bluuebunny/crossref_metadata_2025 huggingface dataset:
The Fluorometric Determination of Thymine Glycol as Criterion of the Treatment of Foodstuffs (Biological Material) with Ionizing Radiation
Authors: Pfeilsticker Konrad , Lucas Jürgen
Published: December 2003

The conservation of foodstuffs by means of ionizing radiation is not permitted in the Federal Republic of Germany. Until now, radiative treatment has been detectable within limits only for dried substances, in particular spices. This communication describes the principle of a new method that is suitable for water‐containing foodstuffs. Thymine glycol (TG) is known to be formed upon irradiation of DNA. When (TG‐containing) DNA is allowed to react with o‐aminobenzaldehyde, TG is converted into 3‐hydroxyquinaldine, which can be detected fluorimetrically. The fluorescence intensity can be correlated with the radiation dose, thereby allowing not only the irradiation to be detected but also its amount to be estimated.
Thermolysis of NH4HCO3—A Simple Route to the Formation of Free Carbonic Acid (H2CO3) in the Gas Phase
Authors: Terlouw Johan K. , Lebrilla Carlito B. , Schwarz Helmut
Published: December 2003

“Carbonic acid cannot exist in the free state.” Now, a surprisingly simple experiment has shown that this statement is wrong. Thermolysis of NH4HCO3 affords, among other products, H2CO3, which is stable as both the neutral molecule and the radical cation. This finding also confirms the results of detailed ab initio and semiempirical calculations, which show that H2CO3 should be stable in the gas phase because the activation energy for its decomposition is large.
Glycolate and Thioglycolate Complexes of Rhenium and Their Oxidative Elimination of Ethylene and of Glycol
Authors: Herrmann Wolfgang A. , Marz Dieter , Herdtweck Eberhardt , Schäfer Adolf , Wagner Werner , Kneuper Heinz‐Josef
Published: December 2003

Selenium dioxide and osmium tetroxide are effective reagents and catalysts for olefin oxidation, although, owing to their toxicity, reservations remain as to their applicability.[1] We are therefore seeking more easily handled metal oxides that are soluble in organic solvents and that are as effective as osmium tetroxide in carrying out stereospecific cis hydroxylation of olefins. The rhenium(VII) oxide 1, which has meanwhile become readily accessible, is a favorable candidate.[2]
[Ph4P]2 [Sb2 I8]: Weak Interactions between Phenyl Groups of the Cation and Antimony Atoms of the Anion
Authors: Pohl Siegfried , Saak Wolfgang , Haase Detlev
Published: December 2003

A cation as donor and an anion as acceptor, that is the unsual feature of the structure of the title compound. These interactions were made possible by arene–antimony catalysts, whereby phenyl rings of the [Ph4P]⊕ ions are η2‐bonded to antimony atoms in the [Sb2I8]2⊕ ion.
Stabilization of the [AlMe2]⊕ Cation by Crown Ethers
Authors: Bott Simon G. , Alvanipour Abbas , Morley S. David , Atwood David A. , Means C. Mitchell , Coleman Anthony W. , Atwood Jerry L.
Published: December 2003

Of interest for Ziegler–Natta systems with donor ligands are the structures of the cations of 1 and 2. The cation [AlMe2]⊕ is encapsulated by crown ethers and in 1 is bound to three of the six oxygen atoms while in 2 it is bound to all five oxygen atoms; the MeAlMe angle in 1 is 141°, in 2 178°. (1 contains [18]crown‐6, 2 contains [15]crown‐5.)
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New Cisplatin Analogues—On the Way to Better Antitumor Agents
Authors: Pasini Alessandro , Zunino Franco
Published: December 2003

The clinical success of cisplatin (cis‐diamminedichloroplatinum(II)) in antitumor chemotherapy has encouraged an all‐out search for analogues with lower toxicity, improved therapeutic index and increased activity. Literally thousands of analogues, obtained by replacement of the ammine‐ and chloro‐ligands by other amines and anionic ligands, respectively, have been systematically screened for activity in experimental tumor models. Some of these analogues have been selected for clinical evaluation, but only very few of them appear to be promising antitumor agents. More recently, cisplatin analogues have been designed and synthesized on the basis of, inter alia, the following considerations: 1) platinum complexes with carrier molecules as ligands should prove useful for achieving increasing drug concentration in tumor tissues; 2) platinum complexes with chemotherapeutic agents as ligands could afford polyfunctional drugs with synergistic action; 3) complexes containing more than one platinum atom might be more effective than complexes containing only one platinum atom; 4) platinum complexes could be used as sensitizers in radiation therapy. In this paper, we shall give a brief account of the “traditional” analogues, and then critically discuss what we believe could be the new trends in the design of cisplatin analogues.
Biological Exploitation of Solar Energy by Photosynthetic Water Splitting
Authors: Renger Gernot
Published: December 2003

The cleavage of water by solar radiation into dioxygen and metabolically bound hydrogen during photosynthesis is of central importance for the existence of higher forms of life on earth. The realization of this process in biological organisms made possible the use of the earth's huge water reservoir for the exploitation of solar energy and, at the same time, led to the creation of an aerobic atmosphere. The dioxygen thereby formed is a powerful oxidant which permits an energetically highly efficient nutrient turnover. In recent years considerable progress has been made in understanding the functional and structural organization of photosynthetic water splitting. This article attempts to give a review of our current state of knowledge with special emphasis on the oxidation of water to O2in biological systems.
Epidermal Growth Factor (Nobel Lecture)
Authors: Cohen Stanley
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.
1,4‐Diphospha‐1,3‐butadienes
Authors: Appel Rolf , Hünerbein Johannes , Siabalis Nikolaos
Published: December 2003

The similar behavior of PC and CC double bonds is revealed, in particular, in compounds containing both types of π bond, for example in phospha‐1,3‐butadienes. Up to three C atoms have been so far replaced by P atoms in various positions of butadiene. It has now been possible—thanks to the use of bulky substituents—to synthesize, for the first time, two 1,4‐diphospha‐1,3‐butadiene derivatives.
Pentabenzylcyclopentadienylthallium(I): Synthesis and Structure of a “Dimeric” Organothallium Compound with TlTl Interaction
Authors: Schumann Herbert , Janiak Christoph , Pickardt Joachim , Börner Ute
Published: December 2003

A protective shield consisting of six benzyl groups envelops a Tl1Tl1 unit in the title compound. The Tl–Tl distance of 3.6 Å does not correspond to that of a normal single bond but does indicate a bonding interaction—perhaps similar to that in Cu1Cu1 and Au1Au1 complexes. Normally, cyclopentadienylthallium complexes do not have a dimeric structure but rather a polymeric structure containing zigzag chains.
The Generation and Characterization of Molecules by Neutralization‐Reionization Mass Spectrometry (NRMS). New Analytical Methods (33)
Authors: Terlouw Johan K. , Schwarz Helmut
Published: December 2003

The diluted gas phase within a mass spectrometer is ideally suited for performing collision experiments of fast moving ions with thermalized target atoms. One important reaction which can be brought about with a given cation is its selective reduction, i. e. its neutralization. This allows the synthesis in tailored experiments of a variety of unusual chemical species, which because of intermolecular processes cannot be prepared or studied in solution or in a matrix. Among the molecules which have been generated in this way are hypervalent species (Rydberg radicals) like H, van der Waals complexes like He2, mono‐ or disubstituted acetylene derivatives XCCY (X  H, Y  OH, NH2; X  Y  OH, NH2), and elusive compounds like carbonic acid (H2CO3) or carbamic acid (NH2CO2H). Collision experiments can also be used advantageously for the structural analysis of the neutral molecule N generated in unimolecular dissociation reactions of the type m → m+N. The dissociation of ionized aniline for example yields, besides C5H cations, neutral isocyanic acid, HNC, and not cyanic acid, HCN. The technique of Neutralization Reionization Mass Spectrometry (NRMS) is performed in the diluted gas phase, a medium where intermolecular interactions are entirely absent. The method may therefore increasingly be applied to the study of molecular species whose reactivity precludes their characterization in matrices. Molecules in specific excited states and fragments of organometallic compounds with unsaturated valencies are two examples of systems which could fruitfully be studied.
First Synthesis of N‐Methylene‐Ynamines (2‐Azabut‐1‐en‐3‐ynes)
Authors: Würthwein Ernst‐Ulrich , Weigmann Reinhard
Published: December 2003

An addition to the family of C3H3N isomers is HCCNCH2, the skeletal unit of the title compounds 3. Compared to the ynamines these first N‐methylene‐ynamines contain an additional conjugated CN bond. The compounds 3 have now been obtained from the cuprate 1 and the oxime esters 2. For a reliable differentiation, the isomeric propargylideneamine PhCCC(Ph)NPh was also synthesized. Quantum mechanical calculations make it possible to predict the energy content of these compounds (a, R  Ph, R′  Me; b, R  Me, R′  H).
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15N‐CPMAS‐NMR Study of the Problem of NH Tautomerism in Crystalline Porphine and Porphycene
Authors: Wehrle Bernd , Limbach Hans‐Heinrich , Köcher Matthias , Ermer Otto , Vogel Emanuel
Published: December 2003

Solid state, variable temperature 15N‐NMR studies make it possible to predict the behavior of the four N atoms and the two internal H atoms in the isomeric porphine and porphycene. The spectra of porphine indicate a statistical disorder of the internal H atoms. In the case of porphycene it can be concluded that there are two non‐equivalent unsymmetric proton transfer systems. Since the NH…N distances are very short, the energy barrier for the rearrangement is very small. Therefore, other than in the case of porphine, in porphycene the interconversion of the tautomers is so rapid that the rate constants for the proton transfer could not be determined by the 15N‐CPMAS‐NMR method.
Di‐Grignard Compounds and Metallacycles
Authors: Bickelhaupt Friedrich
Published: December 2003

Just as the by now famous Grignard reaction can be used to prepare organomagnesium halides from organic monohalides, organic dihalides can be used under certain conditions to prepare di‐Grignard compounds. Difficulties are encountered in particular when the two halide functionalities are separated by only a short carbon chain (i. e. by 1‐3 carbon atoms): in such cases side reactions predominate. It has however recently become possible to obtain such “short” di‐Grignard compounds on a preparatively useful scale. This opens up new perspectives for the synthesis of metallacyclic compounds of other main group and transition metal elements and in particular for metallacyclobutanes. The preparation, structure and application of a selected number of di‐Grignard reagents will be treated in the present review.
Adsorption and Curvature in Highly Dealuminated Zeolites
Authors: Thomasson Ronnie , Lidin Sven , Andersson Sten
Published: December 2003

Our understanding of adsorption processes has been improved by the finding that the adsorption process in zeolites can be described using the concept of intrinsic curvature. This was demonstrated with hydrocarbons and two zeolites. For both zeolites, a linear relation was obtained between integrated heats of adsorption and N · Kav(N = number of valence electrons of the adsorbed molecule; Kav = mean Gaussian curvature of the inner surfaces).
Electrocatalytic Deposition of Polypyrrole
Authors: Oberst Manfred , Beck Fritz
Published: December 2003

In the presence of a small amount of bromide, the deposition of polypyrrole is possible at appreciably higher current densities than in the absence of a catalyst. The mechanism is assumed to involve the reaction of pyrrole with bromine in the form of Br3 to give [Br3]⊖ and the pyrrole radical cation. The latter then undergoes addition at the 2 position with a second radical cation or with a neutral pyrrole molecule, a CC bond being formed. The dimers are readily oxidized further, resulting in the formation of polymers. The [Br3]⊖ ion is reoxidized at the electrode.
Imaging Oxygen Vacancies in Superconducting YBa2Cu3O7–δ by High Resolution Electron Microscopy
Authors: Tang Dong , Zhou Wuzong , Thomas John M.
Published: December 2003

In the new high‐temperature superconductors YBa2Cu3O7–δ, which have a Perowskite like structure, the oxygen vacancies appear to be of special importance. It is generally difficult to image these vacancies by high‐resolution electron microscopy. However, this has now been definitively achieved by extensive calculations using the “multislice” technique. Under certain conditions of formation—sample thicknesses of 60–80 Å, defocusing adjustment between + 600 and −1800 Å, and a resolution of ca. 2.0 Å—the arrangement of the oxygen vacancies could be determined experimentally. All findings support a recently presented structural model of such superconductors.
How Nature Adapts Chemical Structures to Curved Surfaces
Authors: von Schnering Hans Georg , Nesper Reinhard
Published: December 2003

Guided by the symmetry in a natural way, periodic potential surfaces partition the space in solid crystalline compounds. The arrangement of atoms, clusters and molecules obviously follows the (in general) curved shape of these ‘space partitioners’. In structures, the atoms therefore choose only a very limited subset of the infinite set of possible positions. In collective structures the periodic surfaces separate areas of different interactions between atoms, clusters, and molecules. In a certain sense, they can be considered as inner surfaces, a knowledge of which reveals insights into the organization of crystalline matter. There are many good indications that the weakly bonded electrons in the highest occupied orbitals are preferably localized in the region of the space partitioners. Dynamic processes as well can be correlated to the shape of the periodic surfaces. Moreover, the surfaces are didactically very helpful in making accessible the complicated three‐dimensional relations in collective structures because the main features are projected onto (although curved) two‐dimensional creations. The application of periodic potential surfaces to such a variety of compounds as quartz, brass and alpha‐amylose underscores their general significance. Simple qualitative considerations already reveal the manifold relations to animate and inanimate nature through to mathematics, art and architecture. It appears that, in a very universal sense, the adaption of structures to a collective order finds a natural solution through curvature.
Homoatomic d10–d10Interactions: Their Effects on Structure and Chemical and Physical Properties
Authors: Jansen Martin
Published: December 2003

The distinction between valence electrons and essentially inactive core electrons is the basis of many classifying concepts in chemistry. However, it has recently been recognized that this is an oversimplification and should, at least in some areas of chemistry, be modified. Many cases are known where cations with closed d10configurations are subject to homoatomic interactions that influence structure and properties. A characteristic and surprisingly uniform structural feature (e.g., of a number of compounds containing monovalent coinage metals) is a clusterlike assembly of d10cations that corresponds in geometry and bond lengths to fragments of the metal structures themselves. Further evidence for a special type of bonding in such compounds is provided by their physical properties; for example, the absorption in the UV/VIS region shows a drastic redshift and the compounds are often conductors or semiconductors. The d electrons in such cases have obviously lost their pure “core” nature. All bonding models so far proposed for such systems involve mixing of higher orbitals.
The Recoil‐Free Fraction of the γ Resonance in 197Au Mössbauer Spectroscopy; Findings from Measurements of Polynuclear, Mixed‐Valent Ylide Complexes
Authors: Schmidbaur Hubert , Hartmann Christoph , Wagner Friedrich E.
Published: December 2003

The intensities of lines in 197 Au Mössbauer spectra have so far been very hard to interpret and therefore their use in structure elucidations has not been possible. Owing to the rich variety of new gold compounds in various oxidation states and with different coordination geometries, such an interpretation has now become possible. In addition to the isomeric shift (IS) and the quadrupole coupling (QS), the recoil‐free fraction (RFF) has become an important quantity for analyzing the structures of gold compounds.
High‐Pressure Synthesis and Crystal Structure of Neodymium(III) Oxide Peroxide, Nd2O2(O2)
Authors: Range Klaus‐Jürgen , Stadler Maximilian , Rau Franz , Klement Ulrich
Published: December 2003

The most oxygen‐rich compound so far reported for the system neodymiumoxygen was obtained by high‐pressure synthesis from Nd2O3/KO2 mixtures. Respite the composition NDO2, the X‐ray crystal structure analysis revealed at a neodymium(III) oxide peroxide and not a neodymium(iv) dioxide was present. Nd2O2(O2) is remarkably stable at normal pressure and only undergoes transformation to Nd2O3 at 420°C with loss of oxygen.
Molecular Dynamics of Elementary Chemical Reactions (Nobel Lecture)
Authors: Herschbach Dudley R.
Published: December 2003

That his Nobel lecture might be regarded as a tour through a family album is the view expressed by D. R. Herschbach in a note of thanks to his co‐workers. The following citation might serve as a stimulus for reading this lecture: “As urged by my students, on this occasion I want both to view our still youthful field of research from a wider perspective and to recount some favorite instructive episodes from its infancy. I will also briefly discuss several prototype reactions which have served to develop heuristic models and to reveal how electronic structure governs the reaction dynamics.”–Note: The Nobel lectures of Y.‐T. Lee and J. C. Polanyi who were awarded the Nobel prize together with D. R. Herschbach, have already appeared in the October issue of the journal.
Low‐Temperature Photolysis of Peracetylated Dodecanoyl Peroxides of Tartaric Acid and D‐Gluconic Acid in the Solid State—A Diastereoselective Radical Coupling
Authors: Lomölder Rainer , Schäfer Hans J.
Published: December 2003

The title reaction permits carboxylic acids of the chiral pool to be modified at the asymmetric center with retention of configuration. It is not only preparatively simple but also mechanistically interesting. By steric fixation of radicals in the solid state an otherwise unselective coupling becomes a diastereo‐selective one. The chemical yields are 40–60%, the diastereo‐selectivities 90–95% de.
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Molecular Architecture and Function of Polymeric Oriented Systems: Models for the Study of Organization, Surface Recognition, and Dynamics of Biomembranes
Authors: Ringsdorf Helmut , Schlarb Bernhard , Venzmer Joachim
Published: December 2003

The Part and the Whole. The principle of self‐organization for the creation of functional units is not an invention of modern natural sciences. It was already a basic idea of the ancient philosophies in Asia and Europe: only the mutuality of the parts creates the whole and its ability to function. Translated into the language of chemistry this means: the self‐organization of molecules leads to supramolecular systems and is responsible for their functions. Thermotropic and lyotropic liquid crystals are such functional units, formed by self‐organization. As highly oriented systems, they exhibit new properties. The importance of lyotropic liquid crystals for the life sciences has been known for a long time. They are a prerequisite for the development of life and the ability of cells to function. In materials sciences this concept offunction through organizationled to the development of new liquid‐crystalline materials. From the point of view of macromolecular chemistry, this review tries to combine these two different fields and especially hopes to stimulate their interaction and joint treatment. To exemplify this, the molecular architecture of polymeric organized systems will be discussed. Polymeric liquid crystals combine the ability to undergo spontaneous self‐organization–typical of liquid‐crystalline phases–with the polymer‐specific property of stabilizing these ordered states. As new materials, polymeric liquid crystals have already been investigated intensively. As model systems for biomembranes as well as for the simulation of biomembrane processes, they so far have been little discussed. The intention of this review article is to show that polymer science is able to contribute to the simulation of cellular processes such as the stabilization of biomembranes, specific surface recognition, or even the “uncorking” of cells. Polymer science, having an old tradition as an inter‐disciplinary field, can no longer restrict itself to common plastics. Attempts to reach new horizons have already begun. The borderland between liquid crystals and cells will certainly play an important role. Basic requirements to work in this frontier area between organic chemistry, membrane biology, life science, and materials science will be the delight in scientific adventures as well as the courage to go ahead. The most important prerequisite will be the willingness to cooperate with disciplines which so far have not really accepted each other. From this point of view, this review does not aim at giving defined answers. It wants instead to encourage the scientific venture: too often we cling to painfully acquired knowledge, fearing adventures.
Contributions to Organo‐Nickel Chemistry
Authors: Wilke Günther
Published: December 2003

One hundred years of organonickel chemistry have generated a wealth of new knowledge and a history of examples of accidental discoveries which have finally led to industrial applications. The historical development is associated with the names of Mond, Sabatier, Reppe and Ziegler and, with the methods and techniques available today, many of the original discoveries and unsolved problems are attracting renewed attention. For example, 70 years were to go by before a synthesis first conceived by Sabatier could finally be realized. The path leading from nickel‐catalyzed hydrogenation to highly enantioselective homogeneous catalysts is one of the contributions to organonickel chemistry which is described here.
Zeolites: Catalysts for Organic Syntheses
Authors: Hölderich Wolfgang , Hesse Michael , Näumann Fritz
Published: December 2003

Zeolites have proved to be valuable technical catalysts in petrochemistry and in oil processing. The characteristic properties of zeolites, such as acidity, shape‐selectivity and thermal stability also enable them to be used for highly selective synthesis in the fields of chemical intermediates and fine chemicals. This interesting area of application has grown continuously in recent years. The present article summarizes the various standard types of reaction involved in organic syntheses which can be catalyzed by zeolites; these include, inter alia, electrophilic and nucleophilic substitution reactions, isomerization of double bonds and carbon skeletons, as well as addition, elimination and hydrogenation reactions.
Tris(2,2′‐bipyridyl‐5‐sulfonic acid)rhodium(III), an Improved Redox Catalyst for the Light‐Induced and the Electrochemically Initiated Enzymatic Reduction of Carbonyl Compounds
Authors: Franke Marlene , Steckhan Eberhard
Published: December 2003

Unchanged activity after 36 hours is displayed by the light‐induced redox system consisting of nitrilotriethanol, NAD⊕, horse liver alcohol dehydrogenase, cyclohexanone, and [Ru(bpy)3]2⊕, with the title compound [Rh(L3)]3⊕ as electron relay. The electrochemically initiated enzymatic reduction of cyclohexanone in an analogous system also takes place with [Rh(L3)]3⊕ as redox catalyst without the formation of inactive (NAD)2. Since the reduced form of [Rh(L3)]3⊕ does not reduce carbonyl compounds, it is also possible to exploit fully the stereospecificity of the enzyme with the system described here.
Photochemical Reaction of [(CO)4Co(SiEt3)] with Ethylene: Implications for Cobaltcarbonyl‐Catalyzed Hydrosilation of Alkenes
Authors: Seitz Friedrich , Wrighton Mark S.
Published: December 2003

The insertion of a nonactivated alkene into the MSi bond of a catalytically active transition‐metal complex has been demonstrated for the first time. Furthermore, the formation of CH4 instead of SiMe4 in the reaction of [(CO)4CoMe] with Me3SiH is clearly inconsistent with the traditional Chalk–Harrod mechanism for the transition‐metal‐catalyzed hydrosilation of olefins. Both reactions are key steps in a new proposal for the mechanism of this reaction.
Multiphoton‐Ionization‐Mass Spectrometry (MUPI‐MS) [New Analytical Methods (34)]
Authors: Grotemeyer Jürgen , Schlag Edward W.
Published: December 2003

Mass spectrometry is one of the most important analytical tools in chemistry, biology, medicine and related areas. During the past 30 years, methods have been developed, both for the qualitative as well as the quantitative analysis of a wide variety of substances. The introduction of lasers into chemistry has also profited mass spectrometry, since its nonlinear properties and its tunability open up fundamentally new frontiers. Multiphoton ionization mass spectrometry combines UV‐spectroscopy and mass spectrometry, thus providing a two‐dimensional method that enables substance‐specific and even state‐specific analyses. This progress report presents the fundamentals and possibilities of MUPI mass spectrometry and discusses investigations on amino acids, peptides, chlorophylls and sugars. For each substance, multiphoton ionization can be tuned in such a way that only the molecular ion is formed. An increase of laser intensity induces substance‐specific fragmentations of the molecule, thus contributing to a quick and easy identification of the substance.
Microbial and Enzymatic Processes for the Production of Biologically and Chemically Useful Compounds [New Synthetic Methods (69)]
Authors: Yamada Hideaki , Shimizu Sakayu
Published: December 2003

In recent years, the most significant development in the field of synthetic organic chemistry has been the application of biological systems to chemical reactions. Reactions catalyzed by enzymes and enzyme systems display far greater specificities than more conventional organic reactions. Biological and/or enzymatic syntheses and transformations, that is, “microbial transformations,” have great potential. Some of these reactions have already been shown to have useful applications in the fields of synthetic organic chemistry and biotechnology. This article reviews the current status of the rapidly developing field of microbial transformation, the methodology, available technological procedures, and fields of application being described especially in relation to conventional organic synthesis methods.
Reduction and Hydrogenation with the System Hydrocarbon/Carbon [New Synthetic Methods (70)]
Authors: Dockner Toni
Published: December 2003

For the synthesis of organic compounds, reduction is an indispensible reaction type which is also widely used on an industrial scale. In industrial processes hydrogen is usually used as reducing agent, since strong reducing agents like alkali metals and hydrides can only be used to a limited extent for safety and economic reasons. Very economical reducing agents that are convenient to handle and have high potential application are hydrocarbons in presence of carbon. Hydrocarbon/carbon systems can be readily used instead of molecular hydrogen and expensive metal catalysts for the hydrogenation of compounds containing, for example, CC‐, CO‐, or NO‐double bonds. Furthermore, these systems can be used for carrying out reductions which hitherto required strong reducing agents such as zinc, tin, alkali metals and hydrides. Especially suitable as economic sources of hydrogen are refinery products such as vacuum gas oil, fuel oil S or vacuum residue oil. Hydrocarbons are dehydrogenated to unsaturated systems and finally to carbon.
The Pentazadienide Ion as a Ligand in Metal Complexes
Authors: Beck Johannes , Strähle Joachim
Published: December 2003

The pentazadienide ion is a surprisingly versatile ligand. The planar, zigzag N5 chain can utilize its atoms N1, N3, and N5 to complex one, two, or three metal atoms in a monodentate or bidentate fashion. Of particular interest is the ligand's ability to promote short metal‐metal contacts in complexes with d10 metal ions.
Catalytic Activity of Enzymes Altered at Their Active Sites
Authors: Kaiser Emil Thomas
Published: December 2003

Protein engineering has as its goals the design and construction of new peptides and proteins with novel binding and catalytic properties. In one approach to protein engineering, new active sites have been introduced into naturally occurring proteins either by site‐directed mutagenesis or by chemical modification. Providing that important changes in the tertiary structures do not result from such alterations, at least a portion of the binding site of the original protein should be available for the formation of complexes between the altered enzyme and its substrates. Many examples of active‐site mutations have been described, including the generation by us of a cysteine mutant of alkaline phosphatase. A fundamental limitation of the site‐directed mutagenesis methodology is that replacements of residues are restricted to the twenty naturally occurring amino acids. The alternative, chemical modification, is difficult to carry out for the specific replacement of one amino acid by another. However, we have shown that through such modification coenzyme analogues can be introduced covalently into appropriate positions in proteins, allowing us to produce semisynthetic enzymes with catalytic activities radically altered from those of their precursor proteins. In another approach to protein engineering efforts have focused on the construction of systems where, as a first approximation, folding can be neglected and the preparation of secondary structural units is the target. Examples of the successful design of biologically active peptides and proteins along such lines, taken from our own work, include molecules mimicking apolipoproteins, toxins, and many hormones. In recent studies we have progressed to the stage where we are starting to combine the two general approaches to protein engineering we have described and are able to construct small enzymes like ribonuclease T1 and its structural analogues.
Catalytic Fixation of Nitrogen by the Photocatalytic CdS/Pt/RuO2 Particulate System in the Presence of Aqueous [Ru(Hedta)N2]⊖ Complex
Authors: Khan Mirza M. Taqui , Bhardwaj Ramesh C. , Bhardwaj C.
Published: December 2003

The reduction of coordinated N2 to ammonia at atmospheric pressure and room temperature has been the subject of numerous studies. In all cases the reduction proceeds stoichiometrically, requires large quantities of reducing agents, or has to be initiated electrochemically. The photochemical reduction of N2 to NH3 at 1 atm N2 and 30 °C in aqueous solution in the presence of particles of the semiconductor system CdS/Pt/RuO2 (excitation by visible light) and of the anionic dinitrogen‐ruthenium(II) complex [Ru(Hedta)N2]⊖ as catalyst is now reported (Hedta = trianion of ethylenediaminetetraacetic acid).
Extremely Short Non‐Bonding OxygenOxygen Distances: The Crystal Structures of NbBO4, NaNb3O8, and NaTa3O8
Authors: Range Klaus‐Jürgen , Wildenauer Manfred , Heyns Anton M.
Published: December 2003

2.218 Å for OċO in transition metal‐oxygen polyhedra—this is a record! It is found in high‐pressure NaNb3O8. Similar short contacts between O atoms not bound to each other also occur in the structures of the high‐pressure phases NbBO4 (2.222 Å) and NaTa3O8 (2.225 Å). NaNb3O8 and NaTa3O8 crystallize in a novel tunnel structure: dodecahedra and distorted pentagonal bipyramids are so coupled with one another that channels are formed in which the Na⊕ ions are located.
Accurate Determination of the Redox State in High‐Tc Superconductor Oxides
Authors: Maier J. , Murugaraj P. , Lange C. , Rabenau A.
Published: December 2003

The determination of the excess oxygen content in high‐Tc superconductors is especially important. The efficiency of two new methods by which δ is determined directly has been demonstrated using the prototypic semiconductor YBa2Cu3O6.5+δ. In one method the shift of the Fe2⊕/Fe3⊕ concentration ratio is determined, whereas in the other the O2‐pressure after reaction of the oxide ceramic with water is measured.
Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides
Authors: Lamb H. Henry , Gates Bruce C. , Knözinger Helmut
Published: December 2003

Metal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface‐bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid‐base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O2⊖ ions; ion‐pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid‐base pair sites such as [Mg2⊕O2⊖]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface‐bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.
Naked Molecule Chemistry: Different Bonding Modes of CO to FeX2
Authors: Beattie Ian R. , McDermott Susan D. , Mathews Elizabeth A. , Millington Keith R. , Willson Andrew D.
Published: December 2003

Cocondensation of FeCI2 and FeBr2 with CO/Ar leads either to species of the composition FeX2·n CO, whose vco stretching vibration is of higher frequency than that of free CO, or to the hitherto unknown complexes trans‐[Fe(CO)4X2], X = CI, Br. In the case of FeI2, only trans‐[Fe(CO)4I2] has been detected, and in the case of FeF2 exclusively FeF2·n CO adducts are formed, even with pure CO at 35 K. These findings are of significance for an understanding of sorption phenomena.
High Oxidation State Organometallic Chemistry, A Challenge—the Example of Rhenium
Authors: Herrmann Wolfgang A.
Published: December 2003

Homogeneous catalysis as the major industrial outlet of organometallic basic research has been enjoying great benefit from organotransition metal species that promote bond forming between hydrocarbon fragments. Most of the commercially important processes that serve to produce large‐volume organic feedstock chemicals such as linear α‐olefins (Shell Higher Olefins Process), linear aldehydes (hydroformylation), acetaldehyde (Wacker‐Hoechst), acetic acid (Monsanto), adiponitrile (DuPont hydrocyanation of butadiene) operate at low‐valent metal centers. It is thus hardly surprising that by far the most part of organometallic research during the past few decades has been directed towards an understanding and the improvement of these catalytic reactions as well as towards the related stoichiometric chemistry. As a matter of consequence, our present knowledge on high‐valent organotransition metal compound is comparatively shallow, nor do we know much about the chemical relationship and interconvertability of high and low oxidation states within a given class of compounds. In this article I want to point out some ostensibly challenging perspectives of future organometallic research by describing a novel class of high oxidation state organorhenium compounds as well as by speculating on possible generalizations for other transition metals.
The Structures of Perchloric Acid, HClO4, and Its Anhydride, Cl2O7
Authors: Simon Arndt , Borrmann Horst
Published: December 2003

Crystals of the title compounds contain practically isolated HClO4 molecules and Cl2O7 molecules best described as (ClO)2O2⊖. Solid perchloric acid can be described as rods of H‐bridged molecules forming a closest rod packing (O‐HċO: 82 and 203 pm, respectively), solid dichlorine heptoxide as cubic close‐packed oxygen atoms, the neighboring tetrahedral gaps being occupied by chlorine atoms.
Donor‐Acceptor‐Substituted Cyclic π‐Electron Systems—Probes for Theories and Building Blocks for New Materials
Authors: Gompper Rudolf , Wagner Hans‐Ulrich
Published: December 2003

Donor and acceptor substituents stabilize (4n)π‐electron systems and destabilize those with (4n+2)π electrons. The same is true for the transition states of pericyclic reactions, which explains the appearance of dipolar intermediates in symmetry‐allowed cycloadditions and sigmatropic rearrangements. Donor‐acceptor‐substituted semibullvalenes undergo rapid Cope rearrangement, as do tetraazabarbaralanes. In contrast, tetraazasemibullvalenes can not be isolated, since the isomeric tetrazocines always result. The usefulness of the donor‐acceptor concept in preparative chemistry is demonstrated by numerous stable cyclic (4n)π‐electron systems, like donor‐acceptor‐substituted cyclobutadienes, tetraaminobenzene, and p‐benzoquinone dications, benzodiazepinyl anions, and donor‐acceptor‐substituted cyclopentadienyl cations and their heteroatom‐containing analogues. The new compounds are of interest in the fields of organic metals and ferromagnets, nonlinear optics, and dyestuffs, among others.
A Morphology‐Selective Copper Organosol
Authors: Curtis Andrew C. , Duff Daniel G. , Edwards Peter P. , Jefferson David A. , Johnson Brian F. G. , Kirkland Angus I. , Wallace Andrew S.
Published: December 2003

Hexagonal colloidal copper particles with a narrow size distribution are formed in the reduction of an acetonitrile solution of Cu(OAc)2·H2O with hydrazine hydrate in the presence of poly(vinyl‐2‐pyrrolidone). The deep‐red sols, which were characterized by electron microscopy, are extremely stable in an inert gas atmosphere. They can be redispersed even after complete drying. An insoluble red film is however formed when the dried sol is heated for 30 minutes; this process probably involves polymer cross‐linking.
Model of a Solvent‐Shared Ion Pair with NH ⃛C Hydrogen Bonds between Amine and Carbanion—Crystal Structure of [Fluorenyllithium·2 Ethylenediamine]∞
Authors: Buchholz Stefan , Harms Klaus , Marsch Michael , Massa Werner , Boche Gernot
Published: December 2003

Whereas organolithium compounds normally crystallize as contact ion pairs, in [fluorenyllithium·2 ethylenediamine]∞ the cation and the anion are separated by solvent molecules. Especially noteworthy is the X‐ray crystallographic detection of NH ⃛C hydrogen bonds to a “naked” Carbanion. Since anion and cation interact with the same solvent molecules the solid‐state structure of the title compound can serve as model for a solvent‐shared ion pair.
Reaction Mechanisms in Heterogeneous Catalysis; CH Activation as a Case Study
Authors: Maier Wilhelm F.
Published: December 2003

Heterogeneous catalysis is changing from an empirical art to an exact science. The various methods for the analysis of solids and surfaces, constantly refined by materials science and surface science, seem to be almost unlimited. The increasing availability of atomic resolution microscopy as well as synchrotron radiation allows the characterization of catalyst particles, surface structures, surface processes and surface intermediates. We have learned to determine the surface structure sensitivity of catalytic reactions. Thermodynamic and kinetic data of catalytic reactions are now determined routinely. Isotopic exchange and labeling experiments provide information about reactant‐catalyst interactions. How much have we learned through these techniques about the nature or mechanism of heterogeneously catalyzed reactions? The following article attempts to summarize the progress and the problems encountered in mechanistic studies of CH bond formation and activation in a hydrogen atmosphere as an example for the present state of the understanding of reaction mechanisms in heterogeneous catalysis.
The Structure of Lithium Compounds of Sulfones, Sulfoximides, Sulfoxides, Thioethers and 1,3‐Dithianes, Nitriles, Nitro Compounds and Hydrazones
Authors: Boche Gernot
Published: December 2003

Acceptor‐substituted lithio compounds LiACR1R2, in which the acceptor A is an RC(0), NC, RSO2, RS(O)NR, RSO, RS, O2N, or RC(NNR2) group, have long played an important role in organic synthesis. Their significance has grown still further in the last fifteen years, as one has increasingly learnt to employ them successfully in chemo‐, regio‐, diastereo‐and enantioselective reactions. Remarkably, little if anything was known of the structures of these compounds. It is therefore not surprising that interest in the structures of this class of compounds has greatly increased in recent years. In the following review, we shall summarize recent research into the structures of the lithio compounds of sulfones, sulfoximides, sulfoxides, thioethers and 1,3‐dithianes, nitriles, nitro compounds, and hydrazones. Crystal structure determinations from recent years are central to this study. They are supplemented by solution studies and by calculations of structures. Structural similarities and differences between the individual states of aggregation are pointed out wherever possible, as is also the relationship between structure and reactivity.
Perspectives in Photochromism: A Novel System Based on the 1,5‐Electrocyclization of Heteroanalogous Pentadienyl Anions
Authors: Dürr Heinz
Published: December 2003

The molecular process whereby light induces color changes in certain substances, namely photochromism, is currently a topic of active research. Many reactions—mainly monomolecular—have been reexamined in greater detail from the standpoint of photochromism. Those found to be especially suitable for such studies includecis‐transisomerizations, pericyclic reactions, tautomerizations and dissociations. The most important in this group are undoubtedly the electrocyclization reactions. In this paper we present spiro[fluorene‐9,1′‐[1,8a]‐dihydroindolizine] and its bis‐ and tris‐aza analogues as examples of a promising new system with a wide range of variability. Not only the classical uses, but also those in new fields such as frequency doubling of laser radiation, holography or molecular microcircuit chips can now be considered.
Sodium Pentaphosphacyclopentadienide (NaP5) and the Pentaphosphacyclopentadienide Ion (P) : Theoretical Predictions of Molecular Structures, Infrared and Raman Spectra
Authors: Hamilton Tracy P. , Schaefer Henry F.
Published: December 2003

Agreement and contradiction between experiment and theory were found when the spectroscopic properties of cyclo‐P (and NaP5) were derived by SCF calculations using a large basis set. The theory could reproduce a medium‐intense Raman band, but not an intense IR band at ṽ = 815 cm−1. The calculations indicate that this band is more likely due to linear P.
A Chemical Approach to Protein Design—Template‐Assembled Synthetic Proteins (TASP)
Authors: Mutter Manfred , Vuilleumier Stéphane
Published: December 2003

Advances in methodology in both chemistry and molecular biology allow us to take a fresh look at protein science. Chemical synthesis of peptides and site‐directed mutagenesis are now standard research tools, paving the way for the construction of new proteins with tailor‐made structural and functional properties. The decisive hurdle on the way lies not in the synthesis of the molecules proper but rather in a better understanding of the complex folding pathways of polypeptide chains into spatially well‐defined structures. Can the chemist use his synthetic tools to bypass the notorious “folding problem?” In this article, we present a new approach developed in our laboratory, which opens a chemical route to artificial proteins with predetermined three‐dimensional structures, allowing a first step towards the synthesis of new proteins with functional properties.
Structure and Dynamics of Chiral Molecules
Authors: Quack Martin
Published: December 2003

Could there be chiral methane? What is the characteristic structural feature (in a physicochemical sense) of a molecule? This question dates back to Louis Pasteur, the discoverer of molecular chirality, and since the work of van't Hoff and Le Bel is generally considered by chemists as solved. In the present article it is pointed out that there exist fundamentally conflicting theoretical views of the physical origin of molecular chirality. These views predict consequences that could, in principle, be distinguished experimentally, but at present there is no conclusive experimental evidence available. Possible experiments are suggested that test different hypotheses. The importance of the magnitude of the parity‐violating energy difference δEPV in molecules due to the weak nuclear force for both the structure and spectra of chiral molecules and for the kinetics of racemization is discussed. The chemical relaxation rate coefficient of chiral molecules with some appreciable energy of excitation is derived for several limiting cases of a simple statistical mechanical model, which takes ΔEPV into account.
Pep5: Structure Elucidation of a Large Lantibiotic
Authors: Kellner Roland , Jung Günther , Josten Michaela , Kaletta Cortina , Entian Karl‐Dieter , Sahl Hans‐Georg
Published: December 2003

Lantibiotics are peptide antibiotics which contain lanthionine. Pep 5, a strongly basic compound, is produced from Staphylococcus epidermidis 5 by ribosomal synthesis of a precursor peptide and post‐translational modification. The antibiotic activity of Pep 5 rests on the formation of strain‐dependent pores in bacterial membranes. Pep 5 is the hitherto largest lantibiotic (Mr = 3488). It is N‐terminally blocked by a 2‐oxobutyryl residue and contains, besides 25 protein amino acids, dehydrobutyrine (2), meso‐lanthionine (2) and 3‐methyllanthionine (1). Automatic Edman degradation, mass spectrometry of enzymatically obtained fragments and chemical derivatization gave a 34‐peptide sequence with three sulfide rings (lanthionine = S(CH2CHNH2COOH)2).
Challenges in Materials for Health Care Applications
Authors: Williams David F.
Published: December 2003

Important issues face the use of advanced materials in medical applications. It is now possible to replace or augment many tissues and parts of the body by implanted devices, but there are still severe limitations to functions they are able to perform and problems associated with their compatibility with the tissues. Biomaterials of the future need to simulate more closely the tissues they are replacing and both the current position and future outlook are reviewed in this light.