bluuebunny/crossref_metadata_2025_split huggingface dataset:
Metal Complexes of Small Cycloalkynes and Arynes
Authors: Bennett Martin A. , Schwemlein Heinz P. 
Published: December 2003

AbstractCyclooctyne is the smallest unsubstituted cycloalkyne that can be isolated in the free state and it is more reactive than acyclic alkynes towards transition metal complexes. Smaller cycloalkynes such as cycloheptyne, cyclohexyne, benzyne and cyclopentyne, which are transient molecules in the free state, can be stabilized by coordination either to mononuclear, electron‐rich, transition metal‐containing fragments, e.g. [ZrCp2(PMe3)] and M(PR3)2 (M = Ni, Pt), or by formation of dinuclear or polynuclear metal complexes, e.g. [Os3H2(CO)9(C6H4)] and [Pt2(μ‐PPh2)(μ‐C5H6)(PPh3)3]. The alkynes can donate between two and four π‐electrons to the metal centers, the higher number being favored for the early transition metals. The metal‐cycloalkyne and metal‐aryne bonds in the mononuclear complexes readily insert molecules containing CO, CC, CC and CN bonds, a feature that may be useful in organic synthesis. The highly unsaturated species 1, 4‐benzdiyne acts as a bridging ligand between two metal centers in [{Ni(Cy2PCH2CH2PCy2)}2(μ‐C6H2)].
Organic Dications: Gas Phase Experiments and Theory in Concert
Authors: Lammertsma Koop , von Ragué Schleyer Paul , Schwarz Helmut 
Published: December 2003

AbstractKnowledge of structure and bonding are integral parts in the understanding of the chemistry of neutral, radicaloid and positively or negatively charged species. The interplay of molecular bonding and the inherent electrostatic repulsion in doubly‐charged molecules expands our insight in the versatility of bonding in electron deficient molecules. Small organic dications are now routinely detected in the gas phase by a variety of mass spectrometric techniques. Ab initio molecular orbital theory has advanced to the stage of accurately predicting potential energy surfaces. This review highlights the symbiotic relationship between gas‐phase ion and computational studies and gives a rationale for the salient bonding features in doubly‐charged organic cations.
Highly Reactive Intermediates from the Cocondensation Reactions of Iron, Cobalt and Nickel Vapor with Arenes
Authors: Zenneck Ulrich 
Published: December 2003

AbstractIn 1969, P. L. Timms reported the first preparative cocondensation reactions of metal vapors with organic and inorganic substrates. The use of this technique in preparative chemistry soon spread rapidly, but in recent years there has been less activity in this sector. If metal atom reactions are not utilized primarily for the formation of new products, but for the synthesis of highly reactive intermediates, a new synthetic strategy may be developed. Our aims are reaction sequences which, based on an effective cocondensation reaction, lead gradually and selectively to new substance classes. This principle can be illustrated by the example of the cocondensation products of arenes and iron, cobalt, or nickel vapor, which decompose between −70 and −50°C. The classes of products accessible by this method extend from clusters, through π‐complexes, organophosphorus and organoboron cage compounds to pure organic cycloaddition products.
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

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

AbstractOur 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.
The Chemistry of Interstellar Space
Authors: Herbst Eric 
Published: December 2003

AbstractWithin 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.
Nonmolecular Metal Chalcogenide/Halide Solids and Their Molecular Cluster Analogues
Authors: Lee Sonny C. , Holm Richard H. 
Published: December 2003

AbstractA 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

AbstractThe 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.
Advanced Materials: Trends and Possibilities in Liquid Crystalline Polymers
Authors: Economy James 
Published: December 2003

AbstractDuring 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.
Enantioselective Addition of Organometallic Reagents to Carbonyl Compounds: Chirality Transfer, Multiplication, and Amplification
Authors: Noyori Ryoji , Kitamura Masato 
Published: December 2003

AbstractNucleophilic 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

AbstractChemical 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

AbstractThe 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.
Ribonuclease T1: Structure, Function, and Stability
Authors: Pace C. Nick , Heinemann Udo , Hahn Ulrich , Saenger Wolfram 
Published: December 2003

AbstractProteins 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

AbstractAlkylidene 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

AbstractThe 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

AbstractOne 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.
Phytochrome, the Visual Pigment of Plants
Authors: Rüdiger Wolfhart , Thümmler Fritz 
Published: December 2003

AbstractPlants 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.
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

Abstractα‐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.