bluuebunny/crossref_metadata_2025 huggingface dataset:
The electronic structure, spectra, and reactivity of nitrophenols
Authors: Namboodiri K. P. Krishnan , Viswanathan S. , Ganesan R. , Bhasu V. C. Jyothi
Published: December 2004

The all‐valence‐electron CNDO/2 calculations were performed for the three isomeric nitrophenols. Using the newly derived σ‐core charges and subsequently revising the valence‐state ionization potentials and one‐center two‐electron repulsion integrals, Pariser–Parr–Pople (PPP) CI calculations were performed on the title compounds following the Nishimoto–Forster scheme. A better agreement between theory and experiment has been observed in spectral assignments compared to the conventional PPP approach. Information from the CNDO/2 calculations was used to obtain useful electronic structural parameters and to get a quantitative insight into the chemical reactivity of these molecules. All the results were compared with the basic compounds, phenol and nitrobenzene. The electronic spectra of these isomers were recorded in both polar and nonpolar solvents.
Three‐Electron bonds. II. SS and SCL three‐electron bonds
Authors: Clark Timothy
Published: December 2004

Ab initio molecular orbital calculations are reported for the H2SSH· and H2SCl· radicals and for the H2SClH+· radical cation. The two neutral species are found to be very weakly bound van der Waals' complexes, whereas the H2SClH+· radical cation is bound by 11.9 kcal mol−1 at MP2/4‐31G. The importance of charge in σ* radicals is discussed.
Geometry and energy of overcrowded ethylenes. II. Bornanylidene, fenchylidene, and bifluorenylidene derivatives
Authors: Favini G. , Simonetta M. , Sottocornola M. , Todeschini R.
Published: December 2004

The geometry and energy of some bornanylidene, fenchylidene, and bifluorenylidene derivatives have been determined by the molecular mechanics method. Low twisting of the double bond was found for the compounds of the first two series. The difficulties found in the synthesis of 2‐di‐tert‐butylmethylenefenchane can be related to a remarkable strain energy which is only slightly lower than that of tetra‐tert‐butylethylene. A twisted conformation is the preferred one in bifluorenylidene and its 1,1′‐dimethyl derivative (E isomer) in agreement with experimental data. Conformational inversion and E–Z interconversion paths are discussed for the latter compound and theoretical results are compared with dynamic NMR data.
Theoretical studies on “acetylenic zipper” reaction intermediates
Authors: Hass Ernst C. , Mezey Paul G. , Abrams Suzanne R.
Published: December 2004

Nonempirical molecular orbital studies on a series of isomeric octyn‐1‐ols indicated that the anomalous stability of the alkoxide of 3‐octyn‐1‐ol isomer is probably related to stereochemical and polarization properties of the molecule. The calculations correlated with the experimental observation that the least stable isomer should be the terminal alkyne, 7‐octyn‐1‐ol.
Evaluation of MINDO/3 calculated structures. III. Saturated acyclic compounds with chlorine, nitrogen, oxygen, or sulfur
Authors: McManus Samuel P. , Smith Maurice R. , Shafer Steven G.
Published: December 2004

Fifty‐one structures have been calculated by the MINDO/3 method to evaluate the errors introduced by branching and by the presence of the heteroatom. The structures are evaluated by comparisons which reveal that the calculated ΔHf values reflect a bias because of the presence of the heteroatom. With two carbons or more in a chain attached to a heteroatom group, a linear relationship exists which makes possible the calculation of reasonably accurate ΔHf values for unbranched alcohols, primary or secondary amines, ethers, thioethers, thiols, and alkyl chlorides. Branching errors do not seem to be linearly related among the systems. Some errors in calculations of geometries are also discussed.
Computer enumeration and generation of benzenoid hydrocarbons and identification of bay regions
Authors: Knop J. V. , Szymanski K. , Jeric̃ević Z̃. , Trinajstić N.
Published: December 2004

A computer‐oriented algorithm is developed for enumerating and generating all rigorously planar benzenoid hydrocarbons. An additional algorithm is also developed for identification and enumeration of carcinogenic bay regions. The results are reported for undisrupted benzenoid structures up to ten fused rings. The reported numbers of isomeric benzenoids do not agree with the results in the literature, because other authors included in their considerations some polyhexes which are not strictly planar structures (e.g., helicenes) or which represent rings of hexagons (e.g., circulenes). The reported results for enumeration of bay regions fully agree with those of Balasubramanian et al.
Restricted Hartree–Fock approximation. II. Computational aspects of the direct minimization procedure
Authors: Fern Rico J. ÁNdez , Paniagua M. , Fern Alonso J. I. ÁNdez , Fantucci P.
Published: December 2004

A RHF energy minimization procedure based on the treatment outlined in Part I of this series of articles is presented. Test calculations performed on several closed‐ and open‐shell systems show that the present procedure is definitely superior to the conventional SCF methods. In particular, the convergence of this procedure is ensured, the rate of convergency is high, and the computational cost of each cycle is low.
Symmetry properties of chemical graphs. V. Internal rotation in XY⋅3XY⋅2XY3
Authors: Randić Milan
Published: December 2004

Structures XY⋅3XY⋅2XY3 of symmetry C2v (of which propane is an example) are examined and the rearrangement due to the internal rotation of the end groups XY3 studied. The isomerization graph is constructed, various forms of which are displayed and the symmetry of which has been determined. The order of the group is 72. There are nine prime (irreducible) representations (4A + E + 4G) with the following partitioning of the elements into classes: 1, 42, 62, 9, 122, 18. When the mechanism for rearrangement is generalized to include enantiomers, a duplex graph is produced with the order of the group 144 which is isomorphic to the group S2(S3,S2) (generalized wreath product of the symmetric group S2 and S3). The corresponding graph has been constructed and displayed in one of more symmetrical forms. Isomorphism of groups of order 144 is discussed and a procedure is outlined in which correspondence between distinctive combinatorial objects is established by inducing permutations of m elements from available permutations of n elements. The scheme is based on selection of suitable graph invariants in one system and their labeling as m objects which form the basis for representation of the symmetry for the other system.
CHARMM: A program for macromolecular energy, minimization, and dynamics calculations
Authors: Brooks Bernard R. , Bruccoleri Robert E. , Olafson Barry D. , States David J. , Swaminathan S. , Karplus Martin
Published: December 2004

CHARMM (Chemistry at HARvard Macromolecular Mechanics) is a highly flexible computer program which uses empirical energy functions to model macromolecular systems. The program can read or model build structures, energy minimize them by first‐ or second‐derivative techniques, perform a normal mode or molecular dynamics simulation, and analyze the structural, equilibrium, and dynamic properties determined in these calculations. The operations that CHARMM can perform are described, and some implementation details are given. A set of parameters for the empirical energy function and a sample run are included.
Orbital photography
Authors: Hout Robert F. , Pietro William J. , Hehre Warren J.
Published: December 2004

A scheme is presented that enables photographic representation of the (two‐dimensional) surfaces of (three‐dimensional) molecular orbitals and electron density functions. The algorithm has been implemented by a program which is completely general and computationally efficient.
Efficient diffuse function‐augmented basis sets for anion calculations. III. The 3‐21+G basis set for first‐row elements, Li–F
Authors: Clark Timothy , Chandrasekhar Jayaraman , Spitznagel Günther W. , Schleyer Paul Von Ragué
Published: December 2004

The relatively small diffuse function‐augmented basis set, 3‐21+G, is shown to describe anion geometries and proton affinities adequately. The diffuse sp orbital exponents are recommended for general use to augment larger basis sets.
Uracil–4‐Hydroxyuracil tautomerism revisited
Authors: Zielinski Theresa Julia
Published: December 2004

The tautomerism of the uracil‐4‐hydroxyuracil system is examined with the 3–21G basis set and full geometry optimization using gradient techniques. Single‐point calculations at the 6‐31G level were also done and the results compared with the 2‐pyridone–2‐hydroxypyridine system. The best ΔE(taut) obtained in this work is 74.1 kJ/mol.
Ab initio study of catalyzed and uncatalyzed amide bond formation as a model for peptide bond formation: Ammonia‐Glycine reactions
Authors: Oie Tetsuro , Loew Gilda H. , Burt Stanley K. , Macelroy Robert D.
Published: December 2004

As a model reaction for peptide and bond formation, the SN2 reactions between glycine and ammonia have been studied with and without amine catalysis:

using ab initio molecular‐orbital methods. For each of the catalyzed and uncatalyzed reactions, two reaction mechanisms have been examined: a two‐step and a concerted mechanism. The stationary points of each reaction, including intermediate and transition states, have been identified and free energies calculated for all geometry‐optimized reaction species to determine the thermodynamics and kinetics of the reaction. The calculations demonstrate that a second ammonia molecule catalyzes amide bond formation, and that the two‐step mechanism is more favorable than the concerted one for the catalyzed reaction, while for the uncatalyzed reaction both mechanisms are competitive.
Solvent dielectric attenuation of substituent effects. Dependence on boundary representation in prolate spheroidal cavity models
Authors: Ehrenson S.
Published: December 2004

Extension of the inhomogeneous continuum solvent model to prolate spheroidal cavity systems in the context of Kirkwood–Westheimer substituent‐reactivity theory is described. Reasonable effects attributable to electrical saturation and electrostriction, which are modeled by relatively simple spatial dielectric functions outside the solute–solvent boundary, may be demonstrated. It is also shown that choices of proper (i.e., nonaveraged) location of the interacting sites and magnitude of substituent dipole moments are comparably important to the quality of theoretical prediction.
Geometry optimization using symmetry coordinates
Authors: Kondo H. , Jaffé H. H. , Lee H. Y. , Welsh William J.
Published: December 2004

The 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

The 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

Changes 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

A 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

It 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

The 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

Optimized 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

Transition 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

Atomic 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

Ab 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

A 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

A 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

A 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

Interaction 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

The 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

In 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

An 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

The 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

5‐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

The 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

Cyano‐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

Im 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

Die 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

Die 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

An 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

A 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

The 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

The 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

The 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.
A model for polymer fatigue
Authors: Opp D. A. , Skinner D. W. , Wiktorek R. J.
Published: December 2004

Attempts to use low‐cycle fatigue techniques derived from the work of Coffin and Manson to predict the fatigue performance of six diverse structural plastics are discussed. Through an analysis of dynamic property and thermal behavior studies of polymer samples in reversed‐tension fatigue, an improved model for the polymer fatigue process has been proposed. Laboratory test data are presented illustrating the relationship between the various models.
The influence of molecular weight distribution on melt viscosity, melt elasticity, processing behavior and properties of polystyrene
Authors: Thomas D. P. , Hagan R. S.
Published: December 2004

The influence of molecular weight and molecular weight distribution on the melt rheological behavior of two polystrenes of approximately the same weight average molecular weight, but of widely different molecular weight distribution, was determined. Then, using a series of capillaries with different length‐to‐diameter ratios in an Instron Capillary Rheometer, the entrance correction methods of E. B. Bagley and the relationships of W. Philippoff and F. H. Gaskins, the recoverable shear strain (SR) in the melt at the capillary wall for these mono‐ and polydisperse polystyrenes was determined. Shear modulus (G) and normal stress (PN) were calculated using the relationships: G = τRC/SR and PN = 2τRC SR, where τRC is the corrected shear stress at the capillary wall. These are compared to values obtained using a Weissenberg Rheogonimeter. These two polystyrenes were also injection molded into an ASTM specimen mold over a wide range of stock temperature, using a 12 OZ. in‐line reciprocating screw injection press, and evaluated for mechanical property values. The effects of the elasticity parameters (SR G & PN) and their magnitude on the rheology, processability and mechanical properties of these polystyrenes are discussed.
Fine structures and fracture processes in plastic/rubber two ‐phase polymer systems. II. Observation of crazing behaviors under the electron microscope
Authors: Matsuo M.
Published: December 2004

Detailed crazing behavior in several plastic/rubber twophase polymer systems was studied by means of direct observation of ultrathin sections under the electron microscope by employing osmium tetroxide staining and hardening procedure. Samples used are ABS polymer, high ‐impact polystyrene and several PVC/rubber blends. All of the systems investigated showed evidence of stress ‐crazing when under flexural stress. Relationships between the dispersed rubber particles and the crazing behaviors were studied, and the role of rubber particles in the toughening mechanism of plastics was discussed based on these observations.
Natural and artificial weathering of polyethylene plastics
Authors: Howard J. B. , Gilroy H. M.
Published: December 2004

Empirical test data on low‐density polyethylenes exposed to the elements for very long times are compared with results on samples of the same materials subjected to artificial “weathering” in a modified carbon arc machine.

The test results conform to the expression

where ta is hours of accelerated weathering, tn years of natural exposure, and the constants B and k have values of 150 and 2.4, respectively, for the materials and test conditions involved.

Earlier results are confirmed in that optimum weather resistance is found to depend on good dispersion of a fine‐particle black. Protection improves with concentration up to the limit of about 5% by weight imposed by degenerative effects on mechanical properties from higher loadings of the fine channel blacks. Dependence of weatherability on molecular weight (melt index) is noted.
Meaning and measurement of crystallinity in polymers: A Review
Authors: Kavesh S. , Schultz J. M.
Published: December 2004

A useful concept in polymer science is the degree of crystallinity—the fraction of the polymer that exists in a relatively ordered state. Methods of determination of the degree of crystallinity using density, infrared, thermal, N.M.R. and X‐ray measurements are examined in light of modern notions of the structure of semi‐crystalline polymers.
Measurement of water vapor transmission through polyethylene electrical insulation
Authors: Eichhorn R. M.
Published: December 2004

A method is described for measuring the rate of water vapor transmission through thick sections of polyethylene used as insulation on electrical conductors of pure sodium metal. The technique could be generally useful for materials which do not react with sodium, and for cylindrical samples which can be filled with molten sodium in a dry box. For samples with uniform dimensions the results are extremely precise because sensitive electrical measurements are used. Specimens of products in final form can be employed to determine the effects of variations in processing.
Polyvinylchloride melt rheology II—the influence of molecular weight on flow activation energy
Authors: Collins E. A. , Metzger A. P.
Published: December 2004

The flow behaviors of a series of polyvinylchloride (PVC) resins covering a broad range of molecular weight have been examined at several temperatures. It has been shown that the influence of temperature on viscosity depends on the temperature range. That is, the flow activation energy is not constant but can be approximated by two values, one applicable to low temperatures, the other to high temperatures.

The flow activation energy based on viscosities at constant shear rate decreases as the molecular weight increases. In contrast, the flow activation energy from viscosities at constant shear stress increases with molecular weight.

The fact that the activation energy is dual valued does not seem to be associated with the polymer type. Both emulsion and suspension resins exhibit this behavior. Addition of certain modifiers appears to alter the activation energy at lower temperatures. These observations indicate that the shift in the activation energy in the low temperature range is due to a change in the flow mechanism.
Cold forming of plastics part II. Draw forming of laminates containing crosslinkable core
Authors: Prevorsek D. C. , Koch P. J. , Oswald H. J. , Li H. L.
Published: December 2004

The problems encountered with cold formed thermoplastic material, namely, low heat distortion temperature and stress and solvent crazing, can be overcome by using a laminate consisting of a reinforced thermosetting core sandwiched between two drawable face sheets. This process imposes a restriction on the material by the need for a curing cycle to transform the core into a hard, three dimensional network. To take full advantage of the very fast metal forming technique, the curing should be accomplished outside of the forming apparatus and without external restraint. In order to impart a practical shelf life to the laminate at room temperature, the chemical requirements of the core can only be met with an elevated temperature cure. This in turn places an additional restriction on the face sheets, as they will have to withstand the curing conditions without external constraint and without dimensional changes of the formed part. Finally, the face sheets then have to become an integral part of the formed laminate upon curing of the latter, involving a high adhesive strength between the cured core and the face sheets. The criteria for the selection of the materials for various layers are presented as well as the limitations on the construction of the entire laminate.
Kinetics of thermal oxidation of polyolefins—A review
Authors: Reich L. , Jadrnicek B. R. , Stivala S. S.
Published: December 2004

The kinetics of uncatalyzed autoxidations for polyolefin films, such as atactic and isotactic polypropylene and poly‐(butene‐1) are reviewed in light of recent work. Reaction temperatures generally varied from 100 to 150°C and oxygen concentrations from 5 to 100% by volume. A general reaction scheme is suggested and kinetic expressions subsequently derived therefrom have been satisfactorily applied to account for experimental results. Linear relationships between a maximum rate and concentration of oxygen for both low and high concentrations were obtained.

In the case of the catalyzed autoxidations [Co(III) acetylacetonate] the general reaction scheme was modified to take into account the presence of the catalyst. From this modified scheme, various kinetic expressions relating maximum rate and concentrations of oxygen, polymer and the catalyst were derived. First‐order reaction with respect to the concentration of the catalyst was found at low concentrations of the catalyst, and near zero‐order at relatively high catalyst concentrations. A correlation between catalytic activities of metal acetylacetonates [Co(III), Mn(III), Cr(III), Fe(III) and Cu(II)] and the oxidation‐reduction potentials has been established.

Experimental dependencies of maximum carbonyl formation rates as a function of polymer concentration were found to agree well with the theoretical both for catalyzed and uncatalyzed oxidations.
Gas separation with porous anisotropic polymer films
Authors: Bradley D. F. , Baker R. W.
Published: December 2004

The utility of anisotropic ultrafiltration membranes in gas separation has been examined. These membranes consist of a thin (0.2‐2μ) barrier layer supported upon a more porous supporting substructure. Gases permeate these membranes by a Knudsen diffusion mechanism. Permeabilities from 0.1 to 0.4 cm sec−1 cm Hg−1 were obtained. These values are 104 to 105 higher than most conventional gas separation membranes. These membranes may be useful in several gas separation processes.
The influence of molecular weight on the melt rheology of polypropylene
Authors: Thomas Douglas P.
Published: December 2004

Melt viscosity and melt elasticity data were obtained over a broad range of temperatures and shear rates on a series of four polypropylenes of different molecular weight but approximately the same molecular weight distribution. The superposition technique was used with both temperature and molecular weight to shift flow curves for all four materials at three temperatures each along the shear rate axis to generate a master flow curve at a given temperature and molecular weight. For polypropylenes of this type, and molecular weight distribution shift, factors which can be used to extend the useful range of experimentally obtained flow data were determined. The dependency of apparent viscosity on weight average molecular weight at shear stresses as high as 106 dynes/cm2 is shown. The dependency of melt elasticity on molecular weight and temperature is discussed.
Internal pressure behavior of polymers above and below the glass temperature
Authors: Bagley E. B. , Scigliano J. M.
Published: December 2004

It has been well established in the literature that the internal pressure, Pi = (∂E/∂V)T, of a polymer in the glassy state is about half the value expected from the behavior of the polymer just above the glass temperature, Tg. Consideration of this behavior in terms of a recent analysis of factors affecting internal pressures leads to the conclusion that the expression for the total energy of a glass must include a volume‐dependent stored energy term, a term not present above Tg. This stored energy could be associated with actual bond and segment deformations in the glassy state. Brittleness and solvent cracking behavior of glasses will be strongly dependent on this stored elastic energy which can be modified by altering the molding conditions under which the glass is formed.
Weatheirng study of glass‐fiber reinforced polyester sheets by scanning electron microscopy
Authors: Blaga A.
Published: December 2004

Evidence is presented of the main steps in the physical breakdown of glass‐fiber reinforced polyester (GRP) composites on outdoor weathering. The chronological sequence is fiber ridging, rupture of the resin layer covering ridging fibers or fibers running close to the surface, spalling of the resin at the site of failure and subsequent erosion, fiber prominence and formation of a network of microcracks. Breakdown is believed to be caused by a type of stress fatigue imposed on the composite by cyclic variation of humidity and temperature in conjunction with solar radiation, and by the action of water and oxygen. The under side of the exposed GRP sheeting shows only incipient breakdown, indicating that solar radiation is an important factor. Countermeasures suggested to reduce breakdown include techniques to keep fibers away from the surface, use of resins with better thermal and moisture characteristics, and use of resin formulations with the best light stability.
Converging flow of polymer melts in extrusion dies
Authors: Cogswell F. N.
Published: December 2004

Practical extrusion processes often involve geometrically complex dies. Such dies are usually tapered, or streamlined, to achieve maximum output rate under conditions of laminar flow. These converging flows may be analysed in terms of their extensional and simple shear components to calculate the relationships between volume flow rate, pressure drop, and post extrusion swelling. The analysis can also be extended to cover the free convergence as fluid flows from a reservior into a die. Comparisons between predicted and observed data for a series of coni‐cylindrical dies suggest that using this approach the pressure drop/flow rate relationship can be predicted within ±20% and the swell ratio/flow rate relationship within ±10%. Similar treatments have been in use for the last three years in solving such complex flow problems as radial flow in injection moulding and two‐dimensional annular convergence in blow moulding dies.