bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct Identification of the Carbon Skeleton of Organic Compounds using Double Quantum Coherence 13C‐NMR Spectroscopy. The INADEQUATE Pulse Sequence
Authors: Buddrus Joachim , Bauer Hans
Published: December 2003

NMR spectroscopy plays an important part in the determination of the structures of organic compounds. The parameters of importance here are the chemical shifts of the 1H and 13C nuclei and the spin‐spin interactions both between 1H nuclei and between 1H and 13C nuclei. Couplings between 13C nuclei were almost completely neglected until a few years ago, since they were extremely difficult to observe because of the low natural abundance of 13C. However, it is these couplings which afford information directly on the carbon‐carbon connectivities in the molecule. It is now possible to use a special NMR pulse sequence to make these couplings more readily visible: the result of using this sequence is a 13C‐NMR spectrum from which the carbon skeleton concerned can be directly read off. Two‐dimensional spectra in particular are very easy to evaluate. The pulse sequence involved, which bears the somewhat puzzling name INADEQUATE, produces double‐quantum coherences from which the NMR signals of the coupled carbon nuclei can be obtained. In this article the principle of double‐quantum coherence is described and a number of examples for the application of the INADEQUATE pulse sequences to problems in synthetic organic chemistry, biosynthesis and natural products chemistry are presented; in addition, the possibility of applying the INADEQUATE method to other nuclei is considered.
The Nerve Growth Factor: 35 Years Later (Nobel Lecture)
Authors: Levi‐Montalcini Rita
Published: December 2003

Growth factors—for nerves and for epidermal tissue—are the areas of research of last year's recipients of the Nobel Prizes for Medicine. The growth of nerves and of epidermal tissue requires a chemical stimulant. Nerve growth factor (NGF) proved to be a freely diffusing protein that is essential for the normal development of embryos, but which, in excess, results in major disruption of neurogenic processes. NGF has been isolated from several sources, including the salivary glands of mice. Crude extracts of NGF had unexpected side effects, the systematic investigation of which led to the discovery of epidermal growth factor (EGF). EGF, like NGF, is a protein, whose sequence has been determined. The primary signal mediated by EGF is thought, at present, to involve the tyrosine kinase activity of its receptor.
Charge Transfer and Radical Ions in Photochemistry
Authors: Mattay Jochen
Published: December 2003

Bimolecular photoreactions usually occur via several steps involving “complex‐type” intermediates (excimers or exciplexes). Although physical chemists have investigated them for nearly thirty years (Förster, Weller), and postulated their existence in numerous publications, it is only in the last decade that evidence for an exciplex intermediate in cycloadditions (Caldwell) has been obtained, thereby establishing the link between spectroscopy and synthetic photochemistry. Since then, many investigations have confirmed the role of exciplexes as intermediates in bimolecular photoreactions. The photochemically induced charge transfer from a donor to an acceptor substrate determines not only the bond strengths in the primary exciplex but also influences—via the charge‐transfer nature of the complex—the structure of the exciplex and thereby the selectivity of subsequent reactions. Such chemical processes between the donor and the acceptor prevent the reverse transfer of electrons, which usually results in regeneration of the starting materials, thereby only causing the dissipation of energy and occasionally—under specific conditions—the generation of molecules of the starting material in the triplet state. Ionic photodissociation also reduces the probability of a reverse transfer of electrons. This process, which produces solvated radical ions, is promoted by polar solvents and salt effects, particularly special salt effects. By this means it is possible to generate specific radical cations from, for example, alkenes and dienes and to study their reactions. Although in some areas, for example physical and biophysical chemistry, discussions of single electron transfer (SET) processes have long been found in the basic textbooks, organic chemistry lags far behind, although—and this is shown by the numerous recent publications—organic chemists are also aware of the fundamental importance of these processes. In this connection photochemistry plays a central role, since its instruments make it possible to carry out specific SET processes, to analyze them, and to use them in chemical reactions.
Dynamic Processes on the IR Time Scale: Coalescence of CO Stretching Vibrational Bands in [(η4‐Diene)Fe(CO)3] Complexes
Authors: Grevels Friedrich‐Wilhelm , Jacke Jürgen , Klotzbücher Werner E. , Krüger Carl , Seevogel Klaus , Tsay Yi‐Hung
Published: December 2003

Complex chemists should check the temperature dependence of the IR spectra of their compounds more often. The IR spectra of the Fe(CO)3 complexes with norbornadiene and 1,5‐cyclooctadiene at room temperature show only two CO bands, despite Cs symmetry, whereas at ca. 10 K the expected three bands are observed with about equal intensities. That is, a coalescence must take place by a very rapid intramolecular rearrangement whose activation energy is only ca. 2. 2 kcal mol−1.
Iodophosphonium Salts with Unusual Properties and a Structural Alternative for Halophosphoranes
Authors: du Mont Wolf‐Walther , Bätcher Michael , Pohl Siegfried , Saak Wolfgang
Published: December 2003

The NMR spectroscopically monitored course of the reaction of tBu3P with I2 in CH2Cl2 is most unusual: no separate signals are found for the products, and the continuous changing of the chemical shifts and coupling constants does not cease at the “equivalence point” (tBu3PI: I2 = 1:1). This can be explained in terms of the equilibria (a)–(c). Also unusual is the structure of tBu3PI2 in the solid state: tBu3PII.
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Ether Cleavage with Organo‐Alkali‐Metal Compounds and Alkali Metals
Authors: Maercker Adalbert
Published: December 2003

Reactions with organo‐alkali‐metal compounds are mostly carried out in ether‐type solvents. It has long been known that ether cleavage takes place at the same time. The mechanism of the cleavage, in particular the fact that dialkyl ethers can be decomposed by a variety of mechanisms acting simultaneously, has only become clear in the most recent investigations. The observation that even with purely aliphatic ethers a considerable amount of Wittig ether rearrangement can occur is remarkable. Unusual secondary reactions can also occur by means of which alkyl‐ and aryllithium compounds in ether or tetrahydrofuran yield new organolithium compounds—sometimes with rearrangement. The reactions of alkyl aryl ethers with alkali metals are also varied and five different mechanisms for them have been discussed in the literature. It is interesting, for example, that the cleavage of anisole can be directed simply by changing the solvent so that in one case only cleavage of the aryl‐oxygen bond occurs and in the other almost 100% cleavage of the alkyl‐oxygen bond results. The formation of biphenyl as a by‐product upon cleavage of the alkyl‐oxygen bond in anisole was also puzzling. This was later shown to occur not through the dimerization of phenyl radicals but via 2‐methoxybiphenyl. The number of cleavage mechanisms reduces to two if one assumes two different σ*‐radical anions as being intermediates in the kinetically controlled reaction. Comparison of the reactivity of thioethers and ethers reveals not only gradual but also fundamental differences—the cleavage of the thioethers is thermodynamically controlled.
Novel Bimetallic Macrocyclic Polymer Structures Incorporating Deprotonated 2‐Pyrrolidone Bridges: The Crystal Structures of [{Hg2Cu(C4H6NO)4}X2]n(X  NO3 or ClO4)
Authors: Goodgame David M. L. , Williams David J. , Winpenny Richard E. P.
Published: December 2003

Both title compounds are polymers with layers consisting of 32‐membered rings having the composition C8N8O8Hg4Cu4, They are obtained by heating Hg(OAc)2, 2‐pyrrolidone, and Cu(NO3)2 or Cu(ClO4)2 in methanol. The counterions are decisive for maintaining the structure within the corrugated layers. The O atom of an NO occupies an apical site of Cu; the two other O atoms form an HgONOHg bridge. ClO, on the other hand, forms only a CuOClOHg bridge.
Fluorides of Copper, Silver, Gold, and Palladium
Authors: Müller Bernd G.
Published: December 2003

Fluorine, by far the most reactive of the non‐metals, is capable of forming a large number of compounds with nearly all other elements (exceptions (so far): He, Ne, and Ar), even under comparatively “mild” reaction conditions. These compounds usually differ markedly from those of the heavier halogens in composition, structure, and chemical and physical properties. Thus, for example, it is generally quite easy to prepare fluorides containing elements in high oxidation states (often their maximum), as in AgF2, CsAgF4, PdF3, CsAuF6 etc., whereas the corresponding chlorides, bromides or iodides are in many cases (still) unknown. Conversely, the synthesis of fluorides containing these elements in middle or low oxidation states often meets with considerable difficulty, even where it is possible at all, as, e.g., in the case of CuF, AuF, PtF2, SeF2. Finally, there are also some examples of compounds MXn, which with X = F are stable, but with X = Cl are unstable or decompose easily (e.g. CoF3/CoCl3, VF4/VCl4, PbF4/PbCl4, AsF5/AsCl5). Consequently, fluorine compounds are of great general interest.
Conformational Transitions between Enantiomeric 310‐Helices
Authors: Hummel Rolf‐Peter , Toniolo Claudio , Jung Günther
Published: December 2003

The 310‐helix enantiotopermerization of the decapeptide Z‐(Aib)10‐OtBu occurs at 1200 interconversions per second at room temperature. This finding, obtained by 13C‐DNMR measurements, was derived from the coalescence temperature of 265 K for this 3−3‐helix interconversion, observed here for the first time. The corresponding free energy of activation is only 46 kJ mol −1. Aib is the achiral α‐aminoisobutyric acid building block, which is important in drug design.
Reaction of Sn with Single Crystal Au: Textured Fiber Epitaxial Growth of AuSn at Room Temperature
Authors: Haushalter Robert C. , Treacy Michael M. J. , Rice Stephen B.
Published: December 2003

Transmission electron spectroscopic investigations of thin layers of AuSn on single‐crystal Au foils–prepared by dipping the foils for varying lengths of time into ethylenediamine solutions of K4 Sn9–demonstrated epitaxial growth of AuSn crystallites. It has been known for a long time that Sn ions immediately attack gold, but this is the first close characterization of the surface species formed.
Structure and Color of Pentaphenylbismuth
Authors: Schmuck Arno , Buschmann Jürgen , Fuchs Joachim , Seppelt Konrad
Published: December 2003

A violet color and strong dichroism are very unusual features for a compound of a main‐group element. The X‐ray structure analysis performed at −96°C reveals that Bi(C6H5)5, synthesized earlier by Wittig et al., may be described as a square pyramid and is thus very similar to the (colorless!) Sb(C6H5)5. Optical investigations showed that excitation occurs in the basal plane of the square pyramid. A charge‐transfer transition from a ligand to the bismuth atom was proposed.
Polystannanes Ph3Sn‐SnPh3(n = 1–4): A Route to Molecular Metals?
Authors: Adams Stefan , Dräger Martin
Published: December 2003

A diminution in the HOMO‐LUMO energy difference with increasing chain length in the polystannanes 2 mentioned in the title is indicative of a smooth transition between covalently bound polystannanes and metallic tin and thus affords grounds for answering the question posed in the title in the affirmative. The four polystannanes synthesized according to equation (a) are all‐trans‐configurated; compound 2, with n4, contains the longest SnSn bond observed so far.
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A New Mechanism For Superconductivity
Authors: Dewar Michael J. S.
Published: December 2003

The interplay of two Cu‐O vibrations in YBa2Cu3O7 could–at least‐qualitatively–not only explain the surprisingly high critical temperature of this material but also the sensitivity of the critical temperature towards magnetic fields and 16O/18O isotopic exchange. Important is a linear arrangement of metal atoms in two oxidation states (e.g. CuII/CuIII), in'which two coordinated lattice vibrations lead to (directed) electron hopping.
Natural Fats and Oils—Renewable Raw Materials for the Chemical Industry
Authors: Baumann Horst , Bühler Matthias , Fochem Heinz , Hirsinger Frank , Zoebelein Hans , Falbe Jürgen
Published: December 2003

Since last century, the supply of raw materials for the chemical industry has undergone a radical change. Whereas at the beginning of the nineteenth century the demand for basic chemicals was satisfied entirely by renewable raw materials, from about 1850 the chemical industry came to rely increasingly upon coal. In the 1940's, mineral oil started to become increasingly important, and during the past thirty years it has remained by far as the most important source of raw materials. Renewable raw materials are likely to become important again in the future, as the choice of raw materials is now of great significance not only for economic reasons by influencing competitiveness, but also because this choice largely determines the properties of the derivatives produced and their ecological effects. Following the two oil crises of the 1970's, there is also now a growing awareness of the limits of raw material resources, and against the continuing background of agricultural surpluses, chemists are again showing an increasing interest in renewable raw materials. Since the end of the seventies, the Bundesministerium für Forschung und Technologie (Ministry of Research and Technology of the Federal Republic of Germany) has supported research projects on renewable raw materials in universities and in industry. For the chemical industry the use of natural products as raw materials opens up a wider spectrum of synthetic methods and finished products, some of which are not accessible by petrochemical routes.
Flexibility and Rigidity of Proteins and Protein–Pigment Complexes
Authors: Huber Robert
Published: December 2003

Proteins may be rigid or flexible to various degrees as required for optimal function. Flexibility of large parts of a protein, which rearrange or move, is particularly interesting and will be discussed in this article. We differentiate between several categories, although the boundaries between them are diffuse: flexibility of peptide segments, order–disorder transitions of spatially contiguous regions, and domain motions. The domains may be flexibly linked to allow rather unrestricted motions or the motions may be constrained to certain modes. The various categories of large‐scale flexibility will be illustrated with the following examples: (1) Small protein proteinase inhibitors are rather rigid molecules which provide binding surfaces complementary to their cognate proteases but show also limited segmental flexibility and adaptation. (2) Large plasma proteinase inhibitors exhibit large conformational changes after interaction with proteases probably for regulatory purposes. (3) Pancreatic serine proteases employ a disorder–order transition of their activation domain as a means to regulate enzymic activity. (4) Immunoglobulins show rather unrestricted and also hinged domain motions in different parts of the molecule probably to allow binding to antigens in different arrangements. (5) Citrate synthase adopts open and closed forms by a hinged domain motion to bind substrates and release products and to perform the catalytic condensation reaction, respectively. (6) Riboflavin synthase, a bifunctional multienzyme complex, catalyzes two consecutive reactions by means of two subunits, α and β. The β‐subunits form a shell, in which the α‐subunits are enclosed. Diffusional motion of the catalytic intermediates is therefore restricted. In addition, rearrangement of the N‐terminal segment occurs during the assembly of the β‐subunit. In contrast, rigidity is dominant in the structures of the light‐harvesting complexes and the photosynthetic reaction centers involved in photosynthetic light reactions. These are large protein–pigment complexes in which the proteins serve as matrices to hold the pigments in the appropriate conformation and relative arrangement. Since motion would contribute to deactivation of the photoexcited states of the pigments and diminish the efficiency of light‐energy and electron transfer, the functional role of rigidity is easy to rationalize for these proteins.
Structure Elucidation and Artificial Intelligence
Authors: Bremser Wolfgang
Published: December 2003

The difference between the older and the more recent computer systems amounts, in the final analysis, to the difference between an insect and a human being. The insect comes “fully programmed” into the world–programmed by instincts which it follows without using any thought processes. On the other hand, the human being must first learn the correct behavior–however, such learning can develop an impetus all its own: becoming, indeed, proceeding from his own decision and his awareness, the human being can change his former action programs. The creation of a computer system of the X‐th generation will actually far more resemble educating a child than the traditional programming of a computer.
[{Hg3Co(C4H6NO)6}(NO3) 2]n: A Macrobicyclic Bimetallic Chain Polymer Incorporating Deprotonated 2‐Pyrrolidone Bridges
Authors: Goodgame David M. L. , Williams David J. , Winpenny Richard E. P.
Published: December 2003

For corrigendum see DOI:10.1002/anie.198805892

For corrigendum see DOI:10.1002/anie.198805892

For corrigendum see DOI:10.1002/anie.198805892

For corrigendum see DOI:10.1002/anie.198805892

For corrigendum see DOI:10.1002/anie.198805892
Cyclodextrins Increase Surface Tension and Critical Micelle Concentrations of Detergent Solutions
Authors: Saenger Wolfram , Müller‐Fahrnow Anke
Published: December 2003

Because they function as solubility enhancers! Cyclodextrins envelop the hydrophobic part of the detergent molecule in their cavity and thus destroy the formation of micelles as well as of monolayers at the air/water interface. The former increases the critical micelle concentration, the latter the surface tension. The effect is most pronounced when the cyclodextrin cavity and hydrophobic part of the detergent molecule exhibit the tightest fit. Cyclodextrins are thus suitable, inter alia, as antifoaming agents.
Structure of Molecular PNO, Matrix Isolation and ab initio Calculations
Authors: Ahlrichs Reinhart , Schunck Stephan , Schnöckel Hansgeorg
Published: December 2003

Molecular PNO is formed upon photolysis of PN and O3 in an argon matrix. From the IR spectrum its structure can be formulated as \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${\rm P} \begin{array}{c}\hbox{---}\\[-11pt]\hbox{---}\\[-10pt]\raise1pt\hbox{---}\\\end{array}\mathop {\rm N}\limits^\oplus \raise1pt\hbox{---}\mathop {\rm O}\limits^\ominus$\end{document}, since a band at ṽ = 1755 cm−1 can only be assigned to a NO vibration. This structure contradicts chemical intuition, according to which the isomer NPO should be more stable, but is in agreement with the results of quantum chemical calculations, which show that PNO is 22 kJ mol−1 more stable than NPO.
One or Several Pioneers? The Discovery of Noble‐Gas Compounds
Authors: Laszlo Pierre , Schrobilgen Gary J.
Published: December 2003

For the general public, science history and restaurant menus share a labeling style. Like Melba peach, Soubise sauce, or tournedos Rossini, many a scientific discovery is credited to a single thinker or tinkerer. Such ultrasimplification rubs into oblivion other important contributors or codiscoverers. The discovery of noble‐gas compounds conforms to such a stereotyped and mythic view: according to the standard account, Neil S. Bartlett, then at the University of British Columbia, made it a quarter of a century ago on March 23, 1962. We do not attempt here a full and definitive history; it would be premature and we do not have the space. Rather, we shall document the concept and the earlier attempts at synthesis–they antedated Bartlett's success by three decades. The motivation for this note may be summarized in the following questions: (1) What sort of insight and input were those of Linus Pauling? (2) What were the likely causes for failure of the attempt by Yost and Kaye to synthesize noble‐gas fluorides in the 1930s? (3) Were there other researchers who envisioned a noble‐gas reactivity?
Carbon Dioxide as an Alternative C1 Synthetic Unit: Activation by Transition‐Metal Complexes
Authors: Behr Arno
Published: December 2003

The carbon dioxide molecule has been of limited importance as a synthetic unit in organic chemistry. When it is coordinated to transition metals, however, completely new possibilities arise; CO2 can bond to metal complexes in a variety of ways and can enter into insertion and coupling reactions, or become catalytically attached to other substrates. The formation of CC bonds between carbon dioxide and unsaturated hydrocarbons under conditions of homogeneous catalysis makes available new synthetic routes to industrially interesting organic compounds.
Isoleucyl‐tRNA Synthetase: An Enzyme with Several Catalytic Cycles Displaying Variation in Specificity and Energy Consumption
Authors: Freist Wolfgang
Published: December 2003

The translation of DNA via the genetic code into protein is a fundamental process carried out by all living systems. The extreme accuracy with which it is performed requires that the enzymes involved operate with extremely high specificity. It is for this reason that the structure and function of these proteins have been of special interest to molecular biologists and biochemists for the last twenty years. The discrimination between the different amino acids present in the cytosol and their esterification with the correct tRNA is the special task of the aminoacyl‐tRNA synthetases. One of the most difficult discriminations that has to be performed is the specificity for isoleucine. There are many amino acids in the cell that have a structure similar to isoleucine, in particular valine. To solve this problem, the aminoacyl tRNA synthetases have evolved unique proofreading mechanisms. The most detailed investigations of these mechanisms have been on the isoleucyl‐tRNA synthetases from yeast and from Escherichia coli. Both enzymes are single‐chain polypeptides with molecular weights of about 110 000. Sequence homologies with other aminoacyl‐tRNA synthetases seem to occur only within narrow regions, for example in the ATP binding site. The identification of the correct amino acid is achieved in a four‐step process. In the first two steps the hydrophobic interaction of the amino acid side chain with the enzyme is utilized for discrimination. The two hydrolytic proofreading steps that follow make use of the steric hindrance associated with the amino acid side chain. The enzymes are characterized by the special property that slight variations in the reaction conditions cause conformational changes that affect the catalytic cycle. In this way the observed specificity of the enzymes can be changed to almost any value. In general, high specificity is associated with high energy cost, which is reflected in high ATP consumption. Under physiological conditions a medium level of specificity and energy consumption seems to be preferred.
Photochemical Reactions of Ligands in Transition‐Metal Complexes
Authors: Poznyak Anatoli L. , Pavlovski Valeri I.
Published: December 2003

Three types of photochemical reaction of coordination compounds are well‐known and have often been discussed: redox reactions, substitutions, and isomerizations. Less well known is a fourth type of reaction, which occurs fairly frequently: reaction of the ligands themselves. It is the purpose of this article to fill the corresponding gap in the photochemical literature and to summarize our current knowledge of such reactions and of their mechanisms and possible applications.
(Mesityl)2Ge(fluorenylidene)–Stabilization of a Ge–C Double Bond by Charge Transfer into an Aromatic System
Authors: Lazraq Mohamed , Escudié Jean , Couret Claude , Satgé Jacques , Dräger Martin , Dammel Ralph
Published: December 2003

Not only steric shielding but also resonance with a delocalized π‐electron system can help to stabilize nonclassical multiple bonds. Thus, despite high steric demands, 15 C atoms and the Ge atom of the title compound 1 lie nearly in the same plane. The experimental finding is in agreement with the result of a MNDO calculation.
“Proton Sponges” and the Geometry of Hydrogen Bonds: Aromatic Nitrogen Bases with Exceptional Basicities
Authors: Staab Heinz A. , Saupe Thomas
Published: December 2003

Certain aromatic diamines (the “proton sponges”) are found to have exceptionally high basicity constants: this is due to spatial interaction of the basic centers, which are in close proximity. The two factors which are most important in causing this effect are, on the one hand, the extreme steric strain in these systems and the destabilizing effect of the overlap of the nitrogen lone pairs of the neutral diamines and, on the other, the strong NċHċN hydrogen bonds which are formed on monoprotonation and which lead to a considerable relaxation of the steric strain. By the systematic variation of the structures of such aromatic diamines we have been able to study these effects as a function of steric factors, in particular of the geometry and the bond length of the NċHċN hydrogen bonds, by means of X‐ray structural analysis. The hydrophobic shielding of the basic centers and the NċHċN hydrogen bonds, which was characteristic of the “proton sponge” compounds studied previously, is indeed responsible for the extremely low rate of protonation and deprotonation of these compounds; however, it apparently has no influence on their high thermodynamic basicity. The recent synthesis and basicity determination of a new type of “proton sponge” with no hydrophobic shielding whatever show that not only very strong but also kinetically active bases are accessible using the “proton sponge” concept. Their unusual properties, which are discussed here as the result of steric interactions between two basic centers, provide examples of the fact that cooperative steric interactions of reactive structural elements can lead to properties which cannot be derived from an isolated consideration of the various functional groups. Such “proximity effects” are certainly of general importance in chemistry and biochemistry; the study of their structure‐function relationships is worthy of closer consideration.
Atomic Motions in Molecular Crystals from Diffraction Measurements
Authors: Dunitz Jack D. , Maverick Emily F. , Trueblood Kenneth N.
Published: December 2003

The information provided by modern crystal structure analyses is not limited to the atomic arrangement. It also includes, for each atom, a set of quantities known as anisotropic Gaussian displacement parameters (ADP's), which provide information about averaged displacements of atoms from their mean positions. From analysis of these quantities, conclusions can be drawn about the rigid‐body motion of molecules, about large‐amplitude internal molecular motions, and about the identification of any disorder present in the crystals as being mainly dynamic or mainly static in nature. For some crystals, such analyses yield energy barriers to rotation of rigid molecules or molecular fragments that are in good agreement with values obtained by other physical methods.
μ‐Hydroxo‐ and μ‐Oxobis(μ‐acetato)diruthenium Complexes with Weak Intramolecular RuċRu Interactions
Authors: Neubold Peter , Wieghardt Karl , Nuber Bernhard , Weiss Johannes
Published: December 2003

Despite an increase in bond order from 1.0 to 1.5 an expansion of the RuRu bond of 0.084 Å is observed on going from the Ru‐complex 1 to the RuIIIRuIV‐complex 2—an electron is removed from an antibonding molecular orbital. The red‐violet, diamagnetic, dinuclear complex 1 is protonated at the μ‐oxo bridge by strong acids (2 M HCl), resulting in formation of the corresponding μ‐hydroxo complex in which the RuIII centers (d5, low spin) are coupled antiferromagnetically (J = −218 cm−1).
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Enzymes and Curvature
Authors: Blum Zoltan , Lidin Sven , Andersson Sten
Published: December 2003

Future music? That the curvature of specific faces in molecules has an influence on the properties of the molecule is, e.g., well known in the case of zeolites. A unifying description of proteins and enzymes on a similar basis provides a simple explanation of their role in biosystems. A possible application of the proposed concept is in the design of antiviral agents.
N‐Aryl‐O‐acylhydroxylamines: Preparation by O‐Acylation or N → O Transacylation and Reaction with Amines; Model Reactions for Key Steps Connected with the Carcinogenicity of Aromatic Amines
Authors: Boche Gernot , Bosold Ferdinand , Schröder Stefan
Published: December 2003

That aromatic amines Aryl‐NH2 are carcinogenic is (more or less) common knowledge nowadays. N‐Aryl‐O‐acetylhydroxylamines 2, which are active metabolites thereby, and other O‐acylhydroxylamines of this type have now been prepared by O‐acylation of arylhydroxylamines or by transacylation from the hydroxamine acids 1. The compounds 2 react in vitro as electrophilic aminating reagents with amines as nucleophiles to give hydrazines 3. 2 could react analogously in vivo with binucleophiles like DNA.
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Isolation and Structure of [(Fluorenone ⊙⊖){Na⊕(dme)2}]2—a Touchstone for Radical Contact Ion Pairs?
Authors: Bock Hans , Herrmann Hans‐Friedrich , Fenske Dieter , Goesmann Helmut
Published: December 2003

Single crystals of a radical ion pair salt—the title compound—could be grown after stoichiometric reduction of fluorenone with sodium in aprotic solution under argon at –50°C. The structure determination reveals that a dimeric contact ion pair containing an Na2O2 four‐membered ring is present and shows, for the first time, the bonding between radical anions and countercations: The solvated Na⊕ ion is situated above the plane of the molecule and outside of the πCO plane. This finding further supports the assumption of a π spin delocalization from the radical anion to the metal cation, as derived from the positive ENDOR metal couplings of radical ion pairs in solution.
Trimethylplatinum‐Theophylline Hexamer: A Novel Pt6 Heterocycle Containing Both PtN and PtO Bonds
Authors: Lorberth Jörg , Massa Werner , E.‐Essawi Mohy , Labib Lilian
Published: December 2003

A cyclic hexamer of trimethylplatinum units and theophylline anions is a model substance for R‐DNA complexes. The Pt atoms are coordinated in a distorted octahedral fashion with facial arrangement of the Pt‐N and Pt‐O bonding to one theophylline ligand and a further Pt‐N bond to a second theophylline ligand. Theophyiline functions thereby as bridging ligand, which is bidentate on one side and monodentate on the other. The six theophylline ligands form a layer between three Me3Pt groups.
Electron Transfer and Charge Transfer: Twin Themes in Unifying the Mechanisms of Organic and Organometallic Reactions
Authors: Kochi Jay K.
Published: December 2003

The broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer‐sphere and inner‐sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D⊕, A⊖], for which the free energy change can be evaluated from the charge‐transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge‐transfer ion pairs [D⊕, A⊖] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge‐transfer bands of various EDA complexes with a 25‐ps laser pulse. Such studies confirm the radical ion pair [Arene⊕, NO2] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron‐transfer mechanism between arenes and the nitryl cation (NO) electrophile.
New Transition Metal Clusters with Ligands from Main Groups Five and Six
Authors: Fenske Dieter , Ohmer Johannes , Hachgenei Johannes , Merzweiler Kurt
Published: December 2003

In both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three‐ or four‐membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe3)2 (E = S, Se, Te) and E′R(SiMe3)2 (R = Ph, Me, Et; E′ = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations; a variety of reactions in the ligand sphere are also possible.
A New Approach to Ionic Ozonides
Authors: Hesse Werner , Jansen Martin
Published: December 2003

Tetramethylammonium ozonide, N(CH3)4O3, is surprisingly stable! It forms red, transparent crystals, which only decompose above 75 °C. Till now, the thermally very labile alkali‐metal ozonides were thought to be the most stable ionic ozonides. The possibility of developing a preparative chemistry involving ionic ozonides was therefore viewed with pessimism. N(CH3)4O3 was synthesized by double reaction of KO3 with N(CH3)4O2 in NH3. This route to ionic ozonides should prove to be general.
Solubilization of Elemental Sulfur in Water by Cationic and Anionic Surfactants
Authors: Steudel Ralf , Holdt Gabriele
Published: December 2003

The solubility of S8 in water is increased by a factor of 5000 when surfactants are added. This is surprising since elemental sulfur is especially hydrophobic and is neither wetted nor dissolved by water. The solubilization of S8 in aqueous surfactant solutions explains, for example, how sulfur bacteria are able to transport S8 into the cell, where it is oxidized to sulfate: the bacteria secrete surfactant molecules.
High‐Resolution Solid‐State 13C‐NMR Spectroscopy of Polymers [New Analytical Methods (37)]
Authors: Voelkel Rüdiger
Published: December 2003

Until a few years ago, solid‐state nuclear resonance yielded spectra containing broad lines only. Meanwhile, CP/MAS‐NMR spectroscopy has provided a method which gives narrow nuclear resonance lines from a solid‐state specimen as well. Using this technique, it is now possible to produce spectra of “rare” nuclei (13C, 29Si, 15N etc.) which are resolved in terms of chemical structure. The analytical capabilities of NMR spectroscopy can be applied to the solid state: it may be that it is necessary to identify compounds in the solid state because, for example, a solvent would alter the coordination sphere, or that it is desired to monitor chemical reactions in the solid state, for example the baking of an enamel. Where a substance in the solid state is concerned, high‐resolution 13C‐NMR spectroscopy provides not only information about the chemical structure, but also about the solid state itself. To mention just a few examples, information on the conformation, crystal structure and molecular dynamics, as well as molecular miscibility is given. This opens up a broad spectrum of applications, from a statement concerning the crystal modification of an active substance in ready‐to‐use pharmaceutical preparations, e.g. tablets, to the question of whether two polymers are miscible with one another at a molecular level.
Phosphaalkynes—Syntheses, Reactions, Coordination Behavior [New Synthetic Methods (73)]
Authors: Regitz Manfred , Binger Paul
Published: December 2003

Organophosphorus compounds have been applied in two ways in chemical synthesis. They can either be used as a reagent in a step of the synthesis (for example, in the Wittig reaction) or they can be incorporated directly into the target molecule. This second application, in particular, has expanded greatly in the last few years with the preparation of low‐coordination phosphorus compounds. These include the phosphaalkynes, which are of great interest to organic and inorganic chemists. Phosphaalkynes have been employed in the synthesis of heterocyclic compounds, phosphaarenes and their valence isomers, and polycyclic compounds. Further applications have been the use of phosphaalkynes as new ligand systems in complex chemistry and their cyclooligomerization with organometallic reagents. While the chemical properties of phosphaalkynes have little in common with those of nitriles, they are in many ways very similar to those of the isoelectronic acetylenes.
Generation and Characterization of Sulfurous Acid (H2SO3) and of Its Radical Cation as Stable Species in the Gas Phase
Authors: Sülzle Detlev , Verhoeven Marjon , Terlouw Johan K. , Schwarz Helmut
Published: December 2003

Sulfurous acid (H2SO3) and its radical cation H2SO are stable molecules: this has been shown unambiguously by neutralization‐reionization. MS experiments and by ab initio calculations. That these species are non‐existent in solution is not due to intrinsic factors, but to the medium. The key experiment, with deuterated starting material and product, is shown below.
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Iodine‐Induced Stereoselective Carbocyclizations: A New Method for the Synthesis of Cyclohexane and Cyclohexene Derivatives
Authors: Barluenga José , González José M. , Campos Pedro J. , Asensio Gregorio
Published: December 2003

The ready availability of the starting materials and the extensive possibilities for functionalizing the products are the distinguishing features of the title reaction 1 → 2, which is shown exemplarily in equation (a). Further advantages are good yields, high diastereoselectivity, and the simple purification of the carbocyclic products.
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Coordination of H2 and O2 to [OsHCl(CO)(PiPr3)2]: A Catalytically Active M(η2‐H2) Complex
Authors: Esteruelas Miguel A. , Sola Eduardo , Oro Luis A. , Meyer Uwe , Werner Helmut
Published: December 2003

Both H2 and O2 are coordinated to the coordinatively unsaturated comples 1 without bond breaking. While 1·H2 is only stable in a hydrogen atmosphere, 1·O2 does not split off oxygen even under high vacuum. At 60°C in solvents such as benzene, 1,2‐dichloroethane and 2‐propanol 1·H2 catalyzes the reductions of alkenes and ‐unsaturated ketones: thus for example benzylidene‐acetone in 2‐propanol gives benzylacetone and 4‐phenyl‐2‐butanol in a ratio of 54:1.
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The Chemistry of Amino Imino Boranes
Authors: Nöth Heinrich
Published: December 2003

Dicoordinated boron compounds can no longer be considered as “exotic” species in boron chemistry: this is proved by the extensive chemistry of bis(dialkylamino)boron(1+) ions, alkylidene boranes, and imino boranes. The latter are characterized by high reactivity and chemical versatility. A special position is taken by amino imino boranes, whose chemistry is reported here. The presence of an amino group enhances the reactivity of imino boranes on the one hand and increases their kinetic stability on the other. New types of diamino boranes accessible via amino imino boranes often undergo intramolecular ring closure. These heterocycles are well suited to generate new types of cations of boron. Cycloaddition reactions using amino imino boranes open up a new heterocyclic chemistry of boron. The chemistry ofN‐functional amino imino boranes, which is still its infancy, shows much promise.
Electrocatalytic Haloform Reaction: Transformation of Methyl Ketones into Methyl Esters
Authors: Nikishin Gennady I. , Elinson Michail N. , Makhova Irina V.
Published: December 2003

Neither free halogen nor halogenite has to be used for the haloform reaction if it is carried out electrocatalytically. Reduction of 1 (R = Ph, 1‐naphthyl, Me(CH2)5, iPr, PhCHCH) in MeOH in the presence of NaBr at 30 °C and a constant current density of 220 mA cm−2 suffices in order to obtain the methyl ester 2 in 61–68% yield. Under the same conditions ketones such as PhCOEt afford α‐hydroxyacetals, in the case of 1, R = PhCH2, on the other hand, predominantly the Favorski rearrangement product methyl 3‐phenylpropionate is formed.
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NaxMg5−xGa9, a New Intermetallic Phase as a Link Between Zintl Phases and Metals
Authors: Nesper Reinhard
Published: December 2003

NaxMg5−xGa9 crystallizes in a new structure type in which the Ga atoms form a three‐dimensional framework of linked icosahedra and Friauf polyhedra. These two types of polyhedra are typical for intermetallic phases. The compound shows a considerable phase width. The structure determination gives indications that, despite the metallic properties, variable electron numbers cause local changes of the chemical bonding which only take place at a single terminally bound Ga atom.
A New Oxygen Sensor Based on the Thermoelectric Power of Solid Solutions of Nb2O5 and Bi2O3
Authors: Chadwick A. V. , Zhou W. , Thomas J. M.
Published: December 2003

One part of Nb2O5 to 60 parts of Bi2O3 suffices in order to achieve a reaction of Bi2O3 of thermopower Θ sensitive to a change of oxygen pressure. The β‐phase of the solid solution of Nb2O5 in Bi2O3, which is stable below 900 K, is a promising material for oxygen sensors which operate without the necessity of an oxygen reference pressure at relatively low temperatures. The reason for the rapid and reversible change of thermopower as a function of oxygen partial pressure is still unknown.
Self‐Reduction of [Ph4P][Nb(SCH2CH2CH2S)3] with Formation of a Nb Complex
Authors: Tatsumi Kazuyuki , Sekiguchi Yoitsu , Sebata Michiyo , Nakamura Akira , Cramer Roger E. , Chung Thomas
Published: December 2003

The size of the chelate ring determines the reactivity of Nbv complexes: Whereas [Ph4P][Nb(SCH2CH2S)3] is completely stable in acetonitrile, in the case of the homologous title complex a reductive dimerization takes place. The anion 1 formed initially reacts further with methanol to give 2, which was isolated as the Ph4P‐salt; the structure of this anion is shown on the right.

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Can Polystyrene be Optically Active?
Authors: Wulff Günter , Dhal Pradeep K.
Published: December 2003

The first synthesis of optically active polystyrene with [α] as large as −3.5°was achieved by using an optically active template compound (D‐mannitol derivative containing two p‐vinylphenylboronic acid groups). This compound was copolymerized with styrene. The copolymer contains p‐dihydroxyboryl‐substituted distyrene diads of defined configuration, which are separated by atactic polystyrene segments. Reduction of the B(OH)2 groups affords polystyrene. In such an arrangement, approximately 25% of the diad centers are stereogenic and chirotopic (asymmetric), thus contributing to the optical rotation of the molecule.
Stiff‐Chain Polymers—Structure, Phase Behavior, and Properties
Authors: Ballauff Matthias
Published: December 2003

Most of the industrial plastics used up to now consist of flexible macromolecules in which the repeat units are joined together in a nonlinear fashion by rotating bonds. Stiff‐chain polymers, which in the ideal case have a rodlike shape, have received much less attention. The reason for this is that such polymers usually exhibit an exceedingly low solubility. Very often, they are absolutely insoluble and do not melt. However, in recent years these polymers have aroused technical interest because high‐modulus fibers and moldings can be fabricated from them. These favorable mechanical properties are directly related to the nearly parallel arrangement of the rodlike macromolecules in the solid state which is achieved through spinning or extrusion from a liquid crystalline (nematic) state. It is therefore evident that a thorough understanding of the phase behavior and the structure is required for a universal technical utilization of these materials. The great variety of methods used in polymer science today has led to a deeper understanding of stiff‐chain polymers. Experimental results together with theoretical modeling of the phase behavior have direct implications for the practical use of these macromolecular materials.
Functionalized Conducting Polymers—Towards Intelligent Materials
Authors: Garnier Francis
Published: December 2003

Organic conjugated polymers and oligomers constitute a three‐dimensional network of molecular wires, in which all monomeric units can be functionalized with various prosthetic groups. By varying the nature of these groups, specific interactions with external physical or chemical phenomena can be developed in these materials, leading to molecular devices such as sensors, transducers, memories and logic operators. Chemists have already mastered the realization of many of these functional elements, which mimic those existing in organized beings.The further assembly of these elements in multifunctionalized organic conducting polymers and oligomers will represent the next step towards intelligent materials.
Electron Densities, Deformation Densities, and Chemical Bonding
Authors: Schwarz W. H. Eugen , Ruedenberg Klaus , Mensching Lothar , Miller Lance L. , Valtazanos Petros , von Niessen Wolfgang
Published: December 2003

For corrigendum see DOI:10.1002/anie.198910741
Revision and Modification of the Traditional Mechanism of CH/CC Activation by “Naked” Transition‐Metal Ions
Authors: Karraß Sigurd , Eller Karsten , Schulze Christian , Schwarz Helmut
Published: December 2003

Why Fe⊕‐complexed n‐propylamine decomposes into H2, NH3, ethylene and propylene in the mass spectrometer cannot be explained in terms of current concepts. Tandem MS experiments with deuteriated n‐propylamine enable an explanation: H2, NH3 and propylene could be formed via an n‐propyl‐Fe⊕‐NH2 complex, which rearranges to the propene complex. For the formation of ethylene, a route via a four‐membered FeNC2 complex and its conversion into an ethylene complex is possible.
Materials Processing—a Key Factor
Authors: Michaeli Walter
Published: December 2003

Review: Critical changes in the properties of materials due to varying processing conditions are the driving force behind the attempt to understand and control processing effects. Walter Michaeli uses the processing of polymers to emphasize the factors important in directing the design and processing of materials and the advantages of the integration of computers into development systems.
General Aspects and Optimization of Enantioselective Biocatalysis in Organic Solvents: The Use of Lipases [New Synthetic Methods (76)]
Authors: Chen Ching‐Shih , Sih Charles J.
Published: December 2003

Enantioselective biocatalysis in nonaqueous media is becoming increasingly important in preparative synthetic chemistry. This article discusses (1) the general catalytic properties of enzymes in nonaqueous environments, (2) the basic principles that govern lipase‐catalyzed enantioselective esterification and transesterification reactions in organic media for the preparation of optically active acids and alcohols, (3) the determination of kinetic and thermodynamic parameters, and (4) the quantitative analysis of published data.