bluuebunny/crossref_metadata_2025 huggingface dataset:
A Stochastic Modeling Approach to Real-Time Prediction of Queue Overflows
Authors: Sheu Jiuh-Biing
Published: December 2003

Queue overflow is a critical issue in developing queue prediction technologies for applications in Advanced Transportation Management System (ATMS). Conventional queue prediction methods, however, are limited to incident-free queue length prediction where traffic arrivals can be readily obtained using detectors. Despite the problems posed by queue overflow, studies addressing queue-overflow issues, or for predicting queue overflows beyond detectors, appear inadequate. This paper describes an advanced methodology which uses a stochastic system modeling approach and random processes for predicting queue lengths beyond detectors in real time. Lane changing is taken into account in developing the queue-overflow prediction model because lane changing accompanies queue overflow in most cases. A discrete-time, nonlinear stochastic system is specified for modeling the queues and lane changes beyond detectors during queue-overflow occurrence. The noise terms of the recursive equations of the model account for the effects of queues and a variety of arriving volumes on vehicular lane-changing maneuvers during queue-overflow occurrence. The unknown traffic arrivals beyond detectors are predicted employing random processes. In addition, a recursive estimation algorithm for predicting real-time queue overflows is developed utilizing the extended Kalman filtering technique. Preliminary test results indicate that the proposed methodology is promising for real-time prediction of queue overflows. The predicted queue overflows can be used not only in understanding the phenomenon of lane traffic patterns during queue-overflow occurrence, but also in developing related advanced technologies such as real-time road traffic congestion control and management systems.
Procedures for Finding Nondominated Solutions for Multiple Objective Network Programming Problems
Authors: Sun Minghe
Published: December 2003

Procedures for finding nondominated solutions for multiple objective network programming problems are developed and tested. Nondominated solutions are obtained by solving augmented weighted Tchebycheff network programs. The procedures exploit the network structure of the problem in order to speed up the solution process. To use the network structure as much as possible, a weighted-sum network problem and/or a min-max network problem are solved in order to find a basic solution that is close to the optimal solution of the augmented weighted Tchebycheff network program. Starting from this basic solution, the special simplex method for network problems with side constraints is finally applied to solve the augmented weighted Tchebycheff network program. Computational results show that, for the test problems used in this study, up to 70% of computation time can be saved with the proposed procedures as compared with the sole application of the special simplex method for network problems with side constraints. These procedures can be incorporated into any interactive multiple-objective programming procedure which uses sample nondominated solutions to solve multiple-objective network programming problems.
Engine Routing and Scheduling at Industrial In-Plant Railroads
Authors: Lübbecke Marco E. , Zimmermann Uwe T.
Published: December 2003

In-plant railroad engine scheduling involves routing and scheduling decisions for a heterogeneous fleet of switching engines to serve a set of time-window- and capacity-constrained transportation requests. Despite an ever-increasing competition, the current planning is purely by pencil and paper. Our paper describes the mathematical and algorithmic developments for addressing in-plant railroad decision support for scheduling and routing. The problem discussed in our work is related to the multiple-vehicle pickup and delivery problem. Exploiting the structure of admissible schedules of our particular railroad situation, we introduce two formulations of the problem as mixed integer and set partitioning programs. We propose solving the linear programming relaxation of the set partition model by column generation. We focus on the pricing problem stated in the form of a constrained shortest path problem, which is NP complete in the strong sense. A new exact label correcting algorithm is developed that prunes the search space in a novel manner. Heuristically obtained integer solutions of a practical quality are proposed as well. All the claims are demonstrated by computational experiments on both artificial and real-life data. We discuss implementation details as well.
Structure of the Transition Zone Behind Freeway Queues
Authors: Muñoz Juan Carlos , Daganzo Carlos F.
Published: December 2003

Observations of freeway traffic flow are usually quite scattered about an underlying curve when plotted versus density or occupancy. Although increasing the sampling intervals can reduce the scatter, whenever an experiment encompasses a rush hour with transitions in and out of congestion, some outlying data stubbornly remain beneath the “equilibrium” curve. The existence of these nonequilibrium points is a poorly understood phenomenon that appears to contradict the simple kinematic wave (KW) model of traffic flow. This paper provides a tentative explanation of the phenomenon, based on experimental evidence. The evidence was a FIFO queue that grew and receded over two detector stations, generating typical flow-density scatter plots at both locations. The locations were far from other interacting traffic streams. The data revealed that a transition zone where vehicles decelerated gradually existed immediately behind the queue. The transition zone was quite wide (about 1 km at both locations), moved slowly (approximately with the “shock” velocity of KW theory), and as a result spent many minutes over each detector station. Disequilibrium flow-density points arose only when the transition zone was over the detectors, suggesting that the transition zone explains their occurrence. The disequilibrium points drifted gradually from one branch of the curve to the other, as KW theory would have predicted if “shocks” had a characteristic width equal to the dimension of the transition zone. Nothing was found in the data to contradict this view. This paper also shows that in our case, if one neglects the shocks' physical dimension, the position of every vehicle can be predicted with KW theory to within approximately five vehicle spacings. Thus, it appears that KW theory can predict rather accurately traffic behavior at the back of FIFO queues, i.e., when the lanes are equally attractive to all drivers. We end with a discussion offering some perspective on how the findings of this paper related to the traffic thinking found in the current literature.
A Bundle Algorithm Approach for the Aircraft Schedule Recovery Problem During Hub Closures
Authors: Thengvall Benjamin G. , Bard Jonathan F. , Yu Gang
Published: December 2003

A bundle algorithm is presented to solve a multicommodity network model for determining a recoveryplanfor a single carrier with multiple fleets following a hub closure. The algorithm is shown to provide feasible near-optimal solutions much more quickly than can be obtained using a standard commercial mixed-integer programming code (CPLEX). In this application, a bundle method is used to solve a Lagrangian relaxation of the integer programming formulation. The full algorithm includes heuristic techniques for finding feasible solutions from the solutions to the relaxed problems. Extensive computations were performed using data from Continental Airlines. The results show that the proposed approach provides faster times to optimality in some cases and always obtains feasible, near-optimal solutions for larger problems much more quickly than can be found using CPLEX. In addition, while a standard commercial code will provide only one solution, this approach provides multiple high-quality solutions.
Factors Influencing Blind Conflict Risk in En Route Sectors Under Free-Flight Conditions
Authors: Willemain Thomas R.
Published: December 2003

We develop a simulation model of the flight of aircraft through an en route sector. The model is used in two experiments. The first studies the effect of directional clumping on risk. The second is a full factorial design with replication, which simulates a large number of scenarios to assess the impact of key input factors on free-flight risk measures. We measure risk for a given flight by the mean distance of closest approach to other aircraft and the mean number of approaches closer than a specified distance. Using analysis of variance and regression analysis, we determine that the most important influences on blind collision risk are time of entry into the sector and number of aircraft. Also influential are certain orientations of flight paths and the distribution of airspeeds. The least important factor is altitude. The relationship between number of aircraft and the two risk measures can be quantified in regression relationships.
The Isocyanide–Cyanide Rearrangement; Mechanism and Preparative Applications
Authors: Rüchardt Christoph , Meier Michael , Haaf Klaus , Pakusch Joachim , Wolber Erwin K. A. , Müller Barbara
Published: December 2003

Until recently the isocyanide–cyanide rearrangement was of interest almost solely as an example of a unimolecular gas‐phase reaction, and kinetic studies had been carried out in only a few simple cases. Kinetic measurements in solution were made possible only by the discovery and suppression of a parallel free‐radical chain process which leads to the same products. The rate of the isomerization is almost independent of the structure of the starting material and of the substituents present. An exception is provided by extreme steric hindrance in three dimensions which, as in tris‐α‐substituted triptycyl isocyanides, leads to a considerable increase in the activation energy. The results can be interpreted in terms of a purely sigmatropic mechanism, as predicted by ab initio calculations. The preparative application of this rearrangement reaction requires the suppression of side reactions and can best be carried out by flash pyrolysis; yields are then almost quantitative. Allyl isocyanides react without allyl isomerization, optically active isocyanides with complete retention of configuration. New, economically interesting syntheses for the known nonsteroidal anti‐inflammatory drugs ibuprofen and (S)‐naproxene are described. The application of the useful synthetic building blocks, the optically active β‐acyloxy cyanides, which are formed from optically active α‐amino acids, will be demonstrated.
Lise Meitner and Fission: Fallout from the Discovery
Authors: Sime Ruth Lewin
Published: December 2003

Much has been written about Lise Meitner, but she remains on the periphery. Of her pioneering work in nuclear physics, little is said; she is remembered primarily for nuclear fission, a discovery in which she did not share. Especially in Germany the staging seldom varies: Otto Hahn in the spotlight, Fritz Straßmann in his shadow, Lise Meitner in the wings, dimly outlined in reflected light. Her role is open to speculation. One writer sees her work “crowned by the Nobel Prize for Otto Hahn”; [1] another, once director of an institute that bears her name, portrays her as the physicist who obstructed the discovery from the start.[2] Often she is cast as Hahn's Mitarbeiterin;[3] sometimes she is completely invisible, as in one of the world's great science museums, which for 30 years displayed[4] the fission apparatus–equipment assembled by Lise Meitner on a table in her laboratory in her physics section of the Kaiser‐Wilhelm‐Institut für Chemie—without ever mentioning her name.[**] The principals themselves do not agree. In his memoirs, Hahn has remarkably little to say of his closest colleague and friend. Only over a glass of wine, we are told, “konnte ihm die Äusserung entschlüpfen: ‘Ich weiss nicht; ich fürchte, Lischen hätte mir die Uranspaltung verboten.”’[5] (“he might let slip: ‘I don't know; I'm afraid Lischen would have forbidden me to discover fission.”’) Straßmann insists: “Lise Meitner war die geistig Führende in unserem Team gewesen”![6] (“Lise Meitner was the intellectual leader of our team”!) From Lise Meitner herself we have no autobiography. Available, however, is the large collection of letters and documents she has left behind, and these make it possible for us to learn much more.
Polyacetylene in Polyacrylonitrile Matrix: Novel Soluble Matrix Polyacetylenes by Ylide‐Nickel Catalysis
Authors: Starzewski K. Alexander Ostoja , Bayer Günter M.
Published: December 2003

In its crude form, polyacetylene is black, insoluble, nonprocessible, and unstable. Now, ylide‐nickel catalysts have opened up an approach to the preparation of novel polyacetylenes in a stabilizing polyacrylonitrile matrix. These were characterized by IR and UV spectroscopy. The composition of the polymers can be controlled; the degree of polyene conjugation is dependent on the choice of ligand. Even for polyacetylene contents over 50%, the polymers are soluble and easily processed.
d‐Electron Density in Formal d0 Systems from Multiplet Splitting in the Photoelectron Spectrum of Permanganates
Authors: Schmalz Michael , Schöllhorn Robert , Schlögl Robert
Published: December 2003

A total of 3.5 electrons are present in the formally empty d orbitals of manganese in MnO. This was found by analysis of the multiplet splitting in the Mn 3 s photoelectron spectrum. The appearance of this splitting is itself a proof that d electrons are present. As a consequence, the charge at the oxygen ions must be substantially less than − 2. These results are of importance for the description of the charge distribution in high‐temperature superconductors of the oxometallic type.
Oxidation of Alkanes by [Fe(O)OH]⊕ in the Gas Phase—the Role of Iron Oxidation State in CH Activations
Authors: Schröder Detlef , Schwarz Helmut
Published: December 2003

Under the conditions of Fourier transform ion cyclotron resonance, the FeIV oxide [OFeOH]⊕ was induced to react with methane, ethane, n‐butane, and isobutane. These reactions between mass‐selected cations of metal oxides and organic compounds in the gas phase allow control of the stoichiometry and the oxidation states of metal oxides. In order to investigate the reaction mechanisms involved in CH activation, these experiments were also carried out with deuterated compounds.
Lantibiotics—Ribosomally Synthesized Biologically Active Polypeptides containing Sulfide Bridges and α,β‐Didehydroamino Acids
Authors: Jung Günther
Published: December 2003

Lantibiotics are polycyclic peptide antibiotics containing intrachain sulfide bridges, formed from the thioether groups of the amino acids lanthionine and β‐methyllanthionine. They also contain α,β‐unsaturated amino acids such as didehydroalanine and didehydroaminobutyric acid. A knowledge of the lantibiotic biosynthetic steps and the enzymes involved makes possible a gene technological construction of analogous highly modified polypeptides. To the family of lantibiotics belong nisin, an important food preservative, epidermin, a highly specific therapeutic agent against acne, a series of enzyme inhibitors, as well as immunologically interesting active peptides. Lantibiotics are produced by ribosomal synthesis, starting from inactive precursor proteins (prelantibiotics). The latter are post‐translationally converted into the active peptide antibiotics through enzymic modifications. The modifying enzymes effect dehydrations at the serine and threonine residues and stereospecific additions of the cysteine thiol groups to the resulting α,β‐unsaturated double bonds, which lead to the formation of several sulfide bridges. Upon subsequent proteolytic cleavage of the leader peptide, the biologically active lantibiotic is formed. Conformational analyses of the lantibiotics, as well as of their prepeptides, enables one to obtain information about the mechanism and steps of the biosynthesis. Antibodies against synthetic prepeptide sequences, and modern instrumental methods for the analysis of peptides, allow structural elucidation of the biosynthetic intermediates.
Groups of Organic Molecules That Operate Collectively
Authors: Menger Fredric M.
Published: December 2003

A “chemical system” is defined as an assemblage of molecules that collectively does something interesting or useful. The key word here is “collectively”, a word that implies an interdependency and a group behavior that can be quite different from that of individual molecules. Batteries, computer chips, concrete, mayonnaise, shampoo, paint, liquid crystal displays, composites, and viruses are all examples of commonly encountered systems. A host–guest or “supramolecular” complex, on the other hand, would not be considered a system (as defined here), because only two species are involved. A chemical system is multimolecular, a collection of molecules interlocked in a tangle of dependencies. The review delves into a variety of chemical systems investigated by the author, including micelles, water pools, films, vesicles, and polymers. All of them can be categorized as “self‐assembling” or “self‐organizing” in the sense that defined structures arise spontaneously owing to noncovalent forces among the component molecules. Such chemical systems are useful for many purposes, including decontamination of environmentally dangerous substances, drug delivery, and separation of organic compounds.
New Discoveries in the Realm of Metal–Metal Multiple Bonds:[η5‐(C5Me5)Co2], the First Organometallic Multiple‐Bond Complex without Bridging Ligands
Authors: Wachter Joachim
Published: December 2003

Double bond, yes or no? That is the poignant question in the case of the complex [Cp*CoCoCp*] prepared by J. J. Schneider et al. The CoCo distance and the number of electrons suggest a double bond, while the 1H‐NMR spectrum and the poor reactivity toward C2H4 and CO would suggest the contrary. Nevertheless, with this unbridged complex, the prerequisite for the synthesis of further such compounds and for studying the properties of these multibond systems was realized (see also Communication on p. 1124ff.).
Reduction of the Small Subunit of Ribonucleotide Reductase by Diimide: Evidence for the Formation of the Mixed‐Valence FeIIFeIII center
Authors: Gerez Catherine , Gaillard Jacques , Latour Jean‐Marc , Fontecave Marc
Published: December 2003

The diiron(III) center of the protein R2, a subunit of the ribonucleotide reductase, can be efficiently reduced with diimide to the mixed valence FeII/FeIII state, which can be detected EPR spectroscopically. A comparison with the spectra of the mixed valence state of hemerythrin suggests that both Fe atoms are only weakly antiferromagnetically coupled, and are bound via a hydroxo bridge.
SiI2—A Novel Triatomic Molecule with a Relativistic Touch
Authors: Bock Hans , Kremer Matthias , Dolg Michael , Preuß Heinz‐Werner
Published: December 2003

Reaction of pure SiI4 with elemental silicon in a short‐path thermolysis furnace at 1200K affords SiI2, one of the few still unknown of the 1638 triatomic molecules of the 13 most important non‐metals. The assignment of the photoelectron spectrum used for the characterization, which contained seven bands; was accomplished with the help of a quasirelativistic two‐component pseudo‐potential SCF calcuation with DZP basis set; the Koopmans' defects are less than ± 0.33 eV.
Optically Anomalous Crystals
Authors: Kahr Bart , McBride J. Michael
Published: December 2003

The optical symmetry of many crystals is lower than would be expected from their external form and X‐ray diffraction data. Recently such optical anomalies have been attributed to nonequilibrium structures resulting from kinetically controlled crystal growth. Impurities are incorporated to different extents at various surface sites that would otherwise have become symmetry‐related within the bulk crystal. After their discovery by Brewster in 1815, optically anomalous crystals were the subject of lively debate throughout the 19th century among some of the most distinguished contributors to the development of crystallography including Biot, Berzelius, Herschel, Mitscherlich, Frankenheim, Pasteur, Mallard, Klein, Groth, Wyrouboff, Barlow, Brauns, Rinne, Pockels, and Friedel. From a sea of wild speculation two conflicting postulates emerged: that the symmetric form resulted from accidental twinning of segments with lower symmetry, or that the optical peculiarities resulted from stress due to impurities or external perturbations. Neither postulate expresses the present view. Interest in this research waned at the turn of the century, and after 1917 no one pursued Tammann's alternative correct insight. The problem of anomalous double refraction attracted no attention for more than half a century until its recent solution. This review discusses both mineral and organic systems with particular emphasis on the phyllosilicate apophyllite and on 1,5‐dichloro‐2,3‐dinitrobenzene. These nonequilibrium structures relate to questions of crystal order, crystal growth, molecular recognition, and the design of new materials. The review provides a reminder of the enduring value of the polarizing microscope as a research tool in chemistry.
The Stability of Buckminsterfullerene (C60): Experimental Determination of the Heat of Formation
Authors: Beckhaus Hans‐Dieter , Rüchardt Christoph , Kao Michelle , Diederich François , Foote Christopher S.
Published: December 2003

The standard enthalpy of formation of crystalline buckminsterfullerene (C60) is 544.99 ± 1.35 kcalmol−1 which corresponds to 9.08 ± 0.02 kcalmol−1 per C atom. This follows from the calorimetric determination of the heat of combustion of extremely pure C60. Of the several theoretically predicted heats of formation, the value calculated with the MM3 force field (ΔH(g) = 570 kcalmol−1) shows the best agreement with the experimental data.
Crystal and Solution Structures of the Novel Complex of Sodium Ketimide with Two Ketimine Ligands [(tBu2C = NNa)4(HN = CtBu2)2]: Stacking of Complexed and Uncomplexed (NNa)2 Rings to give a Highly Distorted (NNa)4 Cubane
Authors: Clegg William , MacGregor Murray , Mulvey Robert E. , O'Neil Paul A.
Published: December 2003

Structure determinations of organometallic compounds of alkali metals often reveal new surprises: In the case of the sodium ketimide [(tBu2CNNa)4(HNCtBu2)2] 1, nonmetalated ketimine units function surprisingly as monodentate ligands for two of the four Na+ ions in the cubanelike (NNa)4 framework. This contrasts significantly with the well‐known lithium ketimides such as (tBu2CNLi)6, which in general resist complexation. Applying the “ring‐stacking principle” from organolithium compounds, 1 can be interpreted as a twofold stack of (NNa)2 rings.
C60: Buckminsterfullerene, The Celestial Sphere that Fell to Earth
Authors: Kroto Harold W.
Published: December 2003

In 1975–1978 the long‐chained polyynylcyanides, HC5N, HC7N, and HC9N were surprisingly discovered in the cold dark clouds of interstellar space by radioastronomy. The subsequent quest for their source indicated that they were being blown out of red giant, carbon stars. In 1985 carbon‐cluster experiments aimed at simulating the chemistry in such stars confirmed these objects as likely sources. During these cluster studies a serendipitous discovery was made; a stable pure‐carbon species, C60, formed spontaneously in a chaotic plasma produced by a laser focused on a graphite target. A closed spheroidal cage structure was proposed for this molecule, which was to become the third well‐characterized allotrope of carbon and was named buckminsterfullerene. It has taken five years to produce sufficient material to prove the correctness of this conjecture. There may be a timely object lesson in the fact that exciting new and strategically important fields of chemistry and materials science have been discovered overnight due to fundamental research, much of which was unable to attract financial support, and all of which was stimulated by a fascination with the role of carbon in space and stars. In this account, interesting aspects of this discovery, its origins, and its sequel are presented. The story has many facets, some of which relate to the way scientific discoveries are made.
Two‐Dimensional Crystallography of Amphiphilic Molecules at the Air–Water Interface
Authors: Jacquemain Didier , Wolf Sharon Grayer , Leveiller Franck , Deutsch Moshe , Kjaer Kristian , Als‐Nielsen Jens , Lahav Meir , Leiserowitz Leslie
Published: December 2003

The advent of well‐collimated, high‐intensity synchrotron X‐ray sources and the consequent development of surface‐specific X‐ray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the air–liquid interface. These methods allowed for the first time the determination of the in‐plane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence X‐ray diffraction, specular reflectivity, Bragg rods, standing waves, and surface fluorescence techniques, and review recent results obtained from them for Langmuir films. The methods have been successfully applied in the elucidation of the structure of crystalline aggregates of amphiphilic molecules such as alcohols, carboxylic acids and their salts, α‐amino acids, and phospholipids at the water surface. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self‐aggregates as well as structural changes occurring by phase transitions. Furthermore, the surface X‐ray methods shed new light on the structure of the underlying ionic layer of attached solvent or solute species. Examples are given where singly or doubly charged ions bound to the two‐dimensional (2D) crystal form either an ordered or diffuse counterionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals, which had been successfully accomplished by epitaxial‐like crystallization both in organic and inorganic crystals.
Control and Design Principles in Biological Mineralization
Authors: Addadi Lia , Weiner Stephen
Published: December 2003

The control of crystal formation has been developed to a remarkable degree by many organisms. Oriented nucleation, control over crystal morphology, formation of unique composites of proteins and single crystals, and the production of ordered multicrystal arrays, are all well within the realm of biological capability. Understanding the control and design principles in biomineralization is a fascinating subject that may well contribute to the improved fabrication of synthetic materials on the one hand, and to the solution of many serious pathological problems involving mineralization, on the other.
Phosphorus Nitrides—the Cinderella of Phosphorus Chemistry?
Authors: Scherer Otto J.
Published: December 2003

Stamped as long since discovered and therefore completely characterized, simple inorganic molecules like the phosphorus nitrides are often neglected. Yet although P3N5 was synthesized in 1903, its structure is still a mystery. W. Schnick et al. resurrected the chemistry of these compounds in the synthesis of P‐N analogues of known structural types. In this way they succeeded in synthesizing and characterizing the lithium salt of the complex anion P4N (Angew. Chem. 1991, 103, 857; Angew. Chem. Int. Ed. Engl. 1991, 30, 830) and Zn7[P12N24]Cl2, whose [P12N24] framework is analogous to that of the [Al6Si6O24]Cl2 unit of sodalite (communication on p. 213).
Inhibition of Fruit Ripening by Antisense‐RNA Technology
Authors: Eckes Peter
Published: December 2003

Controlled ripening of tomatoes—the application of antisense‐RNA technology by an American research group made this possible. The biosynthesis of ethylene, the signal compound which induces ripening, can be suppressed by these molecular biological methods. The storage and transportation of the green fruits is much easier. Their subsequent exposure to ethylene then triggers ripening. Antisense‐RNA technology can be applied in other areas, for example in medical research.
Microcalorimetry of the Spin‐Transition Systems [Fe(bts)2(NCS)2] and [Fe(2‐pic)3]Cl2· EtOH in the Range 15K < T < 300K
Authors: Jakobi Ralf , Romstedt Harald , Spiering Hartmut , Gütlich Philipp
Published: December 2003

For compounds with poor thermal conductivity and samples of only a few milligrams the heat capacity may be determined as a function of temperature with great accuracy with the microcalorimeter presented here. This was demonstrated on two FeII spin‐transition systems. Comparison with the results of measurements conducted with a Nernst calorimeter on a sample almost 1000 times larger was possible in one case; the agreement of the two data sets was quite good. Simultaneously the change in the thermal conductivity at the spin transition was determined for the first time.
The Neutralization of HeC in the Gas Phase: Compelling Evidence for the Existence of an Endohedral Structure for He@C60
Authors: Weiske Thomas , Wong Thomas , Krätschmer Wolfgang , Terlouw Johan K. , Schwarz Helmut
Published: December 2003

Neutralization‐reionization mass spectrometry was used to show that for both neutral He@C60 and He@C, the noble gas atom is incorporated within the fullerene cavity. The C60H˙+ ions were generated by high‐energy collisions of C with quasi‐stationary 4He target atoms and cleaved in the mass spectrometer by loss of C2 or multiples thereof, as is usual for endohedral MC60 complexes.
Observation and Properties of a Hydrogen Bond to Carbon in a Short‐Lived, Gas‐Phase Complex of Methyl Isocyanide and Hydrogen Fluoride
Authors: Legon A. C. , Lister D. G. , Warner H. E.
Published: December 2003

Through coaxial mixing of a pulsed jet of CH3NC/Ar with a continuous stream of HF/Ar, the complex CH3NCċHF is formed. Since the two components are separate until their expansion in the Fourier‐transform microwave spectrometer, their high reactivity can be suppressed. The rotation spectrum of the complex is characteristic of that of a symmetric top, that is, the C3v symmetric complex contains a linear CNCċHF arrangement.
Calculation of Molecular Dynamics for Peptides in Dimethyl Sulfoxide: Elimination of Vacuum Effects
Authors: Kurz Michael , Mierke Dale F. , Kessler Horst
Published: December 2003

The γ turns not found experimentally in cyclic hexapeptides are also not found in molecular dynamics (MD) simulations if the calculations take the surrounding solvent into consideration. In this report the structures obtained from simulations conducted for the peptide in vacuum were used in subsequent simulations with dimethyl sulfoxide as the solvent. The two γ turns originally obtained disappeared, and only the experimentally proven β turns remained. These results show that the solvent must be explicitly included in MD simulations of peptide conformations in solutions.
Radical Intermediates in the Coenzyme B12 Dependent Methylmalonyl‐CoA Mutase Reaction Shown by ESR Spectroscopy
Authors: Zhao Ymin , Such Peter , Rétey János
Published: December 2003

Only after the addition of the substrate, methylmalonyl‐CoA, to the binary complex of highly purified methylmalonyl‐CoA mutase and coenzyme B12 is a clear ESR signal observed. This proves that radical intermediates that have only been postulated in the past are indeed involved in the enzymatic rearrangement of the carbon skeleton.
A New Fullerene Synthesis
Authors: Peters Gregor , Jansen Martin
Published: December 2003

Larger amounts of fullerenes at the remarkably low temperature of formation of 2700 °C (helium pressure 150 kPa) are provided by a new synthetic method. Cylindrical carbon bodies are inductively heated and evaporated, the resulting fullerenes are extracted from the soot formed, and the product distribution is determined by mass spectrometry.
Benzene and Its Derivatives as Bridging Ligands in Transition‐Metal Complexes
Authors: Wadepohl Hubert
Published: December 2003

Transition‐metal complexes in which two or more metal atoms are bridged by one or more arene ligands led a shadowy existence in comparison to the extensive class of mononuclear arene complexes. Arene bridges can occur in a variety of coordination modes and with almost all of the transition–metal elements of the periodic table. Nowhere else are found so many forms of distorted and bent arene rings. The binuclear compounds can be divided into two classes: adducts which show relatively weak metal–arene bonding and complexes which show strong arene–metal interaction. Most of the adducts are in equilibrium with mononuclear complexes in solution or are only stable in the solid state (often as polymers). In both classes syn and anti coordination occurs; their geometries show a wide variation between the extreme cases of η1 : η1‐bridge and η6 : η6‐triple‐decker structure. Metal surfaces with chemisorbed arenes can be seen as a form of multinuclear arene–metal complexes. On transition‐metal surfaces, benzene can be bonded to one, two, or four surface atoms. Molecular clusters with face‐capping arene ligands that are bonded to three metal atoms have until now mainly been limited to two classes. The arenes bound to {(CO)3M}3 (M = Ru, Os) or (CpCo)3 clusters as μ3‐η2 : η2 : η2 ligands show only a weak trigonal distortion towards a Kekulé structure. Detailed investigations of the molecular structure and ligand dynamics of [(CpCo)3(μ3‐arene)] complexes considerably help the understanding of the bonding of arenes to metal clusters and to metal surfaces.
High Resolution Spectroscopy below the Doppler Width
Authors: Neusser Hans Jürgen , Schlag Edward William
Published: December 2003

Microscopic insights into elementary molecular processes are only feasible through the observation of unique quantum states and the study of their couplings and time‐dependent behavior. In the case of larger chemically relevant molecules, the corresponding transitions lie so close that they mostly overlap and lead to band structures. For example, electronic band spectra of large molecules cannot be resolved into single lines even by high resolution UV‐spectrometers. The reason for this is the Doppler broadening, arising from the random velocity of the molecules in the gas phase. In this article, we want to demonstrate that modern laser spectroscopic methods make it possible to overcome this “Doppler barrier” and to resolve the line structure in band spectra. The selective excitation of unique quantum states yields precise information on the mechanisms responsible for energy redistribution in molecules. Energy redistribution processes represent a crucial first step preceding chemical reactions, which they influence decisively. Very high resolution spectroscopy elucidates the influence of molecular rotation on the energy redistribution. Through Coriolis forces in rotating molecules, vibrations can couple, causing a flow of energy from the selectively excited vibrations to the many other vibrational degrees of freedom. Weak interactions, namely van der Waals interactions and hydrogen bonding, determine the structure of large molecules (biomolecules) and supermolecular systems. Weak intermolecular interactions, given their selectively resolved quantum states, can be analyzed and understood at the microscopic level. “Doppler‐free spectroscopy” with its resulting line spectra furnishes information on the structure, including bond lengths, of van der Waals complexes. With this information, Doppler‐free spectroscopy provides the model parameters indispensable for the quantitative description of complex systems.
Bond‐Stretch Isomers: Fact or Artifact?
Authors: Mayer James M.
Published: December 2003

It's almost a question of faith: Do bond‐stretch isomers exist? This apparent phenomenon first came to light in 1971 when results of X‐ray structural analyses of octahedral molybdenum complexes revealed that compounds with the same arrangement of ligands could differ only, but significantly, in the length of the MO bond. After this and other examples were explained by theory, it seemed that nothing could prevent the final acceptance of the concept. But later experimental studies proved that the different bond lengths were actually an artifact caused by impurities in the crystal. The recent report of the synthesis and characterization of two forms of [Nb(E)Cl3(PMe3)3] (E = O, S) by Gibson, McPartlin et al. (Angewandte Chem. Int. Ed. Engl. 1991, 104, 980), however, appears to have started the debate anew.
A New Structure–Magnetism Relationship for Face‐Sharing Transition‐Metal Complexes with d3–d3 Electronic Configuration
Authors: Niemann Andrea , Bossek Ursula , Wieghardt Karl , Butzlaff Christian , Trautwein Alfred X. , Nuber Bernhard
Published: December 2003

A through‐space interaction and not superexchange of the bridging ligands explains the antiferromagnetic coupling in dinuclear face‐sharing octahedral complexes with d3‐d3 electronic configuration. The metal‐metal distance is the only relevant structural parameter for the spin‐exchange coupling constants of a series of V, Cr, and Mn complexes.
New Intermediate Steps in the Synthesis of Larger Nickel Clusters
Authors: Fenske Dieter , Krautscheid Harald , Müller Michael
Published: December 2003

Phosphane complexes of NiCl2 react with Se(SiMe3)2 affording mixtures of cluster complexes whose structures are dependent on the phosphane ligands. Through the variation of the ligands, complexes 1–6 were isolated and structurally characterized (Phe = C2H4Ph). Cluster 2 incorporates a spirocyclic Ni5 framework, 6 a central Ni13 cuboctahedron.
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Dynamic Forcing, a Method for Evaluating Activity and Selectivity Profiles of RGD (Arg‐Gly‐Asp) Peptides
Authors: Müller Gerhard , Gurrath Marion , Kessler Horst , Timpl Rupert
Published: December 2003

The easy interconversion between two conformations, which is deduced from molecular dynamics simulations, explains why the cyclopeptide 1 is equally well bound to two receptors whereas the very similar cyclopeptide 2 shows a selectivity towards these receptors of ca. 30:1. The method of “dynamic forcing” showed that 1 can very easily be transformed from its most stable conformation into a conformation which corresponds to that of 2. Thus, for the first time a connection between the biological activity of a compound and its conformational freedom has been established.
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Selective Inhibition of Trypanosomal Triosephosphate Isomerase by a Thiopeptide
Authors: Kessler Horst , Matter Hans , Geyer Armin , Diehl Hans‐Jürgen , Köck Matthias , Kurz Guido , Opperdoes Fred. R. , Callens Mia , Wierenga Rik K.
Published: December 2003

One approach to conquering sleeping sickness is the selective inhibition of trypanosomal triosephosphate isomerase, a key enzyme in the pathogen's metabolism, with cyclic hexapeptides. The thionylation of the Phe5‐Phe6 amide bond of the cyclic hexapeptide cyclo(Gly1‐Pro2‐Phe3‐Val4‐Phe5‐Phe6) is a minor modification, but effects a total rearrangement of the intramolecular hydrogen bonds which causes a drastic improvement of inhibitoric activity— maximum effect from minimum chemistry!
Ab Initio Investigations of Structure and Stability of [R3SiAl]4, R = H, Me, tBu
Authors: Schneider Uwe , Ahlrichs Reinhart , Horn Hans , Schäfer Ansgar
Published: December 2003

The second largest molecule so far dealt with by ab initio methods is the tetrameric Al1 compound [tBu3SiAl]4. The calculations yielded structure parameters that are in agreement with those of related compounds, and showed that, above all, the lack of π backbonding in the monomer explains why the tetramer is so stable that it can be sublimed at 180°C without decomposition. In contrast to H3SiAl and Me3SiAl, for tBu3SiAl the SiAl distance is only slightly shortened on tetramerization, which could indicate slight repulsion between the tBu3Si groups in the tetramer. As a result of the nonpolar surface of the tetramer, a high degree of disorder in the crystal is to be expected—a possible explanation for the difficulties encountered in the structure determination.
Multiple Peptide Synthesis Methods and Their Applications. New Synthetic Methods (87)
Authors: Jung Günther , Beck‐Sickinger Annette G.
Published: December 2003

Rapid developments in the biotechnology of new proteins, as well as advances in immunology and the development of pharmaceuticals based on inhibitors and antagonists, have led to immense demands for synthetic peptides. Simultaneous preparation of 100–150 completely different peptides, having chain lengths of up to 20 amino acids can nowadays be achieved using multiple synthesis methods. The yields and qualities of the peptides so obtained are high enough to permit reliable in vivo and in vitro screening for biological activities. Moreover, it is possible to optimize synthetic conditions and to carry out comparative studies on the secondary structures and conformational mapping of proteins. Special multiple synthesis methods facilitate the epitope mapping of larger peptides for diagnostic purposes and for the development of vaccines based on a few hundreds of free or rod‐bound peptides that are useful for immunoassays. Multiple methods of peptide synthesis also enable the preparation of so‐called peptide libraries which could comprise hundreds of thousands of peptides, and by which new perspectives for the screening of lead structures will be opened up. Peptide synthesis using a combination of photolabile protecting groups and photolithographic procedures enables the assembling of peptide libraries on small plates for use in miniature immunoassays. Furthermore, lipopeptide‐antigen conjugates allow both the preparation of peptide‐specific and monoclonal antibodies as well as a complete screening of epitopes of B‐, T‐helper and T‐killer cells. Applications in the areas of AIDS diagnosis, the development of vaccines, and screening for the hormone analogues, demonstrate just some of the possibilities that have been opened up by multiple peptide synthesis methods.
On the Influence of Heteroatoms in α‐ and β‐Functionalized Alkyl Transition‐Metal Compounds
Authors: Steinborn Dirk
Published: December 2003

The first homoleptic trimethylsilylmethyl transition‐metal complexes were synthesized about 20 years ago in the research groups of Wilkinson and Lappert, resulting in a significant advance in the understanding of the thermodynamic and kinetic stability of transition‐metal‐carbon σ bonds. The relatively high thermal stability of these bonds provides a dramatic contrast to the Köbrich lithium carbenoids XCH2Li, known for about 30 years and famous for their extraordinarily high reactivity. Both compound classes illustrate that introduction of a heteroatom into an alkyl ligand can have a profound influence on the structure, stability, and reactivity of an alkylmetal compound, opening the possibility of entirely new reactions. Because of their simple and unambiguous structure, α‐ and β‐heteroatom‐substituted transitionmetal compounds containing the unit [MCH2Y] or [MCH2CH2–Y] (where Y is an element from the 4th–7th main groups or groups 14–17, not including C) are particularly suitable for studying effects of the nature of a heteroatom center (neutral/cationic, coordinatively saturated/Lewis basic) on structure, stability, and reactivity as a function of the type of complex, the nature of M, and the number of methylene groups separating M and Y. The diversity of stabilizing and destabilizing influences emanating from the heteroatom center generally produces a broad spectrum of reactivity and stability. This is reflected in an impressive way in homogeneous catalysis, where functionalized alkylmetal compounds arise as intermediates, as well as in the electronic influences of functionalized alkyl ligands, which are determined largely by the nature of the heteroelement present.
[Ni(PtBu)6]—a Strain on the 18 Electron Rule?
Authors: Dehnicke Kurt
Published: December 2003

On first glance a nickel complex with 22 valence electrons is not in keeping with either the well‐known 18‐electron rule or the electron‐counting rules established for cluster molecules by Wade and Mingos. Such a complex is expected to be very unstable. However, [Ni(PtBu)6] prepared by D. Fenske (Angew. Chem. Int. Ed. Engl. 1992, 31, 323) is stable at room temperature, diamagnetic, and decomposes only slowly in solution. This highlight tries to show how these facts can be interpreted according to the 18‐electron rule.
Spectral Hole Burning at Room Temperature and with a Single Molecule: Two New Perspectives
Authors: Bräuchle Christoph
Published: December 2003

En route to real molecular electronics the “spectral hole burners” among physical chemists have come a long way. Two developments which foster these ambitions are discussed herein: 1. For the first time holes with sufficient thermal stability could be burned in experiments at room temperature by using the 4f electron transitions of Sm in substitutionally disordered microcrystals of SrFCl0.5Br0.5 or Mg0.5Sr0.5FCl0.5 doped with Sm2+. 2. The manipulation of single molecules necessary for real molecular electronics was possible on a perylene‐doped polyethylene film. At T = 1.5 K, spectral hole burning could be carried out selectively several times in succession with a single perylene molecule.
An Opinion on the Heterogeneous Photoreactions of N2 with H2O
Authors: Edwards Jimmie G. , Davies Julian A. , Boucher David L. , Mennad Abdelkader
Published: December 2003

The artificial photosynthesis of ammonia, that is, the fixation of N2 and conversion to NH3, is a fascinating research goal about which there are repeated reports of success in the literature. This communication now shows amongst other results why the obvious methods, in particular the heterogeneous catalysis of the reaction of N2 and H2O on the surface of oxidic compounds, are destined to failure.
Transport and Fate of Organic Compounds in the Global Environment
Authors: Ballschmiter Karlheinz
Published: December 2003

The role of chemistry in our soon‐to‐be global industrial society requires a global perspective for the discussion of the uptake, transport, and conversion of chemical compounds in the environment. The fate of organic compounds in the volume flow of the atmosphere and hydrosphere can be categorized into transport pathways and adjustments of equilibria in the multiphase system atmosphere‐oceans‐land surface. The global volume flow in the atmosphere (wind, areas of high and low pressure) and in the hydrosphere (rivers, circulation of water in lakes, ocean currents) alone would account for the transport of organic compounds if they were stable and if all these compounds were molecularly distributed in these phases. However, this particular case is observed only for volatile chlorofluorocarbons and tetrachloromethane. For most organic compounds, complex adsorption/desorption processes on finely dispersed particle phases in the air and water as well as abiotic and biotic transformations determine the transport behavior.

The global fate of a compound includes its accumulation as well as its decomposition in defined environmental compartments. Both phenomenon may have long‐term consequences in the af‐fected areas; this is indicated by the accumulation of polychlorobiphenyls and analogous compounds in marine mammals and deep‐sea fauna, and is evident in the decomposition of chlorofluorocarbons in the stratosphere. The accepted definition of the level of a risk, the product of the probability of an event and the potential amount of damage, takes on a new dimension when viewed from a global perspective‐especially when one further connects the extent of the damage to the possible means of correction. It is the responsibility of scientists to point out the irreparable regional to global consequences directly and indirectly connected to the production of chemicals and to provide suggestions for prevention. Politicians can only react by applying their methods to limit the damage; however, they cannot generate any solutions based on science.