bluuebunny/crossref_metadata_2025 huggingface dataset:
Materials for Optical Data Storage
Authors: Emmelius Michael , Pawlowski Georg , Vollmann Hansjörg W.
Published: December 2003

The outstanding feature of materials research in the eighties has been the convergence of basic research and practical application, leading to ever shorter cycles of innovation. This is especially true of materials which form the basis of key technologies. The mass storage units of the next generations of computers will be based on optical processes having a storage density which exceeds that of all hitherto known storage techniques that are practicable from the technical standpoint. In view of the fact that since 1982 read‐only memories in the form of compact discs (CD‐ROM) have become very successful in the field of audio electronics, research and development are now concentrated on materials for write‐once (WORM) and erasable (EDRAW) storage systems. Suitable materials for optical data storage are substances in which data markings can be recorded and deleted respectively using semiconductor lasers. Materials development is centered on the synthesis of infrared‐absorbing dyes for WORM memories and the production of rare earth/transition metal alloys for magneto‐optical data recording. An introduction to CD‐ROM technology will be followed by an overview of the state of development of the most important storage materials which are currently available commercially, and then the properties of these materials will be discussed with reference to selected examples.
Image Enhancement—Chemical, Digital, Visual
Authors: Lohmann Joachim
Published: December 2003

Digital image processing has become well known through the high quality of some spectacular satellite images. It has developed into a discipline in its own right within information science and now finds application, not only in space travel, but also in such diverse fields as, for example, robotics, medical imaging, and television technology. Image processing also occurs, however, in photographic films and biological visual systems. In these systems, too, recorded images are manipulated for better recognition. This poses the question as to whether similar principles of image processing are operative in these different image handling systems.
[BP2]3⊖ and [BAs2]3⊖, Zintl Anions with Propadiene Structure
Authors: von Schnering Hans‐Georg , Somer Mehmet , Hartweg Martin , Peters Karl
Published: December 2003

It is not the composition of the compounds K3BX2 (1: X = P, 2: X = As) that enforces the propadiene‐like structure on their anions [XBX]3⊖ which was established by X‐ray structure analyses and vibration spectra. Bond lengths of dB−P = 176.7 and dB−As = 186.8 pm were determined; these correspond to a Pauling bond order of 1.9. The compounds 1 and 2, the first ternary boron phosphides and arsenides, are accessible at 1000–1100 K from the elements or by use of BP and BAs, respectively.
Fractals and Related Hierarchical Models in Polymer Science
Authors: Blumen Alexander , Schnörer Horst
Published: December 2003

Polymer science, an interdisciplinary science well‐rooted in organic chemistry and in materials science, encompasses an inordinate number and diversity of substance classes and thus has far‐reaching applications. Interestingly, polymers also represent a great challenge to the theoreticians, since their theoretical treatment often necessitates appropriate extensions of the classical methods from solid state physics and from statistical physics. Thus, new concepts often have to be invoked when considering the special properties of polymers. In this review we concentrate on one of the modern concepts in the theory of polymers, namely on scaling. Scaling is closely associated with new developments in the field of fractals and of hierarchical structures. Such concepts are invaluable for the modeling of complex geometries and for describing dynamical processes in polymeric materials. Here, we focus on a presentation of these ideas and we outline examples of recent research in which these concepts have been successfully applied.
Synthesis of Tetrachloroammonium Hexafluoroarsenate, NCl AsF
Authors: Minkwitz Rolf , Bernstein Dirk , Sawodny Wolfgang
Published: December 2003

The ab initio calculation of an energy minimum for NCl provided the incentive for attempting the synthesis of the title compound by oxidative chlorination of NC13 [Eq. (a)]. The salt thereby obtained is stable below −40°C and, unlike NCl3 nonexplosive. The valence force constant of NCl determined from the vibrational spectrum is ca. 15% smaller than that of CCl4. This finding may be ascribed to the increased ligand repulsion in the NCl ion.
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Structure and Color of Substituted Pentaphenylbismuth
Authors: Schmuck Arno , Seppelt Konrad , Pyykkö Pekka
Published: December 2003

The square‐pyramidal structure is one reason for the intense color of pen‐taphenylbismuth and many of its derivatives, such as 1. The easy electronic excitation is made possible, in this case, by a low‐lying LUMO, which is oriented toward the sixth coordination site of the octahedron. Noteworthy are the lack of shielding by the 4f electrons and the relativistic effects. The trigonal‐bipyramidal derivative 2 is orange instead of violet.
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Molecular Recognition with Model Systems
Authors: Rebek Julius
Published: December 2003

How structures fit together is the principal domain of molecular recognition, and current studies are evolving from the host–guest chemistry of ions to interactions between two molecules. Recent advances in the synthesis of sizable concave molecules, especially those featuring convergent functional groups, make it possible to bind smaller convex structures with considerable selectivity. One result is that hydrogen bonding can be addressed in model systems. The present review emphasizes the use of cleftlike structures as a means of probing the forces involved in nucleic acid recognition. The application of such molecules to the catalysis of chemical reactions, particularly those involved in self‐replicating systems, is also described. Some implications for future pharmaceutical agents are suggested as a result of access to synthetic receptors for biologically relevant targets.
Continuous Generation of NADH from NAD⊕ and Formate Using a Homogeneous Catalyst with Enhanced Molecular Weight in a Membrane Reactor
Authors: Steckhan Eberhard , Herrmann Sabine , Ruppert Romain , Thömmes Jörg , Wandrey Christian
Published: December 2003

Bonding the RhIII complex 1 to polyethylene glycol (MW 20 000) leads to a high molecular weight redox catalyst suitable for continuous generation of the cofactors NADH and NADPH. With the aid of a newly designed flow‐through membrane reactor it has proven possible to achieve an NAD⊕ conversion of 40–45% (turnover number 16.9 h−1, space‐time yield 44.3mmol L−1 d−1). Since the reactivity towards polyethylene glycol‐bound NAD⊕ remains constant, this catalyst is also suited to use in a continuously operating enzyme‐membrane reactor.
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Water‐Soluble Metal Complexes of the Sulfonated Triphenylphosphane TPPTS: Preparation of the Pure Compounds and their Use in Catalysis
Authors: Herrmann Wolfgang A. , Kulpe Jürgen A. , Kellner Jürgen , Riepl Herbert , Bahrmann Helmut , Konkol Werner
Published: December 2003

Gel permeation chromatography is the key to isolation of pure TPPTS 1, as well as the complexes [LXMy(TPPTS)z· (H2O)3z], for which 1 serves as starting material. These complexes are of considerable interest with respect to homogeneous catalysis, since their water solubility permits them to be separated easily from hydrophobic organic reaction products. Charge concentration around the periphery of the molecule is presumably responsible for the fact that TPPTS complexes such as 2 often display lower coordination numbers than their triphenylphosphane analogues.
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Unraveling Overlapping Multiplets in Two‐Dimensional NMR Correlation Spectra by Selective Inversion of Coupling Partners
Authors: Emsley Lyndon , Huber Peter , Bodenhausen Geoffrey
Published: December 2003

Clean and simple two‐dimensional multiplet pallerns instead of overlapping multiplets can be generated in an NMR experiment by employing selective pulses that single out small segments of a complete spectrum. The pattern of the selected multiplet is in this case manipulated by altering the apparent signs of the coupling constants. The technique is suitable not only for separating overlapping signals, but also for testing hypotheses associated with particular coupling networks.
Synthesis and Structure of Ph2P(S)N  TiCl2 · 3C5H5N, the First Imidotitanium Complex
Authors: Roesky Herbert W. , Voelker Heike , Witt Michael , Noltemeyer Mathias
Published: December 2003

Coordination of electron pair donors at the titanium atom leads to stabilization of imidotitanium complexes, which are of interest as catalysts for C‐H activations and polymerizations. Thus, reaction of the thiophosphinic acid amide 1 with TiCl4 and subsequent treatment of the initial product with pyridine enables the synthesis of the orange‐red, crystalline title compound 2. The titanium atom exhibits distorted octahedral coordination; the length of the Ti‐N bond is 172.0(2) pm.
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Bis(pentamethylcyclopentadienyl)silylene as Ligand in Gold(I) Chloride Complexes
Authors: Jutzi Peter , Möhrke Andreas
Published: December 2003

The first real silylene complex was obtained by reaction of decamethylsilicocene 1 with carbonylgold(I) chloride. To reduce the electron deficit at the silicon in 2, one of the two cyctopentadienyl ligands remains π‐bonded. The deep‐red silylene complex 2 resembles the Fischer carbene complexes. Addition of pyridine or tert‐butyl isocyanide leads to the donor‐stabilized silylene complexes of the type (Me5C5)2Si(donor)–AuCl, in which both cyclopentadienyl ligands are σ‐bound.
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Aspartic and Glutamic Acid as Ligands to Alkali and Alkaline‐Earth Metals: Structural Chemistry as Related to Magnesium Therapy
Authors: Schmidbaur Hubert , Classen Hans Georg , Helbig Joachim
Published: December 2003

Magnesium and other alkaline‐earth and alkali metals play an important role in the biochemistry of virtually all living organisms. In contrast to “trace elements” these “common metals” are distributed in most cells and tissues, often in considerable concentrations, and a constant supply is indispensable for unrestricted performance of biological functions. In contemporary medicine, various forms of magnesium therapy are recommended in order to compensate for magnesium deficiency due to certain dysfunctions as well as to loss from excessive transpiration. In addition, there are numerous indications for prophylaxis. Magnesium is usually administered orally in the form of “complexes” with “natural” anions derived from I‐aspartic, L‐glutamic, L‐pyroglutamic (pidolic), citric, and orotic acid. Analytical, electrochemical, and spectroscopic investigations of aqueous solutions of these Pharmaceuticals, together with X‐ray diffraction studies of crystalline samples, have given a detailed knowledge of these coordination compounds. Only certain combinations can be addressed as magnesium complexes, whereas others are only present as aquo complexes both in aqueous solutions and in the solid state. Thus, a delicate balance of metal complexation and hydrogen bonding exists for the components of the drugs, which is relevant for the metal carrier functions of the organic ligands. Compared with the more flexible stereochemistry of Ca, Sr, Ba, MnII, Zn, Li, Na, and K complexes, which have coordination numbers between 4 and 9, octahedral hexacoordination is found exclusively for magnesium, the most stereoselective of these metals.
Cooperative Interactions in Molecular Recognition–the Binding of Diamines to a Tetracarboxylic Acid
Authors: Ebmeyer Frank , Rebek Julius
Published: December 2003

No Absract.
Nonradioactive Assay of DNA Hybridization by DNA‐Template‐Mediated Formation of a Ternary TbIII Complex in Pure Liquid Phase
Authors: Oser Andreas , Valet Günther
Published: December 2003

No Absract.
Organic Synthesis in the Age of Computers
Authors: Hendrickson James B.
Published: December 2003

The computer affords four areas of interest to synthetic chemists: (1) storage and retrieval of information; (2) complex calculations of shape and reaction dynamics; (3) artificial intelligence (AI) to solve synthetic and analytical problems; (4) overall direction of multiple laboratory experiments. Our work began 30 years ago initiating molecular geometry calculations to match experimental results. The explosive growth of computer power since will be illustrated. Synthesis design (as AI) requires an efficient protocol to search huge trees. The SYNGEN program is based on simplification and systematization, first of skeletal dissection, then of structure and reaction characterization, for generation of optimal synthesis routes. The program will be be described and its future development outlined. Finally, a perspective on the future of all four areas will be sketched, noting the necessary attendant growth in systematic use by chemists and in sympathetic interfaces by software designers.
CC Bond Formation by Addition of Carbenium Ions to Alkenes: Kinetics and Mechanism
Authors: Mayr Herbert
Published: December 2003

The addition of carbenium ions to CC double bonds, a key step in many syntheses in organic and macromolecular chemistry, is analyzed using the Lewis acid promoted reactions of alkyl chlorides with alkenes as an example. Stereochemical and kinetic experiments suggest that the transition state is slightly bridged and product‐like. Rearrangements of the carbenium ions that result from the electrophilic attack can be minimized by adding salts with nucleophilic counter ions. The thermodynamics of the addition reactions are analyzed, and the conditions necessary in order to observe the back reaction (i.e. the Grob fragmentation) are discussed. Multiparameter equations that predict rate constants are derived from kinetic studies on the reactivities of carbenium ions and alkenes. Reactivity‐selectivity relationships over a reactivity range that covers eight orders of magnitude show that the structure of the transition state is only changed by variation of substituents in the immediate vicinity of the reaction center.
Oxidative Degradation of Aromatic Pollutants by Chemical Models of Ligninase Based on Porphyrin Complexes
Authors: Labat Gilles , Seris Jean‐Louis , Meunier Bernard
Published: December 2003

Microbially nondegradable or only difficultly degradable chlorophenols and methoxylated arenes are oxidized to quinones in a few minutes by a peroxidase model system. This model system is obtained by reaction of a sulfonated metalloporphyrin complex with KHSO5. It also oxidizes arenes such as 1,3,5‐trimethoxybenzene, which is attacked neither by ligninase nor by radish peroxidase.
Triisopropylamine: A Sterically Overcrowded Molecule with a Flattened NC3 Pyramid and a “p‐Type” Nitrogen Electron Pair
Authors: Bock Hans , Goebel Ilka , Havlas Zdenek , Liedle Siegfried , Oberhammer Heinz
Published: December 2003

An extremely low first ionization energy, a reversible oxidizability, and an electron pair localized on the nitrogen atom in the radical cation characterizes triisopropylamine. The structure inferred from these properties, which is also supported by quantum mechanical calculations, is a flattened NX3 pyramid. Electron diffraction studies in the gas phase confirm the structure: the CNC angle is 119.2°!
3‐Amino‐2H‐Azirines. Synthons for α,α‐Disubstituted α‐Amino Acids in Heterocycle and Peptide Synthesis [New Analytical Methods (43)]
Authors: Heimgartner Heinz
Published: December 2003

The recent upswing in peptide chemistry has been accompanied by an increasing interest in nonproteinogenic amino acids. These include the α,α‐disubstituted glycines, the best known of which is Aib (2‐aminoisobutyric acid, 2‐methylalanine). These α‐amino acids occur in natural oligopeptides such as the peptaibols, a class of membrane‐active ionophores that has been isolated from fungal cultures. The twofold substitution at the α‐C atom of the amino acids severely restricts the conformational freedom of the peptides and causes particular secondary structures to be favored; thus, α, α‐disubstituted α‐amino acids induce the formation of β turns or helices. 3‐Amino‐2H‐azirines are ideal synthons for the construction of oligopeptides, cyclic peptides and depsipeptides (peptolides) containing such α,α‐disubstituted α‐amino acids. The presence of the ring strain in these molecules means that they can be used in peptide coupling without the need for additional activating reagents. Using 3‐amino‐2H‐azirines a large array of heterocycles containing α, α‐disubstituted α‐amino acids as structural elements within their skeleton can be synthesized. The driving force in these reactions is the release of the strain on the three‐membered ring, which usually takes place in a ring‐expansion reaction. The mechanistic elucidation of these reactions, which can be quite complex, contains some surprises.
Piezoelectric Liquid‐Crystalline Elastomers
Authors: Wendorff J. H.
Published: December 2003

Piezoelectric materials have no inversion center in the crystal structure–this prerequisite for the piezoelectric effect is also a feature of cholesteric and chiral smectic (C*) liquid‐crystal phases. This arouses expectations that the development of elastomers in which the network properties are combined with the existence of these phases will lead to an enormous expansion in the application of piezoelectric polymers. The main advantages over piezoelectric ceramics are the ease with which elements of any desired shape can be manufactured, the good mechanical properties, and the chemical inertness.
Novel Polymeric Materials from Monodisperse Copolypeptides by Biotechnological Methods
Authors: Ritter Helmut
Published: December 2003

“The ox can do it better.” This casual, yet time‐tested remark by Emil Fischer with reference to the synthesis of peptides has stimulated almost a generation of preparative chemists to take up the challenge. And, “the ox” has recently met with increasing competition through the introduction of modern biotechnological methods. Though these methods have largely been limited to the successful synthesis of active substances in the pharmaceutical and agricultural sectors in recent years, a breakthrough now seems to have been made by D. A. Tirrell et al. in the preparation of new materials. A cyclopeptide with exactly 14 repeat units of an undecapeptide sequence has been prepared which has a glass temperature of 182°C, decomposes above 250°C, and from which homogeneous films can be cast.
Formation of Colloidal Transition Metals in Organic Phases and Their Application in Catalysis
Authors: Bönnemann Helmut , Brijoux Werner , Brinkmann Rainer , Joußen Thomas , Korall Barbara , Dinjus Eckard
Published: December 2003

Reduction of metal salts with tetraalkylammonium hydrotriorganoborates in organic phases yields a very narrow size range of metal colloids of the elements of groups 6–11. Adsorbed on carrier surfaces, these colloids are highly active hydrogenation catalysts whose activity can be further increased by doping with low‐valent organotitanium compounds.
Helices, Supramolecular Chemistry, and Metal‐directed Self‐Assembly
Authors: Constable Edwin C.
Published: December 2003

Their synthetic applications and potential as models, as well as simple aesthetic pleasure, are the main reasons for interest in helical compounds. The preparation of several double‐helical molecules made up of complexed metal ions was previously reported. Now a triple‐helical molecule, in which two metal ions with a preference for octahedral coordination are complexed with three bis‐(bidentate) ligands, has been synthesized by Williams et al. The geometry dictated by the ligands in this triple helix keep the two metal atoms 8 Å apart (see also p. 1490).
Methyltrioxorhenium as Catalyst for Olefin Metathesis
Authors: Herrmann Wolfgang A. , Wagner Werner , Flessner Uwe N. , Volkhardt Ursula , Komber Hartmut
Published: December 2003

No cocatalysts are needed as additives when methyltrioxorhenium (MTO) supported on acidic carriers is employed to catalyze the metathesis of functionalized olefins. A typical system is MTO/Al2O3−SiO2, which is active, for instance, in the metathesis of ally1 halides, allylsilanes, unsaturated carboxylates, and nitriles. MTO in combination with RnAlCl3‐n is a homogeneous catalyst in ring‐opening polymerizations (RCH3, C2H5; n = 1,2).
Bond‐Stretch Isomers: Fact not Fiction
Authors: Gütlich Philipp , Goodwin Harold A. , Hendrickson David N.
Published: December 2003

The conversion from the low‐spin to the high‐spin state in spin‐crossover transition metal complexes leads to bond‐length isomers, since the distance between the metal center and the coordinating ligand atom necessarily increases by 0.04–0.28Å. The coexistence of two such isomers and the effect of temperature and irradiation on their interconversion has been demonstrated by Mössbauer spectroscopy.
Silane Nitrile: Matrix Isolation, Adduct with Hydrogen
Authors: Maier Günther , Glatthaar Jörg
Published: December 2003

How do carbon and silicon chemistry differ? For example, the cyanide⇌isocyanide equilibrium is completely reversed in the sila series. According to calculations silane nitrile (2) is higher in energy than its isomer 1, which has been known for much longer. The matrix isolation of 2 has now been achieved; 2 is the first silicon compound of the nitrile type.
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Enzyme Mimics
Authors: Kirby Anthony J.
Published: December 2003

The enzyme, the reaction mechanism, or substrate binding can be the basis for enzyme mimics. Together with a short overview of the approaches employed for enzyme imitation, in particular the results of two recent important papers on this topic are discussed: the “pepzymes” of Atassi and Manshouri and the “micelle procedure” of Menger and Fei (see also Angew. Chem. Int. Ed. 1994, 106, 346).
Activation of Glutathione Reductase by Light: A Novel Approach to Design Redox Photo‐Enzymes
Authors: Willner Itamar , Zahavy Eran
Published: December 2003

Light‐induced electron transfer within a protein, coupled with chemical transformations, was achieved with the glutathione reductase modified by eosin isothiocyanate. The dye functions both as an initiator for the electron transfer and as a relay unit in the protein backbone that facilitates the electrical communication with the redox center at which the oxidized glutathione is reduced.
A Novel Open‐Framework Cobalt Phosphate Containing a Tetrahedrally Coordinated Cobalt(II) Center: CoPO4 · 0.5 C2H10N2
Authors: Chen Jiesheng , Natarajan Srinivasan , Thomas John Meurig , Jones Richard H. , Hursthouse Michael B.
Published: December 2003

A network of alternating CoO4 and PO4 tetrahedra linked through corners is formed by the blue title compound. The CoII centers are exclusively in a tetrahedral environment, as the compound's color indicates. The channels, whose pore openings (with respect to Co and P atoms only) are regular octahedra in one direction, but compressed eight‐membered rings in the others, lie parallel to the crystallographic axes.
Tetrakis(hypersilyl)dithallium(TlTl): A Divalent Thallium Compound
Authors: Henkel Sonja , Klinkhammer Karl Wilhelm , Schwarz Wolfgang
Published: December 2003

A TlTl bond 291.4 pm in length is the prominent structural feature of the thallium(II) compound 1, the product of the reaction of RbSi(SiMe3)3 with TlN(SiMe3)2. The red compound 1, which is thermally unstable in solution, can be stored for longer periods in the solid state.
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New Sulfur‐ and Selenium‐Bridged Copper Clusters; Ab Initio Calculations on [Cu2nSen(PH3)m] Clusters
Authors: Dehnen Stefanie , Schäfer Ansgar , Fenske Dieter , Ahlrichs Reinhart
Published: December 2003

The synthesis of new, medium‐sized copper clusters with unusual structures and a successful combination of experiment and theoretical calculation—this is offered by the present communication. The reaction of CuAc with PR2R′ (R, R′  Et, Ph) and E(SiMe3)2 (E  S, Se) leads to the clusters 1–4. Several of these might be intermediates in the synthesis of larger cluster complexes. The structure of [Cu12Se6(PH3)8] calculated by ab initio methods agrees with the structure of 1 within the limits of the methods.
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Donor‐Stabilized Aluminum(II) Bromide
Authors: Mocker Mario , Robl Christian , Schnöckel Hansgeorg
Published: December 2003

Cryochemically prepared AlBr and AlBr3 react in anisole to form donor‐stabilized AlBr2, which precipitates in the form of yellow needles with the composition Al2Br4 · 2 anisole. In the solid state the compound is stable at room temperature with regard to disproportionation into Al and Al‴ species. The Al‐Al distance in the adduct (252.7pm) is roughly as short as that in the radical anion [Al2R4].− (RCH(SiMe3)2), which contains an ALAL a bond (253 pm) strengthened by π bond contributions.
Effects of 35Cl/37Cl, 13C Residual Dipolar Coupling on the Variable‐Temperature 13C CP/MAS NMR Spectra of Solid, Chlorinated Sodium Acetates
Authors: Alarcón Sergio H. , Olivieri Alejandro C. , Carss Stephen A. , Harris Robin K.
Published: December 2003

Agreement between theory and practice: Solid‐state NMR studies on organic compounds with CH2Cl, CHCl2, and CCl3 groups show that residual dipolar coupling between 35Cl/37Cl and 13C nuclei influences the splitting of the signals depending on the magnetic field strength B. When molecular motions are slow enough to prevent self‐decoupling, signals for C nuclei affected by a single Cl nucleus appear as symmetric doublets at B0 >5 T and as asymmetric triplets at lower fields.
Magnetic Spin Effects on Photooxidation Quantum Yields of RuII‐tris(bipyridine) Type Complexes in Magnetic Fields up to 17.5 Tesla
Authors: Bürßner Dieter , Wolff Hans‐Joachim , Steiner Ulrich E.
Published: December 2003

Spin‐control of the rate of electron transfer processes can also be detected in systems with very strong spin‐orbit coupling such as in the ruthenium complex [Ru(bpy)3]2+, bipy = 2, 2′‐bipyridine. From the influence of an external magnetic field, which was varied continuously up to the extremely high fields of 17.5 Tesla, the kinetic details of the very fast photooxidation of [Ru(bpy)3]2+ by methylviologen were determined quantitatively.
Specific Binding of Antibodies to DNA through Combinatorial Antibody Libraries
Authors: Famulok Michael , Faulhammer Dirk
Published: December 2003

A much broader range of applications for antibody techniques should result from a new method that allows the production of a large quantity of diverse antibodies without the immunization of animals. Expression on phage surfaces is used to produce antibodies that recognize double‐stranded DNA with specificity. This is but the latest demonstration of the tremendous potential of antibody methods.
O‐Glycoside Syntheses under Neutral Conditions in Concentrated Solutions of LiClO4 in Organic Solvents
Authors: Waldmann Herbert , Böhm Gerd , Schmid Uschi , Röttele Herbert
Published: December 2003

Under mild, neutral conditions syntheses of O‐glycosides are possible when the glycosyl donor is treated with a glycosyl acceptor in 1 M solutions of LiClO4 in organic solvents. The vital difference between this and the usual processes is that no promoters like heavy metal salts, alkylating agents, and strong Lewis acids are needed.
Cyanide–Isocyanide Isomerism in CN‐Bridged Organometallic Complexes
Authors: Zhu Nianyong , Vahrenkamp Heinrich
Published: December 2003

The inertness of the organometallic building blocks makes it surprisingly easy to prepare the two isomeric forms of CN‐bridged heterodinuclear complexes like 1 a and 1 b. Compounds like these should contribute to the understanding of “molecular Prussian Blue”, dppe = 1,2‐bis(diphenylphosphano)ethane.
Taking Fullerenes from Large Molecules to Supramolecules
Authors: Constable Edwin C.
Published: December 2003

New perspectives for the purification of fullerenes, as well as for the separation of homoelemental allotropic modifications in general, have arisen from recent developments in host–guest chemistry with fullerenes as guests. Relevant research from the groups led by Yoshida, Atwood, and Shinkai is discussed here.
The Crystal Structures of Base‐Free, Monomeric Arylcopper(I) and Arylsilver(I) Compounds; Two Cases of Mistaken Identity?
Authors: Haaland Arne , Rypdal Kristin , Verne Hans Peter , Scherer Wolfgang , Thiel Werner Richard
Published: December 2003

New evidence suggests that the monomeric aryl metal compounds 2,4,6‐Ph3C6H2M (M = Cu, Ag) may be mixtures of 2,4,6‐Ph3C6H2M and 2,4,6‐Ph3C6H2Br or merely the aryl bromide. Although the aryl metal compounds appeared to be well characterized in 1988 by spectroscopic and X‐ray crystallographic methods, doubts arising from theoretical considerations—in particular CCu and CAg distances of presumably the same length—led to reexamination of the X‐ray data.
Sterically Strained Porphyrins—Influence of Core Protonation and Peripheral Substitution on the Conformation of Tetra‐meso‐, Octa‐β‐, and Dodeca‐Substituted Porphyrin Dications
Authors: Senge Mathias O. , Forsyth Timothy P. , Nguyen Liem T. , Smith Kevin M.
Published: December 2003

The saddle‐shaped porphyrin with the largest deviation from planarity observed to date is found in the crystal structure of [H4obrtpp][CF3COO]2 · 0.5 CHCl3 (H2obrtpp = octabromotetraphenylporphyrin). The pyrrole rings in the porphyrin are twisted by 62° with respect to each other, and the β‐pyrrole C atoms are displaced by over 1.5 Å from the mean plane of the porphyrin ring. This extreme distortion is caused by the bulky peripheral substituents on the porphyrin ring and the four protons at the core of the structure.
Covalent Inorganic Azides
Authors: Tornieporth‐Oetting Inis C. , Klapötke Thomas M.
Published: December 2003

The chemistry of covalent inorganic azides originated with the synthesis of aqueous HN3 solutions by Tony Curtis in 1890. A little later, in 1900, it proved possible to prepare iodine azide, IN3, as the first member of the meanwhile complete series of halogen azides. Since then it has been possible to synthesize, in addition to HN3 and the stable salt H2NSbF, azide compounds of elements from Groups 13 to 17. In these compounds the N3 moiety acts as a pseudohalogen and is primarily covalently coordinated to the nonmetal. Only a few organic azides, however, as well as HN3, H2N, and all halogen azides have been thoroughly studied with respect to structure and bonding. The combined application of diffraction methods (X‐ray and electron diffraction) and microwave spectroscopy together with quantum chemical approaches such as ab initio SCF and density functional calculations have led in the last few years to an improved understanding of the molecular properties of numerous nonmetal azides, almost all of which are explosive. This interaction of theory and experiment has greatly enhanced the development of azide chemistry and has led to realistic expectations for the synthesis of as yet unknown nonmetal azides.
Solventothermal Synthesis of Solid‐State Chalcogenidometalates
Authors: Sheldrick William S. , Wachhold Michael
Published: December 2003

Development of solventothermal methods in the intermediate‐temperature (100–600°C) range has been motivated by current interest in the design of solid‐state compounds with open framework structures. The preparation of nanoporous chalcogenidometalates by this technique is particularly promising, as a combination of the typical ion‐exchanging and catalytic features of zeolite‐like phases with the unique optoelectronic properties of transition metal and group 13–16 chalcogenides could afford novel multifunctional materials. Superheated strongly polarizing solvents such as water, methanol, and amines at temperatures in the range 110–200°C provide reaction conditions adequate to greatly enhance solubility, diffusion, and crystallization but still mild enough to leave molecular building blocks such as chains and rings intact to participate in the formation of polymeric anions. Such intermediate temperatures often allow the isolation of metastable solid‐state compounds and thermodynamically stable phases that decompose at higher temperatures. The dimensionality and pore size of the anion network can be influenced by the choice of a suitable structure‐directing alkali metal or alkyl‐ammonium cation. Methanolothermal conditions have been found to be particularly suitable for the preparation of selenido‐ and telluridometalates, and have afforded a new class of very tellurium‐rich alkali metal tellurides such as RbTe6 and Cs3Te22. This article presents a comprehensive review of the application of mild solventothermal techniques to the preparation of heavier group 14–16 and transition metal chalcogenidometalates.
The Chemistry of Spider Toxins and Spider Silk
Authors: Schulz Stefan
Published: December 2003

Spiders have a bad reputation because of their predacious and hidden way of life. Nevertheless, they developed a finely tuned system, based on the use of venom and silk, which enabled them to compete successfully in evolution. Only recently has the mystery of the chemistry of their silk and venom begun to unravel, although the first investigations were performed at the beginning of this century and in the fifties. The silk and toxins are made of proteins, but low molecular weight compounds are also present, too. Among these small molecules are compounds not known from any other natural source. For this reason the following article will focus on the structure and function of the low molecular weight components of the silk and the venom.
Unifying the Current Data on the Mechanism of Cleavage–Transesterification of RNA
Authors: Perreault Denise M. , Anslyn Eric V.
Published: December 2003

The various mechanisms by which RNA is hydrolyzed are currently under intense investigation. The first step in the hydrolysis pathway is a cleavage–transesterification in which a 2′‐OH group attacks a 3′,5′‐phosphodiester linkage with departure of the 5′ group. The second step involves the opening of a 2′,3′‐cyclic phosphodiester. Complications in these steps arise from multiple possible pathways involving specific acid and base as well as general acid and base catalysis. In addition, controversy exists concerning the protonation states of the phosphodiesters and any intermediate phosphoranes under various experimental conditions. A summary of mechanistic studies involving general and specific acid/base catalysis of RNA hydrolysis and the hydrolysis of RNA analogs is presented herein, along with the interpretations given by the original authors. Included are theoretical calculations, kinetic studies, pKa determinations, isotope effects, Hammett and Brønsted correlations, and model studies. Recent analyses of the mechanism of RNase A are also briefly reviewed. Two limiting mechanisms for the cleavage–transesterification step that unify the data in the literature and differ only in the role of the phosphorane and its protonation state are given at the outset. An analysis of the literature studies supporting these mechanisms is then provided.
Titanosilicates: Recent Developments in Synthesis and Use as Oxidation Catalysts
Authors: Murugavel Ramaswamy , Roesky Herbert W.
Published: December 2003

The more than two hundred papers that have appeared in 1996 in a relatively narrow field clearly show that something is happening in the area of titanosilicates, which are used as heterogeneous oxidation catalysts. Particularly important are those titanosilicate compounds which have a zeolite‐type structure and those molecular precursors that serve as model substances.
The Phenylacetyl Group—The First Amino Protecting Group That Can Be Removed Enzymatically from Oligonucleotides in Solution and on a Solid Support
Authors: Waldmann Herbert , Reidel Armin
Published: December 2003

Penicillin G acylase from E. coli was used to cleave the phenylacetyl group—the first enzyme‐labile protecting group for the amino function of nucleobases. Now protected oligonucleotides can be unmasked both in solution and on solid supports with this versatile enyzme under mild conditions (pH 7, room temperature). These results may lead to the development of new enzymatic reactions in oligonucleotide and solid‐phase chemistry as well as in combinatorial chemistry.
Enantioselective Synthesis with Lithium/(−)‐Sparteine Carbanion Pairs
Authors: Hoppe Dieter , Hense Thomas
Published: December 2003

“Chiral carbanions”—that is, enentiomerically enriched lithium–carbanion pairs in which the carbanionic center carries the chiral information—were regarded until recently as “exotic species.” In the past ten years it has become clear that they can, in fact, play a meaningful role in enantioselective synthesis, since substitution for lithium occurs here stereospecifically, usually with retention of configuration. They are also more readily and commonly accessible than was originally assumed. The trick lies in the use of lithium cations with chiral ligands, whether in the form of alkyllithium species used as bases in kinetically controlled, enantiotopically discriminating deprotonation, or in thermodynamically controlled equilibration in configurationally labile epimeric ion pairs. The lupine alkaloid (−)‐sparteine has shown itself admirably suited as a chiral bidentate ligand, and its efficiency and breadth of application are so far unsurpassed. This contribution constitutes an overview of the preparation of chiral reagents, convering primarily “umpoled” synthons such as homoenolates. 1‐oxyalkanides with a broad pattern of substitution, and α‐aminobenzyl anions.
Homoleptic Metal Carbonyl Cations of the Electron‐Rich Metals: Their Generation in Superacid Media Together with Their Spectroscopic and Structural Characterization
Authors: Willner Helge , Aubke Friedhelm
Published: December 2003

Homoleptic carbonyl cations of the electron‐rich metals in Groups 8 through 12 are the newest members of the large family of transition metal carbonyls. They can be distinguished from typical metal carbonyl complexes in several respects. Their synthesis entails carbonylation of metal salts in such superacids as fluorosulfuric acid and “magic acid” HSO3FSbF5. Thermally stable salts with [Sb2F11]− as counterion are obtained with antimony pentafluoride as reaction medium. Both the [Sb2F11]− anion and superacid reaction media have previously found little application in the organometallic chemistry of d‐block elements. Also unprecedented in metal carbonyl chemistry are the coordination geometries with coordination numbers 4 (square‐planar coordination) and 2 (linear coordination) for the cation. Formal oxidation states of the metals, and the charges of the complex cations, extend from + 1 to +3: thus CO is largely σ‐bonded to the metal, and the CO bond is strongly polarized. Minimal metal → CO π‐backbonding and a positive partial charge on carbon are manifested in long MC bonds, short CO bonds, high frequencies for CO stretching vibrations (up to 2300 cm−1), and small 13C NMR chemical shifts (up to δc, = 121). Prominent examples of these unusual homoleptic carbonyl cations, which were recently the subject of a Highlight in this journal, include the first carbonyl cation of a p‐block metal [Hg(CO)2]2+, the first trivalent carbonyl cation [Ir(CO)6]3+, and the first multiply charged carbonyl cation of a 3d metal [Fe(CO)6]2+. In this overview we propose to (a) outline the historical origins of cationic metal carbonyls and their methods of synthesis; (b) present a summary of the general field of carbonyl cations, which has developed over a yery short period of time; (c) discuss the structural and spectroscopic characteritics of metal–CO bonding; (d) discuss the special significance associated with reaction media and the [Sb2F11]− anion; and (e) point to the most recent results and anticipated future developments.
Immobilization of horseradish peroxidase by entrapment in natural polysaccharide
Authors: Shukla Sachin P. , Modi Kalpana , Ghosh P. K. , Devi Surekha
Published: December 2003

Horseradish peroxidase (HRP), which catalyzes oxidation reduction reactions of large number of substrates, was entrapped in K‐carrageenan beads using polyethyleneimine as hardening agent. The heat and storage stability was found to be better for entrapped horseradish peroxidase than free enzyme. The entrapped enzyme showed 50% retention of its activity after 4 cycles. Effective diffusion coefficient for diffusion of hydroquinone into K‐carrageenan beads was found to be 0.27 × 10−10 m2/s during enzyme‐catalyzed oxidation of hydroquinone. Kinetic parameters calculated from Lineweaver–Burke plots were observed to be Km = 8 × 10−5 and Vmax = 1.53 for free enzyme, and Km = 8.3 × 10−5 and Vmax = 2.18 for entrapped enzyme when enzyme concentration was kept constant and Km = 4 × 10−11 and Vmax = 0.45 for free enzyme and Km = 4.5 × 10−11 and Vmax = 0.58 for entrapped enzyme when substrate concentration was kept constant. This indicates that there is no conformational change during entrapment. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2063–2071, 2004
Conductive polymer composites: Influence of extrusion conditions on positive temperature coefficient effect of poly(butylene terephthalate)/poly(olefin)–carbon black blends
Authors: Feller J. F.
Published: December 2003

Processing conditions determine both the morphology and the carbon black (CB) distribution in conductive polymer composites (CPCs). Good control of these parameters is essential to obtain reproducible electrical properties. We found that for extruded tapes of poly(butylene terephthalate) (PBT)/poly(olefin) (OLE)–CB blends, an increase of both processing temperature and screw speed leads to a significant decrease of resistivity. This shift factor, more important with PBT/poly(ethylene)–CB than with PBT/poly(ethylene‐co‐ethyl acrylate)–CB, is attributed to viscosity variations of all compounds. A decrease of viscosity promotes CB aggregation at the PBT/OLE interface and creates larger conductive channels by coalescence. Nevertheless, the CB concentration effect appears predominant compared to the morphology effect to explain resistivity variations. This interesting finding can be used either to improve reproducibility of electrical properties or to adjust electrical properties without changing CPC formulation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2151–2157, 2004
Poly(dimethylsiloxane) coatings for controlled drug release. II. Mechanism of the crosslinking reaction in emulsion
Authors: Gao Zongming , Schulze Nahrup Julia , Mark James E. , Sakr Adel
Published: December 2003

Earlier work focused on the crosslinking of hydroxyl‐terminated poly(dimethylsiloxane) (PDMS) particles in a stable latex suitable for spraying onto drug tablets, and established the conditions for eliminating the usual toxic catalysts that would be unacceptable in such pharmaceutical coating materials. Use of these coatings for controlling the rate of release of a drug, however, requires a better understanding of their properties and thus clarification of the mechanism through which the crosslinking occurs. The present study approaches this goal by documenting the effects of anionic, cationic, and nonionic surfactants at various concentrations, and in acidic, neutral, or basic media. FTIR spectroscopy was used to monitor the transportation of crosslinker from the water phase into the hydrophobic PDMS phase. The results suggest a possible mechanism for the crosslinking in sufficient detail to be used to optimize the coatings for drug‐release applications. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2186–2194, 2004
Dynamics of lipase catalyzed enzymatic degradation of poly(bisphenol‐A carbonate)
Authors: Sivalingam G. , Madras Giridhar
Published: December 2003

Poly(bisphenol‐A carbonate) (PC) was degraded in solution at various temperatures (26–70°C) by different lipases, Candida Rugosa (CR), Hog Pancreas (HP), Lipolase (LL), and Novozyme (NV), in various solvents. The degradation of PC was monitored using gel permeation chromatography (GPC). The molecular weights of oligomers obtained were around ∼1,400 irrespective of the lipases used. Continuous distribution kinetics was proposed to determine the rate coefficients of the polymers and deactivation of the enzyme. The FTIR analysis of the polymer before and after degradation showed that there is large reduction of carbonate linkages and generation of hydroxyl and acidic groups in the broken chains. The optimal temperatures for HP and other lipases were 50 and 60°C, respectively. HP showed higher degradation activity at lower temperatures and the overall degradability of the lipases was in the order of LL > CR > NV > HP. The effect of viscosity and polarity of the solvents on the degradation of the polycarbonate was also investigated. While the degradation rate decreased with viscosity, it increased with polarity for the solvents. © 2003 Wiley Periodicals, In6c. J Appl Polym Sci 91: 2391–2396, 2004
Dielectric and conductivity studies on cobalt phthalocyanine tetramers
Authors: Saravanan S. , Joseph Mathai C. , Anantharaman M. R. , Venkatachalam S. , Prabhakaran P. V.
Published: December 2003

Electrically conductive organic and metalloorganic polymers are of great interest and they have applications in electronic, optical, photonic, photoelectric, electrochemical, and dielectric devices. Tetrameric cobalt phthalocyanine was prepared by conventional chemical method. The dielectric permittivity of the tetrameric cobalt phthalocyanine sample was evaluated from the observed capacitance values in the frequency range 100 KHz to 5 MHz and in the temperature range of 300 to 383°K. It is found that the system obeys the Maxwell Wagner relaxation of space charge phenomenon. Further, from the permittivity studies AC conductivity was evaluated. The values of AC conductivity and DC conductivity were compared. Activation energy was calculated. To understand the conduction mechanism Mott's variable range hopping model was applied to the system. The T−1/4 behavior of the DC conductivity along with the values of Mott's Temperature (T0), density of states at the Fermi energy N (EF), and range of hopping R and hopping energy W indicate that the transport of charge carriers are by three‐dimensional variable range hopping. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2529–2535, 2004
Precipitation of PMMA/PCL blends using supercritical carbon dioxide
Authors: Vega‐González A. , Domingo C. , Elvira C. , Subra P.
Published: December 2003

Solutions of a poly(methyl methacrylate)–poly(ε‐Caprolactone) (PMMA/PCL) polymer blend in dichloromethane (DCM) and mixtures of the same polymer blend and cholesterol in DCM were sprayed into supercritical carbon dioxide. Carbon dioxide was contacted with 0.23–1 wt % polymer solutions and with 0.3–1 wt % polymer plus 0.1–0.6 wt % cholesterol solutions in a continuous mode of operation. Pressure and temperature were constant for almost all of the experiments, 11 MPa and 314 K, respectively. Fibrous networks composed of many smaller microfibrils were obtained by spraying the different solutions through a conical nozzle into concurrently flowing supercritical carbon dioxide. This morphology suggests such an important degree of agglomeration that primary particles are no longer discernible. Processing the polymers with CO2 leads to the removal of contaminants as the precipitate was free of monomer and initiator. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2422–2426, 2004