bluuebunny/crossref_metadata_2025_split huggingface dataset:
Determination of the Wiener molecular branching index for the general tree
Authors: Canfield E. R. , Robinson R. W. , Rouvray D. H. 
Published: December 2004

AbstractThe many applications of the distance matrix, D(G), and the Wiener branching index, W(G), in chemistry are briefly outlined. W(G) is defined as one half the sum of all the entries in D(G). A recursion formula is developed enabling W(G) to be evaluated for any molecule whose graph G exists in the form of a tree. This formula, which represents the first general recursion formula for trees of any kind, is valid irrespective of the valence of the vertices of G or of the degree of branching in G. Several closed expressions giving W(G) for special classes of tree molecules are derived from the general formula. One illustrative worked example is also presented. Finally, it is shown how the presence of an arbitrary number of heteroatoms in tree‐like molecules can readily be accommodated within our general formula by appropriately weighting the vertices and edges of G.
Structural features of benzylic carbanions. A theoretical study
Authors: Lipkowitz Kenny B. , Uhegbu Christopher , Naylor Adel M. , Vance Robert 
Published: December 2004

AbstractBenzylic anions were studied with the semiempirical MNDO molecular orbital method. Structural changes were analyzed as metal counterions and solvents were allowed to interact with the benzylic carbon. Generally, benzylic carbanions were found to be trigonal planar but became pyramidal or bent when metals were included in the calculation.
Interaction energy studies of pyrrolopyrimidine nucleoside antibiotics: Tubercidin
Authors: Sanyal Nitish K. , Ojha Rajendra Prasad , Roychoudhury M. 
Published: December 2004

AbstractOn the basis of intermolecular interaction energy studies, the possibility of incorporation of a pyrrolopyrimidine nucleoside analog has been examined. Both stacking as well as in‐plane (hydrogen bonding) interactions have been considered for computing the association energy of analogous base (tubercidin) with nucleic acid bases and base pairs. The molecular charge distribution has been computed by CNDO/2 method while the interaction energy has been computed using second‐order perturbation theory. The energy values and the sites of association of the tubercidin base in the complex formation, obtained from the minimum energy configurations, have been compared with the corresponding energy values as well as the sites of association of nucleic acid bases in the transcription process. The theoretical results thus obtained have been discussed in the light of a model developed earlier, and an attempt has been made to correlate the results with the observed biological activity of tubercidin.
Symmetry properties of chemical graphs. IX. The valence tautomerism in the P73− skeleton
Authors: Randić Milan , Oakland David O. , Klein Douglas J. 
Published: December 2004

AbstractWe investigate the intramolecular rearrangement of the P73− ion in which PP bonds are continuously broken and reestablished resulting in a dynamic structure with all seven phosphorus atoms being chemically equivalent. This leads to 7/3 = 1680 valence tautomers. The P73− ion is a typical fluctuating molecule, distantly related to C10H10 bullvalene. In order to determine the symmetry of the process we first consider the problem of the construction of the rearrangement graph. Initial steps toward this goal are illustrated, and additional characterizations of the graph are derived after its construction by computer. A number of properties of this rather complicated graph having 1680 vertices and 2520 edges are discussed: in particular the occurrence of various cycles, the maximal distance between vertices, the count of neighbors at different distance, and finally its symmetry.
Mindo/3 and mndo calculations of closed‐ and open‐shell cations containing C, H, N, and O
Authors: Halim Herman , Heinrich Nikolaus , Koch Wolfram , Schmidt Jochen , Frenking Gernot 
Published: December 2004

AbstractHeats of formation of 119 closed‐ and open‐shell carbocations calculated by the semiempirical quantum chemical methods MINDO/3 and MNDO are reported and compared with experimental data. With proper consideration of failures in specific areas, both methods can be used for the thermodynamics of carbocations containing C, H, N, and O. MINDO/3 predicts unrealistic values for nitrogen containing cations with nitrogen multiple bonds and is not suited for closed‐shell cations containing oxygen. Saturated acyclic hydrocarbon radical cations often are computed with abnormally long CC bonds by MNDO. Otherwise, the standard deviation of the two methods is not very different, being in the range of ±13 kcal/mol. MINDO/3 tends to overestimate the cation stabilities, whereas MNDO calculates cations usually too high in energy. Some of the errors which were found in the calculations of the ions are related to the computed values for the parent neutral structures, but others are not.
A method for the machine detection of near equivalence of major substructures in a molecule
Authors: Bersohn Malcolm , Fujiwara Shizuo , Fujiwara Yuzuru 
Published: December 2004

AbstractThe use of the concept of near equivalence of substructures in a computer program concerned with organic synthesis requires a concrete definition of «nearness» and an efficient method for implementing the concept. Such a method has been devised and its use is described and examples are given.
Molecular mechanics and molecular shape. III. Surface area and cross‐sectional areas of organic molecules
Authors: Meyer A. Y. 
Published: December 2004

AbstractA nonanalytical procedure is proposed to calculate the surface area and cross‐sectional areas of molecules. Results are reviewed for about 70 species comprising alkanes, acyclic and cyclic, alkyl halides, and alcohols. By regression analysis, relationships are detected between surface areas or segment areas and physical properties. Computed cross‐sectional areas are confronted with adsorption values. Four ways are proposed to define quantitatively the intuitive notion of «molecular globularity».
Comparison of sequences as a method for evaluation of the molecular similarity
Authors: Jerman‐Blaẑiĉ B. , Fabiĉ I. , Randić M. 
Published: December 2004

AbstractString comparison techniques were developed and applied for measuring the molecular similarity of chemical structures. The molecular structures were encoded as a sequence of numbers representing counts of paths of different lengths. The similarity index between two compounds was calculated as the difference between the gains of information derived through comparison of the corresponding molecular path sequences. Ranks between the structures of the studied data base obtained according to this similarity were used as basic data for deriving correspondences between the elements of the set of compounds. The method was applied on a group of 41 barbiturates. Correlation equations were calculated for different groups of compounds grouped according to the displayed similarity. The correlation equations and the corresponding statistics were obtained using standard computer programs. Special algorithm for computing the similarity index and the correlation matrix (outlined very briefly) was developed and implemented on VAX 11/750.
Vectorization of a classical trajectory code on a floating point systems, Inc. Model 164 attached processor
Authors: Kraus Wayne A. , Wagner Albert F. 
Published: December 2004

AbstractA triatomic classical trajectory code has been modified by extensive vectorization of the algorithms to achieve much improved performance on an FPS 164 attached processor. Extensive timings on both the FPS 164 and a VAX 11/780 with floating point accelerator are presented as a function of the number of trajectories simultaneously run. The timing tests involve a potential energy surface of the LEPS variety and trajectories with 1000 time steps. The results indicate that vectorization results in timing improvements on both the VAX and the FPS. For larger numbers of trajectories run simultaneously, up to a factor of 25 improvement in speed occurs between VAX and FPS vectorized code.
Rate constants for the bimolecular self‐reaction of tertbutyl radicals in nalkane solvents
Authors: Schuh H. , Fischer H. 
Published: December 2004

AbstractRate constants for the self‐reaction of tertbutyl radicals in six nalkanes are determined as functions of temperature by kinetic electron‐spin resonance spectroscopy and from rates of product formation. They are well described by a Smoluchowski–Stokes–Einstein treatment including effects of microfriction and spin statistical factors, indicating complete diffusion control. The ratio of disproportionation to combination products is found to depend on temperature and solvent. This is tentatively ascribed to reorientational effects in the encounter pairs.
Rate constants for the reaction of O(3P) atoms with CH2 = CHF, CH2 = CHCl, and CH2 = CHBr at 298 ± 2°K
Authors: Atkinson R. , Pitts J. N. 
Published: December 2004

AbstractAbsolute rate constants for the reaction of O(3P) atoms with CH2 = CHF, CH2 = CHCl, and CH2 = CHBr have been obtained at 298 ± 2°K using a modulation phase shift technique. The rate constants (k2 × 10−8 l./mole · sec) obtained are: CH2 = CHF (1.61 ± 0.20), CH2 = CHCl (2.54 ± 0.26), and CH2 = CHBr (2.45 ± 0.25). These rate constants are lower than those determined by discharge flow techniques, but that for CH2 = CHF is in good agreement with relative rate measurements.
Deuterium isotope cage effects in the dismutation of alkoxyl radicals. Significance for the mode of alkylperoxyl termination
Authors: Mendenhall G. D. , Quinga E. M. Y. 
Published: December 2004

AbstractDecomposition of (R1R2CXON=)2 (R1 = R2 = Ph; R1 = R2 = pClC6H4; R1 = Ph, R2 = CH3; X = H or D) in hydrocarbon solvents at 31–75°C afforded R1R2CO in 6–8% lower yield when X = D than when X = H. This result is ascribed to cage escape of secondary alkoxyl radicals in competition with dismutation. Deuterium isotope effects for alkoxyl cage dismutation of 1.35–2.4 are estimated from the data. It is suggested that this same competition may account for the experimentally slower termination rate constants in solution of primary and secondary alkylperoxyl radicals on α‐deuteration.
Über den Einfluß der Wärmebehandlung auf die Korrosionsbeständigkeit der Magnesium‐Aluminiumlegierung Mg‐Al 7
Authors: Bulian W. 
Published: December 2004

AbstractEs wurde der Einfluß der Wärmebehandlung, und zwar sowohl einer Homogenisierungsglühung wie auch einer Warmaushärtung auf die Korrosionseigenschaften der Mg‐Knetlegierung Mg‐Al 7 untersucht. Die Korrosionsversuche wurden mit der Eudiometerprobe angestellt, geprüft wurde mit 0,6 %iger wäßriger NaCl‐Lösung.
Schmierung und Oberflächenbehandlung mit reinstem Molybdändisulfid
Authors: Weiss F. 
Published: December 2004

AbstractReinheits‐ und Zerteilungsgrad als wesentliche Kriterien der Wirksamkeit bei extremen Beanspruchungen—Schmierwirkung bedingt durch den lamellaren Aufbau der gitterförmigen kristalle — Hohe chemische und thermische Stabilität — Herstellung festhaftender Molybdänsulfid‐Überzüge durch Verwendung verschiedener Träger.Es wurden Beispiele aus der Praxis über die Anwendung der MOLYKOTE‐Produkte gegeben und dabei die Art der Aufbringung des Schmierfilms auf die verschiedensten Bauelemente beschrieben. — Je intensiver solche Schmierfilme aufgebracht werden können, desto wirksamer sind sie. — In vielen Fällen hat ein Einarbeiten von MOLYKOTE‐Pulver in die Werkstoffe, z. B. Sintermetalle, Kunststoffe usw. eine Herabsetzung des Reibungskoeffizienten und Verminderung des Verschleißes herbeigeführt. — Die eigentlichen Einsatzgebiete dieser neuen Schmierstoffe liegen vor allen auf dem Gebiete der Grenz‐ bzw. Teilschmierung dort, wo unter extremen Druck‐ und Temperaturbedingungen die üblichen Schmiermittel auf Öl‐ und Fettbasis naturbedingt nicht voll befriedigen können.
Über die Oxydations‐ und Korrosionsbeständigkeit von Zirkonium
Authors: Köhler Walther 
Published: December 2004

AbstractZirkonium, das als Oxydationsmittel, Legierungselement und Röhrenbaustoff zunehmende Verwendung findet, zeichnet sich, ähnlich wie Titan, durch hohe Oxydations‐ und Korrosionsbeständigkeit aus. Die Arbeit gibt an Hand des neueren amerikanischen Schrifttums einen zeitgemäßen Überblick über die wichtigsten physikalischen Eigenschaften von Zirkonium, insbesondere über den Einfluß von Verformung und Glühbehandlung im Verlaufe der Halbzeugfertigung sowie über die thermische und chemische Beständigkeit des Zirkoniums gegen etwa 45 verschiedene angreifende Mittel.
Materialeinsparung durch aktiven Verschleißschutz
Authors: Wahl W. 
Published: December 2004

AbstractDurch Abnutzung entstehen in den verschiedensten Bereichen der Technik erhebliche Materialverluste hauptsächlich bei Metall:Metall‐ und Metall:Mineral‐Berührung. Der dadurch entstehende beträchtliche Materialverlust nimmt zu durch die Erhöhung des spezifischen Durchsatzes eingesetzter Maschinen, des Arbeitens im Verbundsystem, höherer Rohstoffausbeutung, Einführung moderner Technologien bei Rohstoffgewinnung und Recycling und dem zunehmenden Wunsch, die durch solche Schäden entstehenden Folgekosten zu vermindern. Dabei ist nicht echte Rohstoffknappheit maßgebend sondern vielmehr die in Zukunft sich verändernde Relation der Steigerungsraten von Rohstoffkosten und Lohnkosten. Materialeinsparungen sind durch Anwendung der heute bekannten Grundlagen auf dem Verschleißgebiet möglich. Im einzelnen umfassen sie die Maßnahmen:– Einsparen von Material bei Neuteilen,– Einsetzen höherwertiger Werkstoffe,– Einsetzen von Verbundlösungen und– Regenerieren abgenutzter Teile.Zahlreiche praktische Beispiele aus dem Bereich des Metall:Mineral‐ Verschleißes werden erläutert.Durch diese Maßnahmen hat der Ingenieur die Möglichkeit, Rohstoffe einzusparen.
Bedeutung der Unregelmäßigkeit Gauß'scher Zufallsfolgen für die Betriebsfestigkeit
Authors: Gassner E. , Lowak H. 
Published: December 2004

AbstractDie zum Teil widersprüchlichen Angaben im Schrifttum über den Einfluß des Unregelmäßigkeitsfaktors sind vermutlich darauf zurückzuführen, daß bei einschlägigen Untersuchungen neben dem Unregelmäßigkeitsfaktor auch noch andere Parameter unbeabsichtigt variiert wurden. Um zu einer Klärung beizutragen, wurden unter Zufallsfolgen von Axiallasten mit Merkmalen eines stationären Gaußprozesses Betriebsfestigkeits‐ Versuche an einem Kerbstab und an einer einschnittigen Fügung aus der Legierung AlCuMg2 durchgeführt und der Einfluß der Unregelmäßigkeit auf die Lebensdauer bestimmt. Die untersuchten Unregelmäßigkeitsfaktoren überdecken mit l = 0,99,0,7 und 0,3 den für die Betriebsfestigkeit interessanten Anwendungsbereich. Das gewählte Verfahren zur Generierung der Zufallsfolgen gewährleistet, daß der Unregelmäßigkeitsfaktor die einzige Variable unter den üblichen Parametern der Zufallsfolge bleibt.
Keramische Werkstoffe für Brennkammern von Fahrzeuggasturbinen
Authors: Grunke R. 
Published: December 2004

AbstractDamit Fahrzeuggasturbinen im spezifischen Verbrauch mit den heute verwendeten Dieselmotoren konkurrieren können, sind hohe Temperaturen im Brennkammerbereich erforderlich; diese können mit Metallbrennkammern nicht realisiert werden. Als Brennkammerwerkstoff bieten sich hochtemperaturbeständige Sonderkeramiken an. Bei MTU wurden die in Frage kommenden Keramiken unter betriebsnahen Bedingungen auf ihre Eignung als Brennkammerwerkstoff untersucht. Besonders bewährt hàt sich siliziuminfiltriertes Siliziumkarbid. Bei weiterer Entwicklung ist auch die Verwendung anderer Keramiken dankbar.Seit mehr als 40 Jahren wird versucht, anstelle von Hubkolbenmotoren Antriebsmaschinen mit ausschließlich rotierenden Teilen zu verwenden. Die vom Prinzip her einfache Arbeitsweise der Rotationsmaschinen war immer wieder ein Ansporn dazu, die damit verbundenen technologischen Schwierigkeiten in Angriff zu nehmen. So konnte in der Luftfahrt die Gasturbine mit ihrem günstigen Leistungsgewicht den Kolbenmotor in der Leistungsklasse über 500 PS völlig verdrängen.Anfang der 50er Jahre begann man in den USA und in Großbritannien mit der Entwicklung von Fahrzeuggasturbinen. Die Schwierigkeiten, die dabei auftraten, sind bis heute noch nicht befriedigend gelöst. Bedingt durch die kleinen Abmessungen und ein ungünstiges Teillastverhalten entstehen Wirkungsgradverluste, die nur durch hohe Verbrennungstemperaturen ausgeglichen werden können. Damit zum Beispiel eine LKW‐Gasturbine mit 350 PS ähnliche Verbrauchswerte wie ein Dieselmotor erreicht, sind Turbineneintrittstemperaturen von etwa 1400 °C erforderlich. Da aus Gründen der Kosten und des Wirkungsgrades aufwendige Kühlkonzepte, wie sie in Fluggasturbinen eingesetzt werden, nicht in Frage kommen, ist die Realisierung dieser hohen Temperatur mit Metallteilen nicht möglich. Bereits bei einer Brennkammereintrittstemperatur von 1000°C werden etwa 5% der angesaugten Luft zur Brennkammerkühlung verwendet, was den spezifischen Verbrauch um etwa 8 g/PS h erhöht (1).Diese Problematik sowie die beginnende Rohstoffverknappung und Rohstoffverteuerung führten zu der‐an sich nicht neuen‐Überlegung, die Heißteile einer Fahrzeugturbine aus Keramik zu bauen. Bereits während des zweiten Weltkrieges sowie in den 50er und 60er Jahren wurde versucht, Alumininumoxid für Turbinenschaufeln einzusetzen, was u. a. an der mangelnden Thermoschockbeständigkeit von Al2O3 scheiterte. Nun sind in den letzten Jahren, zunächst in USA und dann auch in Deutschland, die Bemühungen um den Einsatz von Keramik in Gasturbinen wieder verstärkt worden. Die Entwicklungsarbeiten sind z. Zt. in vollem Gange, wobei sich jetzt erste Tendenzen abzeichnen und auch einzelne Teilerfolge erzielt wurden (2).
The effect of design and operating conditions on melting in plasticating extruders
Authors: Tadmor Zehev , Klein Imrich 
Published: December 2004

AbstractA previously proposed but further modified theoretical model for melting in plasticating extruders, in the form of a computer program, which predicts the amount of unmelted polymer at any point in the extruder, was used to simulate the effect of geometrical and operating variables on the melting performance of the extruder. The results indicate in increasing screw length required to complete melting with increasing throughput and a decreasing length of melting with increasing frequency of screw rotation. They further indicate the existence of an optimum barrel temperature for a maximum rate of melting, an optimum number of threads for a maximum melting rate, and a significant decrease in the rate of melting with increasing flight clearance. The effect of other geometrical variables and of operating conditions on the rate of melting and power consumption are also discussed.
The simulation of the plasticating screw extrusion process with a computer programmed theoretical model
Authors: Klein Imrich , Tadmor Zehev 
Published: December 2004

AbstractA fundamental mathematical model of the plasticating extruder was assembled by combining a previously reported melting model with an improved melt pumping model. The validity of the model was tested with numerous experiments on 2 1/2 in. and 8 in. diameter extruders. The experiments were performed with low and high density polyethylene, plasticized polyvinyl chloride, rigid polyvinyl chloride powder, polypropylene, ABS and nylon 66. The mathematical model proved to be excellent in predicting pressure and plastic temperature profiles in the extruder channel, temperature fluctuation, solids content, and power consumption.
Resins and reinforced plastic laminates for continuous use at 650°f—II
Authors: Freeman J. H. , Frost L. W. , Bower G. M. , Traynor E. J. , Burgman H. A. , Ruffing C. R. 
Published: December 2004

AbstractThis paper is the third in a series describing work done under Air Force sponsorship to develop high temperature polyimide laminates for radomes and other parts for supersonic aircraft.Twelve resin compositions are described and evaluated. Most were prepared from benzophenonetetracarboxylic dianhydride and one or more aromatic diamines. Modifying linkages such as amide, ether, benzimidazole, and oxadiazole were present in some cases.A variety of precure and pressing conditions were studied, best results being obtained with a fully cured prepreg pressed at about 700°F and 200–1000 psi. Most of the laminates were made with E glass cloth, but some work was done also with S glass, Refrasil, and carbon cloth.Initial flexural strengths on E glass of 35–60,000 psi at room temperature, and 20–40,000 psi at 600°F were observed for the better resins. On S glass, slightly higher strengths were observed together with values of about 10,000 psi at 700–1000°F. Aging data on E glass laminates show retention of at least 10,000 psi flexural strength at temperature for about 250 hrs. at 650°F, 1000 hrs. at 600°F, 5000 hrs. at 527°F, and 30,000 hrs. at 482°F.Dielectric constant and dissipation factor at 10 Gc were found to be nearly constant at 3.5–4.4 and 0.005–0.01, respectively, for temperatures up to 662°F and for aging up to 1000 hrs. at 600°F. At frequencies of 60 c and 1 Kc, an increase of dissipation factor with temperature was found and was used as a measure of Tg.
The extrusion of polymers below their melting temperatures by the application of high pressures
Authors: Buckley A. , Long H. A. 
Published: December 2004

AbstractA number of polymers have been “cold” extruded (i.e. at temperatures well below their normal melting temperatures) by the application of relatively high pressures. In all cases extrudates of relatively large cross‐sectional area have been produced, and non‐circular cross‐sections have also been extruded. The extruded products has been evaluated and the results discussed. In general it has been found that whilst extrusion gives an oriented product, the increase in tensile modulus is much less than for a comparably drawn fibre. This phenomenon has been discussed in terms of the changes in crystalline structure produced by extrusion.
Effect of melt history on polymer crystallization
Authors: Collier J. R. , Neal L. M. 
Published: December 2004

AbstractAs discussed in this paper, melt treatment prior to crystallization, apparently proceeds by the following stages: partial melting; an insensitive region; and at higher temperatures, deactivation of nucleation sites. The deactivation‐region onset temperature is quite dependent on the subsequent crystallization conditions and may not be observed in some polymers during crystallization from the melt. In addition, those polymers capable of being quenched directly from the melt to a non‐crystalline glassy state and subsequently crystallized by reheating to above the glass transition, do not exhibit any more than a partial melting type melt treatment effect. The deactivation regime absence is a result of the homogeneous nucleation that occurs during crystallization from the quenched glassy state at temperatures slightly above the glass transition.Use of a metal‐ or glass‐constraining medium does mask (at least partially) the effect of melt history upon crystallization from the melt. In addition to the masking effect of a constraining medium, some of the controversy in the literature pertaining to the existence of a melt treatment phenomenon may arise due to degredation of some polymers prior to the onset of the deactivation regime. The crystallization conditions employed are also quite influential on the possible effect melt treatment can have since the melt history phenomenon is noted by its effect on subsequent crystallization.