry, IR and NMR spectroscopy, and cyclic voltammetry all support the formulation of the adduct as C60H6 (morph)6. The amine is covalently added to six pyracylene units. The small activation barrier to the migration is astounding, especially as large changes in the C60 framework are involved.
Flash Photolytic Generation and Study of Ynamines. First Observation of Primary and Secondary Ynamines in Solution
Authors: Chiang Yvonne , Grant Andrew S. , Kresge A. Jerry , Pruszynski Przemyslaw , Schepp Norman P. , Wirz Jakob
Published: December 2003

Primary and secondary ynamines (1 and 2, respectively) have been observed in solution for the first time, and the mechanism of their subsequent reactions were studied. The acid‐ and base‐catalyzed isomerizations of primary and secondary ynamines afford ketenimines which tautomerize to nitriles or to carboxylic acids. Tertiary ynamines (3) are hydrated directly to amines.
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Surface Fractality of Dendrimers
Authors: Farin Dina , Avnir David
Published: December 2003

The surface of the polyamidoamine (PAMAM) starburst dendrimers of Tomalia et al. is indeed fractal. Two independent analyses yielded as fractal dimension D 2.41 ± 0.04 and 2.42 ± 0.07. The fractal dimension of several enzymes lie in this region, so the starburst dendrimers will probably mimic biomolecules.
A Metal‐Ion‐ and Cofactor‐independent Enzymatic Redox Reaction: Halogenation by Bacterial Nonheme Haloperoxidases
Authors: Haag Theodor , Lingens Franz , van Pée Karl‐Heinz
Published: December 2003

Although the chemistry of the halogens is very well known, we know relatively little about biochemical halogenations. The mode of action of a bacterial non‐heme haloperoxidase is explained here for the first time. These enzymes catalyze the oxidation of halide ions (chloride, bromide, iodide) with hydrogen peroxide without the participation of the metal ions of cofactors in the electron transfer. The redox reaction in these enzymes takes place on a methionine residue of the protein chain by the reaction cycle methionine → methionine sulfoxide → halogenated methionine → methionine.
Chemoenzymatic Syntheses of Sialyloligosaccharides with Immobilized Sialidase
Authors: Thiem Joachim , Sauerbrei Bernd
Published: December 2003

The α‐glycosidic transfer of N‐acetylneuraminic acid (Neu5Ac) to the terminally bound galactose of the acceptor molecule is achieved with immobilized sialidase obtained from Vibrio cholerae. This is the first application of this enzyme in the catalysis of transglycosylation, which produces (2–6)‐ and (2–3)‐linked sialyloligosaccharides. These molecules often cannot be prepared by feasible synthetic methods, yet their physiological importance in glycoproteins and ‐lipids continue to generate interest.
Microporous Silica Films
Authors: Maier Wilhelm F. , Wiedorn Michael , Schramm Herbert O.
Published: December 2003

A narrow range of pore radii, pore diameters of 6 ± 2 Å, and a porosity of 14% characterize the silicon dioxide films which can be made by the electron beam evaporation of quartz. Thus they offer an alternative to zeolite films for the separation of liquids and gases, and applications in nonlinear optics and microelectronic sensors.
Switch Peptides: pH‐Induced α‐Helix to β‐Sheet Transitions of Bis‐amphiphilic Oligopeptides
Authors: Mutter Manfred , Gassmann R. , Buttkus U. , Altmann K.‐H.
Published: December 2003

The interconversion of two secondary structure types (α‐helix ⇆ β‐sheet) induced by external conditions such as pH, temperature, concentration, and solvent could be proved in a new class of amphiphilic peptides (switch peptides) in aqueous solution. The pH‐dependent conformational changes were studied by CD spectroscopy. Special attention was given to the investigations of four newly synthesized peptides with similar sequences.
New Approaches to the Use of Amino Acids as Chiral Building Blocks in Organic Synthesis [New Synthetic Methods (85)]
Authors: Reetz Manfred T.
Published: December 2003

α‐Amino acids protected at nitrogen in quite different ways can be transformed without racemization into the corresponding α‐amino aldehydes. Provided one chooses the right protecting groups (e.g., two benzyl residues on nitrogen) it is possible for the first time to carry out Grignard‐like, aldol, and Me3SiCN additions, and hetero‐Diels–Alder reactions with a high degree of nonchelation control. If the reactions of classical carbanions turn out to be nonselective, transmetalation, for example into organotitanium reagents, constitutes an important tool in controlling stereoselectivity. Diastereoselectivity can be reversed by specific variation of the protecting group and reagent. “Protecting‐group tuning”, “metal tuning”, and “ligand tuning” are very useful in reactions of α‐amino aldimines as well. The α‐amino aldehydes can also be converted by Wittig reactions into electron‐poor γ‐amino olefins, which in turn undergo stereoselective cuprate, Michael, and cycloadditions.
1,4‐Dihydropyridines: Effects of Chirality and Conformation on the Calcium Antagonist and Calcium Agonist Activities
Authors: Goldmann Siegfried , Stoltefuss Jürgen
Published: December 2003

4‐Aryl‐l,4‐dihydropyridine‐3,5‐dicarboxylic diesters of the nifedipine type have become almost indispensable for the treatment of cardiovascular diseases since they first appeared on the market in 1975. There are some twenty derivatives currently under clinical development worldwide and work in this area is continuing undiminished. The 1,4‐dihydropyridines are the most effective of the calcium antagonists or calcium channel blockers. They are valued not only for their pharmacological effect, but also as a tool for the investigation of the calcium channel, particularly since the discovery that this class also includes compounds that have exactly the opposite action profile and are known as calcium agonists. There are even instances in which this reversal of activity is found between enantiomers. In view of the importance of chirality to pharmacological activity, the present article will describe methods for the separation of enantiomers, point out the structural differences between calcium antagonists and calcium agonists, and attempt to explain the difference in their behavior.
Silicon Nitride—From Powder Synthesis to Ceramic Materials
Authors: Lange Horst , Wötting Gerhard , Winter Gerhard
Published: December 2003

Silicon nitride is a ceramic material of great interest to advanced engine construction and mechanical engineering owing to an outstanding combination of favorable properties like high mechanical strength at high temperatures, corrosion and wear resistance, great hardness, and low density. The material is based on high‐quality Si3N4 powders, which are shaped and sintered to the ceramic component. This overview outlines the properties required for Si3N4 powders suitable for advanced ceramics. Processes in commerical use and those under development for the production of high quality Si3N4 powders are discussed as well as material manufacturing processes and material properties. By steadily improving powder quality, material properties, and the economy of powder and component production, chemistry and chemical technology play a major role in recent efforts to create a solid fundament for broad applications of silicon nitride ceramics.

“If one would surrender to geological phantasies, one could imagine that during the formation of our planet, when elements combined to the compounds making up its crust and mountain ranges, silicon reacted with nitrogen, and the still red‐hot nitrogen‐silicon, on contact with water, may have decomposed to silicic acid and ammonia. Thus, ammonia may have been formed originally and nitrogen thereby introduced into the forming organic compounds when living nature first started to appear.”

H. Sainte‐Claire Deville and F. Wöhler

“Ueber die directe Bildung des Stickstoffsiliciums” in: Ann. Chem. Pharm. 34 (1859) 248: “If one would surrender to geological phantasies, one could imagine that during the formation of our planet, when elements combined to the compounds making up its crust and mountain ranges, silicon reacted with nitrogen, and the still red‐hot nitrogen‐silicon, on contact with water, may have decomposed to silicic acid and ammonia. Thus, ammonia may have been formed originally and nitrogen thereby introduced into the forming organic compounds when living nature first started to appear.”
Routes to New Aromatic Polycarbonates with Special Material Properties
Authors: Freitag Dieter , Fengler Gerd , Morbitzer Leo
Published: December 2003

Polycarbonate together with the acrylonitrile/butadiene/styrene copolymers, and the poly amides constitute the leading groups of engineering thermoplastics; for polycarbonate in particular, continued dynamic growth is prophesized.

The underlying reason for this lies not only in its outstanding combination of technical properties and excellent price/performance balance, but also in the chemical and physical potential inherent in the basic structure of polycarbonate. The following review demonstrates with examples how this potential can be used in the development of new polycarbonates through the incorporation of alternative monomers, changes in the linear structure, end group variation, addition of special additives, and blending. The main emphasis of this articles lies in the development of polycarbonates resistant to high temperature, with a good balance of technologically valuable properties. For scientific and practical interests a new criterion is offered for polycarbonates having both high‐temperature stability and impact strength.

“One must not only make a discovery, but also realize that a discovery has been made.”

Hermann Schnell [1]
Glycolipids as Immunomodulators: Syntheses and Properties
Authors: Lockhoff Oswald
Published: December 2003

Although the most successful vaccines still consist of attenuated or killed pathogens, it is evident that genetic engineering will eventually provide pure antigens in sufficient quantities. To achieve a protective effect, however, the immune response to these structures, which are only weakly immunogenic, must be activated by adjuvants. The discovery of chemically defined, well‐tolerated. synthetic adjuvants is therefore especially important in effective immunoprophylaxis. A very promising class of compounds in this respect are synthetic glycosylamides, with two long alkyl chains consisting of alkylamines and fatty acids. Unlike natural glycolipids, which exert an immunosuppressive effect, the new glycolipid analogues described here show marked immunostimulation. In vaccination experiments with various antigens they provide efficient protection. Their mode of action, which differs from that of known adjuvants, makes glycolipid analogues particularly interesting for immunization of patients with defective T lymphocyte function, for example AIDS patients.
Herbicides in Photosynthesis Research
Authors: Draber Wilfried , Tietjen Klaus , Kluth Joachim F. , Trebst Achim
Published: December 2003

Photosynthesis and herbicide research have a long common history. Soon after the introduction of a new group of highly effective herbicides in 1956, the substituted aryl ureas, their mode of action was recognized to be the inhibition of light‐driven electron flow and photosynthetic oxygen evolution. Many subsequent highly successful commercial herbicides, including those from other chemical classes, act by this mechanism. The study of their interaction with the photosynthetic system at the molecular level is a prime example of how rapidly advances in unraveling the mechanism of photosynthesis can be translated into practical uses. At the same time the herbicides continued to be valuable and efficient tools in photosynthesis research. In the past herbicides were instrumental in establishing many principal features of the biochemistry and biophysics of photosynthesis, in particular the role of plastoquinone, which is displaced from its binding pocket in one of the protein subunits of the photosystem II. Their further use in elucidating the molecular biology of photosynthesis is illustrated by the recent importance of herbicide‐tolerant mutants for determining structural aspects of photosystem II. We will review the past and present state of the interaction of herbicide and photosynthesis research and will provide a model for the orientation of herbicides within the three dimensional structure of their target, the D1 protein of photosystem II.
Methyltrioxorhenium as Catalyst for Olefin Oxidation
Authors: Herrmann Wolfgang A. , Fischer Richard W , Marz Dieter W.
Published: December 2003

The pronounced Lewis acidity explains the exceptional activity of the title compound as catalyst for olefin epoxidations with hydrogen peroxide. The system MTO/tBuOH/H2O2 transforms olefins selectively and efficiently into epoxides under very mild conditions (−30 to + 60°C, but normally at room temperature). In general, 0.1 to 1 mol% MTO is sufficient to achieve high turnovers. Variation of the alkyl group and addition of cocatalysts (e.g. amines) should influence activity and stereoselectivity.
Analysis of Polymer Latices by Small‐Angle X‐Ray Scattering
Authors: Grunder Ralf , Kim Young Sam , Ballauff Matthias , Kranz Dietmar , Müller Hans‐Georg
Published: December 2003

Analysis—requiring no pretreatment—of the core‐shell morphology of polymer latices is achieved by X‐ray small‐angle scattering in combination with ultra‐centrifuge sedimentation. The latter technique is required for the determination of the particle size distribution. For larger scattering angles (g > 0.1 nm−1), the measured and calculated scattering intensities agree quantitatively; the differences for the smallest g values may be attributed to interparticle interferences and give information on the interaction of the particles.
Surface Organometallic Chemistry on Oxides: Selective Catalytic Low‐Temperature Hydrogenolysis of Alkanes by a Highly Electrophilic Zirconium Hydride Complex Supported on Silica
Authors: Lecuyer Christine , Quignard Francoise , Choplin Agnès , Olivier Danielle , Basset Jean‐Marie
Published: December 2003

At 50°C already the hydrogenolysis of neopentane, isobutane, and propane is catalyzed by the silica‐supported zirconium hydride [( ⦠ SiO)3ZrH]. The reaction selectively produces methane together with isobutane, propane, and ethane, respectively. The activity is attributable to the high electrophilicity of the 8‐electron do complex and is not observed for nonsupported, discrete zirconium hydrides. Moreover, the attachment to the support prevents a bimolecular deactivation.
[(Ph3PAu)6(dppeAu2)(AuCl)4Pd], an Icosahedral Au12 Cluster with a Central Pd Atom
Authors: Laupp Martin , Strähle Joachim
Published: December 2003

Photolysis of the azido complexes [Ph3PAuN3] and [(dppe)Pd(N3)2] or [(Ph3P)2Pd(N3)2] in the presence of [Ph3PAuCl] affords the new heterodimetal clusters 1 and 2, in which the twelve gold atoms form an icosahedron around a central Pd atom. These first icosahedral Au12 clusters containing a heteroatom in their center are isoelectronic with the Au13 cluster [(PhMe2PAu)10(AuCl)2Au]3+.
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Neutral Gold(I) Fluoride Does Indeed Exist
Authors: Schröder Detlef , Hrušák Jan , Tornieporth‐Oetting Inis C. , Klapötke Thomas M. , Schwarz Helmut
Published: December 2003

AuF+ and AuF‐ can be neutralized in the gas phase. Neutralization‐reionization mass spectrometry thus provides proof of the existence of neutral AuF, which was predicted by theory. The upper and lower limits for the bond dissociation energy could also be deduced: 68 kcal mol−1 < BDE < 85 kcal mol−1. The AuF ions are generated by electron impact ionization from AuF3.