bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geometry optimization using symmetry coordinates
Authors: Kondo H. , Jaffé H. H. , Lee H. Y. , Welsh William J. 
Published: December 2004

AbstractThe use of symmetry coordinates (SC) in geometry optimization is discussed. A computer program incorporating the use of sc, together with analytical calculation of the gradient and quadratic acceleration, is described. Also reported are careful test results on a series of small molecules and typical results with a long series of molecules up to quite large size (40–60 atoms).
Determination of reactivity by MO theory. 27. Molecular orbital study of the gas‐phase decarboxylation of but‐3‐enoic acid
Authors: Lee Ikchoon , Cho Jeoung Ki , Lee Bon‐Su 
Published: December 2004

AbstractThe MINDO/3 calculations were performed on the potential energy profile involved in the equilibrium

Optimized structures of stable molecules and transition states have been determined; thermodynamic stabilities of pure acids and barriers indicated that the equilibrium can be set up from any acids. It was argued that direct decarboxylation is only conceivable from (I), since in this process a 1, 5‐hydrogen shift is involved, whereas a higher barrier process of 1, 3‐hydrogen shift is required in direct decarboxylations from other acids. Direct interconversion of (I) and (III) was found to be unfavorable due to a high barrier involved.
A directing effect of n–π* transitions on electronic charges
Authors: Tewari R. D. , Mishra P. C. 
Published: December 2004

AbstractChanges of electronic charge distribution following the lowest singlet n–π* transitions of benzaldehyde, benzamide, benzoic acid, the phthalaldehydes, pyridine, and the diazines have been examined using the CNDO‐S/CI method. A Singlet n–π* transition is found as a general rule to direct electronic charges to the para position of the atom in the ring that carries the lone pair or to which the substituent carrying the lone pair is attached.
Remarks on the analysis of torsional energy surfaces of cycloheptane and cyclooctane by molecular mechanics
Authors: Ivanov Petko M. , Ōsawa Eiji 
Published: December 2004

AbstractA modified version (MM2′) of the Allinger's 1977 force field is checked against cycloheptane and cyclooctane. Cycloheptane is characterized by two pseudorotating itineraries, chair/twist‐chair and boat/twist‐boat, separated by a barrier of 8.5 kcal mol−1. The activation energy in the C/TC pseudorotation is estimated to be 0.96 kcal mol−1, while B and TB transform into each other freely at an energy level 3.8 kcal mol−1 above the global energy minimum (TC). With cyclooctane the lowest energy is calculated for the boat‐chair form which participates in a pseudorotational process with TBC through a saddle point lying 3.5 kcal mol−1 above BC. The chair/chair and boat/boat families contain only one local minimum, crown and BB, respectively, on the MM2′ surface. The results are presented as an illustration for quick coverage of torsional energy surface by two‐bond driver calculation with the block‐diagonal Newton–Raphson minimization, followed by the force search of stationary points by full‐matrix Newton–Raphson optimization.
The Lagrange multiplier method for manipulating geometries. Implementation and applications in molecular mechanics
Authors: van de Graaf B. , Baas J. M. A. 
Published: December 2004

AbstractIt is shown that a Lagrange multiplier method to constrain one or several internal coordinates, or averages and combinations of these, is easily implemented in a molecular mechanics computer program that uses Newton–Raphson (NR) minimization. Results are given for constraints on nonbonded distances and torsion angles. When a potential energy surface is to be explored, it is much better to constrain the average of three torsion angles around a bond than to constrain a single torsion angle. Certain conversions can only be achieved when averages of torsion angles around different bonds are constrained. Combinations of constraints have been applied to evaluate differences between calculated and observed geometries and to obtain transition states for relatively large molecules from results for smaller molecules at relatively low costs. The efficieny of the combination of the Lagrange multiplier method and NR minimization in terms of computing time can be rated as good.
A molecular mechanics treatment of the anomeric effect
Authors: Nørskov‐Lauritsen Leif , Allinger Norman L. 
Published: December 2004

AbstractThe tendency of CO bond lengths to change as a function of the torsional angles at an acetal carbon has been included in a new version of the molecular mechanics program MM2(82), based on the observed behavior of molecules of this class as indicated by ab initio calculations and experimental structural data. The experimental geometries and energies are reasonably well reproduced.
Optimized monopole expansions for the representation of the electrostatic properties of the nucleic acids
Authors: Lavery Richard , Zakrzewska Krystyna , Pullman Alberte 
Published: December 2004

AbstractOptimized monopole expansions for the subunits of the nucleic acids are developed by a reparametrizatio of the Hückel–Del Re procedure designed to reproduce closely the electrostatic properties obtained with precise overlap multipole expansions. It is shown that satisfactory values of both potential and field may be obtained for different DNA conformations and for a transfer RNA. The charge redistribution occuring between the subunits upon forming the nucleic acids is also investigated by ab initio calculations and accounted for in developing the new parametrization.
Theoretical study of the gas‐phase reaction of hydrogen isocyanate with water
Authors: Lee Ikchoon , Lee Bon‐Su , Choi Jae Young 
Published: December 2004

AbstractTransition states for the two probable pathways for the gas‐phase hydrolysis of hydrogen isocyanate have been determined using the MINDO/3 method. Activation barriers obtained showed that the one‐step mechanism is preferred to the two‐step mechanism involving an intermediate. It was shown that the reaction of polymeric water has lower activation barrier than the reaction of monomeric water. Energy and charge decomposition analysis showed that in the former less energy is required in the deformation of molecules for the transition state (TS) formation due to the cyclic flow of electronic charges in the TS.
Charge calculations in molecular mechanics. III: Amino acids and peptides
Authors: Abraham Raymond J. , Hudson Brian 
Published: December 2004

AbstractAtomic charges obtained with a previously published charge scheme are given for amino acids and peptides. In order to do this, a method of handling charged species with the basic scheme2,3 has been developed. The charges obtained for alkylammonium ions and carboxylate ions with the scheme are presented and compared with CNDO and ab initio values. The calculated experimental dipole moments of the zwitterionic forms of glycine, alanine and β‐alanine are then discussed. Finally, the atomic charges obtained for the naturally occurring amino acids are given, both in the form of the N‐acetyl‐N′‐methyl amino acid amides, used as models for the amino acid residues in enzymes, and as the free zwitterionic amino acids. The charges obtained show a good correlation with n. m. r. chemical shifts of both carbon and hydrogen atoms.
Ab initio investigation of the potential energy surfaces of C2H2F2 and C2H2F2+
Authors: Frenking Gernot , Koch Wolfram , Schaale Michael 
Published: December 2004

AbstractAb initio MO calculations have been performed for neutral and cationic C2H2F2 structures. Olefinic and carbene structures are investigated for the neutral isomers, while olefinic, carbene, and fluoronium‐type cations are found. Stability orders and rotational barriers are discussed in terms of orbital and Coulomb interaction. Contrary to previous studies, the higher stability of the geminal isomers is interpreted to be caused by Coulomb attraction.
Computer program for finding all possible cycles in graphs
Authors: Balaban Alexandru T. , Filip Petru , Balaban Teodor‐Silviu 
Published: December 2004

AbstractA new approach is presented for identifying all possible cycles in graphs. Input data are the total numbers of vertices and edges, as well as the vertex adjacencies using arbitrary vertex numbering. A homeomorphically reduced graph (HRG) is constructed by ignoring vertices of degree less than three. The algorithm is based on successive generation of possible edge‐combinations in the HRG. If a combination yields a cycle, it is either printed or stored and then finally printed in a list of all possible cycles arranged in the order of increasing ring size. A unique numbering of the cycle is used. The computer program is listed and exemplified. Computing times are given.
Computer generation of king and color polynomials of graphs and lattices and their applications to statistical mechanics
Authors: Balasubramanian K. , Ramaraj R. 
Published: December 2004

AbstractA computer program is developed in Pascal for the generation of king and color polynomials of graphs. The king polynomial was defined by Motoyama and Hosoya and was shown to be useful in dimer statistics, enumeration of Kekulé structures, etc. We show that the king polynomial of a lattice is the same as the color polynomial of the associated dualist graph, where the color polynomial is defined here as the number of ways of coloring the vertices of a graph with one type of color (say, green) such that two adjacent vertices are not colored with the same color. Applications of these polynomials to exact finite method of lattice statistics are outlined.
Solvent‐Effect investigations through the use of an extended born equation
Authors: Gersten Joel I. , Sapse Anne Marie 
Published: December 2004

AbstractA corrected Born equation, which describes the solute‐dipolar solvent interaction by placing the charges of the atoms of the solute at their position in space inside a spherical cavity of the solvent, is used to calculate the solvent effect on three systems: the hydrated OH− ion, the lithiated aziridine, and the lithiated oxirane. It is found that the solvent effect stabilizes the addition of a water molecule to the OH−(H2O)3 system with the additional water acting as a proton donor. It is also found that the lithiation energy of aziridine and oxirane which are found to be 47 and 43 kcal/mol in gas‐phase, respectively, decrease to 32 and 23 kcal/mol in water solution.
Interaction energy studies of pyrrolopyrimidine nucleoside antibiotics: Sangivamycin
Authors: Sanyal Nitish K. , Ojha Rajendra Prasad , Roychoudhury M. , Tiwari S. N. 
Published: December 2004

AbstractInteraction energy calculations have been employed to study the biological activity of sangivamycin. The interaction energy values and the site of association of the analog have been compared with those for the nucleic acid bases. A comparative estimation of the results with those of other pyrrolopyrimidine nucleosides (Tubercidin and Toyocamycin) has been made. These studies suggest that the activity of pyrrolopyrimidine analogs is of the following order
Toyocamycin > Tubercidin > Sangivamycin
which is in general agreement with the experimental results.
Abstraction by hydrogen atoms from ethylene, propylene, butene‐1, and cis‐ and trans‐butene‐2
Authors: Falconer W. E. , Sunder W. A. 
Published: December 2004

AbstractThe position of abstraction by H atoms from ethylene, propylene, butene‐1, and cis‐ and trans‐butene‐2 and the rates of abstraction relative to addition have been measured at 25°C. Only allylic abstraction was observed. From ethylene, abstraction relative to addition was ≤3×10−4. For propylene, butene‐1, cis‐butene‐2, and trans‐butene‐2, abstraction occurred on 0.2%, 1.6%, 1.5%, and 0.9% of the reactive encounters, if dis‐proprotionation‐combination ratios for allyl and alkyl radicals are similar to those for alkyl–alkyl pairs.
Radical reactions of highly polar molecules. Chlorination of halopropanes
Authors: Moore Leonard O. , Rectenwald Charles E. , Clark Jared W. 
Published: December 2004

AbstractIn polychloro‐2,2‐difluoropropanes, where a large inductive effect results in relatively low reactivity to chlorination, the presence of additional chlorine causes an increased reactivity of the hydrogens geminal to the chlorine and a reduced reactivity of the remote (third carbon) hydrogens. The ways in which resonance, inductive effect, and steric crowding interact in the rates of chlorination of polar molecules are discussed.
An absolute measurement of the rate constant for ethyl radical combination
Authors: Golden D. M. , Choo K. Y. , Perona, M. J. , Piszkiewicz L. W. 
Published: December 2004

AbstractAn absolute value of kr of ethyl radicals at 860 ± 17°K of 4.5 × 109 M−1·sec−1 was determined under VLPP conditions, where the value of kr/kr∞ should be about 1/2. Thus kr∞(M−1·sec−1) ∼ 1010 at 860°K. An error of as much as a factor of 2 in kr would be surprising, but possible.The value of 1010M−1·sec−1 seems to be a factor of from 2 to 5 too high to be compatible with extensive data on the reverse reaction and the accepted thermochemistry. Changes in the heat of formation and entropy of the ethyl radical can change the situation somewhat, but even these changes when applied to the work of Hiatt and Benson [3] indicate that ethyl combination should be ∼ 109.3 M−1·sec−1. More work is necessary if a better value is desired.
The kinetics of the anation reaction of aquopentaamminecobalt(III) by malonate in acidic aqueous solution
Authors: Joubert P. R. , Van Eldik R. 
Published: December 2004

AbstractThe Co(NH3)5OH23+ ion reacts with malonate to form Co(NH3)5O2CCH2CO2H2+ or Co(NH3)5O2CCH2CO2+, depending on the pH of the reaction solution. The kinetics of this anation reaction have been studied as a function of [H+] for the acidity range 1.5 ≤ pH ≤ 6.0 in the temperature range of 60 to 80°C, the [total malonate] ≤ 0.5 M, and the ionic strength 1.0M. The anation by malonic acid follows second‐order kinetics, the rate constant being 8.0 × 10−5 M−1·sec−1 at 70°C, and the anations by bimalonate (Q1, k1) and malonate ion (Q2, k2) are consistent with an Id mechanism. Typical values at 70°C for the ion pair formation constants are Q1 = 1.3, Q2 = 5.4M−1; and for the interchange rate constants k1 = 5.3 × 10−4; k2 = 7.3 × 10−4 sec−1. The activation parameters for the various rate constants are reported and the results discussed with reference to previously reported data for similar systems.
Anchimeric assistance in the gas phase dehydrohalogenation of 5‐chloro‐2‐methylpent‐2‐ene
Authors: Chuchani Gabriel , Martin Ignacio , Alonso Miguel E. , Jano Patricia 
Published: December 2004

Abstract5‐Chloro‐2‐methylpent‐2‐ene decomposes at temperatures of 370–420°C with initial pressures from 61to 158 torr to yield hydrogen chloride and a mixture of methylpentadiene isomers. In a static system, with seasoned vessel and propene inhibitor, the reaction is homogeneous, unimolecular, and of the first order. The rate coefficient is expressed by the following Arrhenius equation: log k (sec−1) = (13.43 ± 0.30) − (215.0 ± 3.7) kJ/mol/2.303RT. The result ofthe present work additionally supports the participation of the neighboringaliphatic olefinic double bond in the rate of HCl elimination of alkenyl chlorides in the gas phase. Moreover, it also confirms the three‐membered conformation as the most favored structure for anchimeric assistance.
On the Michaelis–Menten behavior of pyridinium chloro chromate oxidations: Oxidation of dimethyl, dipropyl, and diphenyl sulfide in chlorobenzene–nitrobenzene mixtures
Authors: Panigrahi G. P. , Mahapatro D. D. 
Published: December 2004

AbstractThe kinetics of oxidation of dimethyl, dipropyl, and diphenyl sulfides by pyridinium chloro chromate in chlorobenzene–nitrobenzene mixtures are reported. The rate data show Michaelis–Menten behavior. The oxidation process is catalyzed by the organic acids like dichloro and trichloro acetic acids. The rate‐determining step appears to be a unimolecular decomposition of a complex of the reactants.
Rate constants for the bimolecular self‐reaction of cyano‐substituted alkyl radicals in solution
Authors: Korth H. G. , Lommes P. , Sicking W. , Sustmann R. 
Published: December 2004

AbstractCyano‐substituted methyl radicals (cyanomethyl–hand 2‐cyano‐2‐propyl radicals) and syn‐and anti‐1‐cyano‐allyl radicals were generated, and their recombination kinetics in solution were investigated between −50 and +50°C by time‐resolved electron‐spin‐resonance spectroscopy. The comparison of the activation energies for recombination with the activation energies of the solution viscosities proves that the dimerizations of the radicals are diffusion controlled with rate constants on the order of 108–109M−1·s−1. In the case of cyanomethyl radicals an additional pseudo‐first‐order process, hydrogen abstraction, was detected and analyzed kinetically. Product analyses support the kinetic measurements.
Die Bedeutung der Anstrichschäden für den Korrosionsschutz
Authors: Würth K. 
Published: December 2004

AbstractIm Vorstehenden wurde ein Überblick über die verschiedenen Arten der Anstrichschäden und ihre Ursachen gegeben. Da sich sehr häufig mehrere Einflüsse auswirken, ist es bei auftretenden Schäden oft schwer oder gar nicht möglich festzustellen, ob die Ursache des Versagens beim Grundmetall oder beim Anstrichstoff, bei der Verarbeitung oder in der Bauweise zu suchen ist. Vielfach fehlt es auch noch an dem not‐wendigen Beobachtungsmaterial, weil die Häufigkeit der Schäden mit der geographischen und örtlichen Lage zusammenhängt. Es wäre daher sehr zu wünschen, daß alle Beobachtungen über Anstrichschäden sorgsam gesammelt und für die notwendigen Forschungen zur Verfügung gestellt werden. Besonders wertvoll sind photographische Aufnahmen, da hinsichtlich der Bezeichnungsweise fur die beobachteten Anstrichschäden keine Einheitlich besteht.
Über das passive und aktive Verhalten von Blei im Erdreich
Authors: Markovic T. , Dugi Z. 
Published: December 2004

AbstractDie Bedingungen nach denen die Korrosion von Blei im Boden in Abhängigkeit von dem Verhältniswert „Wasser zu Bodenluft”︁ verläft, werden abgeleitet und mathematisch formuliert. Im aktiven Zustand verläuft der Gewichtsverlust nach einer Exponential‐, im passiven Zustand nach einer linearen Funktion.Blei, zeigt in verschiedenen Böden ein passives Verhalten das durch HCO3‐Ionen in ein aktives übergehen kann. In einem einer Kohlensäureatmosphäre ausgesetztem Boden verhält sich Blei passiv.
Über den Mechanismus der Korrosion des Eisens in Erdreich
Authors: Markovic T. , Dugi Z. , Sribar B. 
Published: December 2004

AbstractDie Korrosion des Eisens im Erdreich wird in Abhängigkeit von dem Verhältniswert „Wasser zu Bodenluft”︁ erklärt. In dem stromliefernden Element: Pt/Boden + Wasser/Fe, zeigt Fe je nach dem Sättigungszustand des Bodens an Wasser ein aktives oder passives Verhalten. Die Korrosionsgeschwindigkeit des Eisens ist vom pH‐Wert des Bodens abhängig, wenn die Korrosion an der Eisenoberfläche bei Sauerstoffüberschuß stattfindet. — Laborversuche mit Weichstahl‐Versuchsproben ergaben, daß die Korrosion des Eisens in einem an Wasser ungesättigten Boden als eine Reaktion 1. Ordnung, in einem mit Wasser gesättigten Boden dagegen nach dem Diffusionsgesetz abläuft.
Gezieltes Beizen von Apparaten aus chemisch beständigen Stählen
Authors: Risch K. , Althen W. 
Published: December 2004

AbstractAn Schadensfällen und durch Korrosionsversuche wird gezeigt, daß die chemische Beständigkeit der austenitischen Stähle in mehrfacher Weise von ihrem Oberflächenzustand abhängt. Beim Entfernen von Oxidbelägen, als Voraussetzung für eine gute Passivschicht, liefert das Beizen das beste Ergebnis. Oberflächennahe Zug‐Eigenspannungen, die durch eine mechanische Bearbeitung, z. B. Drehen, Bohren, Schleifen, eingebracht werden, können höhere Werte annehmen als die durch Verformen und Schweißen verursachten Spannungen. Als Folge davon tritt oft nur im Bereich einer Oberflächenbearbeitung Spannungsrißkorrosion auf. Weil die zugehörigen Spannungen unter der Oberfläche schnell abfallen, ist es meist möglich, sie durch ein abtragendes Beizen so weit abzubauen, daß Schäden dieser Art unterbleiben. Der notwendige Beizabtrag ist durch Versuche zu ermitteln und das Beizen entsprechend zu überwachen.
Structure‐mechanical property relationships in ethylene methacrylic acid copolymers and their salts
Authors: MacKnight W. J. , Kajiyama T. , McKenna L. 
Published: December 2004

AbstractA dynamic mechanical study has been made of an ethylenemethacrylic acid copolymer containing 4.1 mole per cent of methacrylic acid units and its sodium, lithium and calcium salts. On the basis of the results and related physical chemical studies it is proposed that the structures of the ionized copolymers consist of three distinct phases—a crystalline polyethylene phase, an amorphous polyethylene phase, and an ionic phase consisting of ionic domains. It is further proposed that the unionized acid copolymer consists of two phases—a crystalline polyethylene phase and an amorphous phase consisting of polyethylene crosslined with hydrogen bonded, carboxylic acid dimers.
Stress‐strain behavior of the craze
Authors: Kambour R. P. 
Published: December 2004

AbstractThe constitution and properties of crazes in glassy polymers and their relation to crack propagation are reviewed. New evidence is discussed which shows the craze to be much softer than the parent polymer but capable of sustaining large stresses and strains up to the point of failure. Craze failure is much more dependent on polymer molecular weight than is craze formation, and this difference is reflected in changes in both fracture surface morphology and crack toughness with molecular weight. Finally craze mechanical properties are thought to be integrally related to the mechanical behavior of high impact plastics.
Particle size and molecular weight effects on the melt flow of emulsion PVC
Authors: Berens A. R. , Folt V. L. 
Published: December 2004

AbstractThe melt flow behavior of straight emulsion‐polymerized PVC in a capillary extrusion rheometer has been found to depend upon both the molecular weight and the particle size of the sample. Observations of flow‐rate, post‐extrusion swell, and extrudate appearance, as functions of extrusion temperature and pressure, suggest that both molecular deformation and particle slippage are involved in the flow mechanism. The relative importance of these two modes of flow varies with extrusion conditions and with the PVC molecular weight and particle size. Particle slippage is favored by large particle size, high molecular weight, and low temperature and by a shear stress above a critical yield value. Apparent melt viscosity, swelling, and roughness are minimized under conditions corresponding to the maximum contribution of particle slippage. In the proper range of temperature and shear rate, straight emulsion PVC yields smooth, low‐swell extrudates of excellent physical properties.
Reciprocity failure in films of flash exposed dichromate sensitized polyvinyl alcohol
Authors: Leiga A. G. , Walder R. A. 
Published: December 2004

AbstractThe validity of the Busen‐Roscoe reciprocity law in flash exposed dichromate sensitized polyvinyl alcohol (PVA) films was investigated. It was found that the flashlamp operating conditions were an important parameter in an exposure system, since a spectral distribution shift of the lamp output could be obtained by changing the pulse duration from 0.3 to 2.0 msec. The apparent reciprocity failure obtained by varying the pulse duration while maintaining the energy stored in the capacitor bank fixed was consistent with the spectral distribution shift in the lamp output. When this shift was accounted for there was only a negligible reciprocity failure found over the range of these flash exposures; however, comparison with a steady mercury are lamp showed that the flash exposure cross‐linked the PVA about 200 times more efficiently.