bluuebunny/crossref_metadata_2025 huggingface dataset:
Divinylketone: Conformational preferences. Molecular geometry and vibrational force field in the ground state
Authors: Skancke Per N.
Published: December 2004

The conformational preferences of the molecule 1,4‐pentadien‐3‐one (divinylketone) have been studied by ab initio molecular orbital calculations and discussed in terms of interaction between molecular fragments. The calculations predict a molecular ground state having a fully coplanar s‐cis, s‐cis conformation. In addition, we find three other structures that represent local minima on the energy surface. These are a fully coplanar s‐cis, s‐trans form, and two nonplanar s‐trans, s‐trans forms having symmetries C2 and C1h, respectively. The energies of these forms relative to the ground state are 7.5, 19.2, and 35.9 kJ/mol, respectively. The coplanar s‐trans, s‐trans form represents a saddle point on the energy surface. All conformers and the saddle point have been completely geometry optimized by the gradient technique. For the ground state a complete in‐plane harmonic force field has been evaluated.
Standard deviation in reduced errors (SDRE): A criterion to assess the relative overall quality of approximate wavefunctions
Authors: Zeiss G. D. , Whitehead M. A.
Published: December 2004

The standard deviation in reduced errors (SDRE) quantitatively measures the similarity between an approximate and exact wavefunction. It is the Euclidean distance between the vector of the one‐electron property expectation values calculated using the approximate and exact expectation values.
Interactions of molecules with nucleic acids. VII. Evaluation and presentation of steric contours and molecules in bonding sites
Authors: Miller Kenneth J. , Kowalczyk Paul , Segmuller Wolfgang , Walker George
Published: December 2004

A technique to generate steric contours is developed and applied to the presentation of molecules that interact with DNA. The contours are generated by a Taylor's expansion in an extrapolation procedure designed to avoid infinite derivatives and correct for reversal in the direction of the drawing of the curve. A solution to the hidden line problem in computer graphics is presented for the case of steric contours ordered in parallel planes. The techniques developed in this article are demonstrated with stereographic orthographic projections for B‐DNA, for intercalation complexes of lucanthone and actinomycin‐D with DNA, and for the binding of benzo[a]pyrene diol epoxides (BPDEs) to guanine. The latter example illustrates the value of steric contours in demonstrating the lack of fit of BPDEs to B‐DNA and by the need to unwind DNA to accommodate this molecule. Electrostatic contours in planes may be generated by this same procedure.
Through‐Space effects on vicinal proton spin–spin coupling constants mediated via hetero atoms: Nonequivalence of cis couplings in five‐membered rings
Authors: De Leeuw Frank A. A. M. , Van Beuzekom Aart A. , Altona Cornelis
Published: December 2004

Theoretical studies are presented, aimed at the elucidation of through‐space effects exerted by bridge‐head oxygen and nitrogen atoms on cisoidal proton spin–spin coupling constants in 7‐hetero‐substituted norbornanes. The finite perturbation theory intermediate neglect of differential overlap (INDO) self‐consistent field (SCF) molecular orbital (MO) method, modified according to the description given by Barfield [J. Am. Chem. Soc., 102, 1 (1980)], was employed. It is predicted that the known nonequivalence between Jendo–endo and Jexo–exo in norbornanes, which is in part mediated by the bridgehead methylene group (Barfield transmission effect), also occurs in 7‐heteronorbornanes. A trend is recognized in which the nonequivalence induced by oxygen is rather smaller than that induced by the rear lobe of the NH bond or by the nitrogen lone pair. It is shown that the Barfield effect also explains the observed nonequivalence between cisoidal HCβCγH and HCγCδH couplings in prolines. The calculations also predict similar, though smaller, effects on cisoidal couplings in the tetrahydrofuranyl ring system. Special attention was paid to the practical consequences of the existence of this effect with regard to the pseudorotation analysis of the five‐membered sugar ring in nucleic acids. It is found that, in addition to the recently introduced correction for electronegativity and orientation of substituents [Haasnoot, de Leeuw, and Altona, Tetrahedron 36, 2783 (1980)], a cos2 dependency of 3Jcis upon the phase angle of pseudorotation must be taken into account, especially in the case of deoxyribofuranoses.
Ab initio studies of structural features not easily amenable to experiment. 25. Conformational analysis of methyl propanoate and comparison with the methyl ester of glycine
Authors: Klimkowski V. J. , Scarsdale J. N. , Schäfer Lothar
Published: December 2004

The molecular geometries of three conformations of methyl propanoate (MEP) (CCCO torsions of 0°, 120°, and 180°) and the potential‐energy surfaces of MEP (CCCO torsions) and of the methyl ester of glycine (MEG) (NCCO torsions) have been determined by ab initio gradient calculations at the 4‐21G level. MEP has conformational energy minima at 0° and 120° of the CCCO torsion, while the 60–90° range and 180° are energy maxima. For MEG there are two minima (at 0° and 180°) and one barrier to NCCO rotation in the 60–90° range. The NCCO barrier height is about twice as high (4 kcal/mol) as the CCCO barrier. The 180° NCCO minimum is characteristically wide and flat allowing for considerable flexibility of the NCCO torsion in the 150–210° range. This flexibility could be of potential importance for polypeptide systems, since the NCCO angles of helical forms are usually found in this region. The molecular structures of the methyl ester group CH3OC(O)CHRR′ in several systems are compared and found to be rather constant when R  H and R′  H, CH3, CH3CH2; or when R  NH2 and R′  H, CH3, or CH(CH3)2.
A theoretical study of α‐substituted isopropyl and cyclopropyl anions
Authors: Hopkinson A. C. , McKinney M. A. , Lien M. H.
Published: December 2004

Ab initio molecular orbital theory has been used to probe the effect of the substituent X on the structures, strain energies, stabilization energies, inversion barriers, and proton affinities of carbanions CH3CX CH and cis‐C3H4X−, where X = H, F, CN, and NC. All geometries have been optimized with a 3‐21G basis set, and the parent anions (X = H) were also optimized with the same basis set with a diffuse function added (i.e. the 3‐21 + G basis set). The anions, with the exception of the α‐cyanoisopropyl anion, are pyramidal. The out‐of‐plane angle, α, for the pyramidal anions decreases in the order F > H ≈ NC > CN, and the barriers to inversion follow the same order with the cyclopropyl anions consistently having higher barriers than the isopropyl anions. The substituents strongly stabilize the anions with the stabilization energy following the order CN > NC > F. The cyano group slightly reduces the strain energy of cyclopropane, but the isocyano and fluoro substituents are weakly and strongly destabilizing, respectively. The pyramidal cyclopropyl anions are less strained than the cyclopropanes except when the substituent is a cyano group where the strain energies are reversed but are very similar. The planar anions all have higher strain energies than the cyclopropanes.
Interactions of molecules with nucleic acids. IX. Evaluation and presentation of electrostatic contours on a steric surface with the removal of hidden lines
Authors: Miller Kenneth J. , Kowalczyk Paul J.
Published: December 2004

A technique to generate electrostatic contours on a steric surface is presented and applied to the presentation of molecules that interact with DNA. A set of electrostatic points at predetermined values along with their derivatives are obtained on the steric contours as they are generated. The steric contours are generated in a set of parallel planes. Points with given electrostatic values are then connected between and within the contours mathematically with a Taylor's expansion and two rules: the first to tentatively line up points that can be connected, and the second to check to insure that the remaining points can be connected. This method insures that contours will not cross by requiring that a possible connection of two points leaves an even number of remaining points for each electrostatic value in isolated regions of unused points bounded by points that have already been connected. The hidden line algorithm used previously to draw molecules in a space‐filling model within the context of steric contours is applied to the complete problem of the presentation of a molecule bound to DNA with steric contours in parallel planes, and with electrostatic contours drawn on this steric surface.
Conformational properties of α‐ and β‐azabicyclane opiates. The effect of conformation on pharmacological activity
Authors: Froimowitz Mark , Salva P. , Hite G. J. , Gianutsos G. , Suzdak P. , Heyman R.
Published: December 2004

Conformational energy calculations using the MM2 (molecular mechanics II) program are reported for diastereoisomeric α‐ and β‐azabicyclanes (3‐methyl‐9‐methoxy‐9‐phenyl‐3‐azabicyclo [3.3.1] nonanes) which are prototypical phenyl‐axial and phenyl‐equatorial opiates. After energy minimization, both compounds are found to prefer a chair–chair conformation of the piperidine and cyclohexane rings with two mirror image orientations of the phenyl and methoxyl groups. The calculations also indicate that the equilibrium conformations of the phenyl and methoxyl groups are very similar in the two diastereoisomers. A morphine‐like conformation of the phenyl group was found to be very unfavorable because of a high barrier to rotation of the phenyl group. This does not have an apparent effect on opiate agonist properties, but does result in a significant weakening of the antagonist activity of the N‐allyl derivative of α‐azabicyclane. This is discussed in terms of a model for phenyl‐axial and phenyl‐equatorial opiate substrates which accounts for both similarities and differences in their structure–activity relationships. A detailed comparison is also made between the computed structures and those observed by x‐ray crystallography with excellent agreement between the two.
Molecular mechanics force‐field parameterization procedures
Authors: Hopfinger A. J. , Pearlstein R. A.
Published: December 2004

A set of procedures and guidelines are presented for the estimation of bond length, bond angle, and torsional potential constants for molecular mechanics force fields. The force field constants are ultimately derived by “subtracting” nonbonded molecular mechanics energies from corresponding molecular orbital energies using a model compound containing the chemical structure to be parameterized. Case study examples of bond length, bond angle, and torsional rotation force field parameterizations are presented. A general discussion of molecular mechanics force field parameterization strategy is included for reference and completeness. Finally, a curve‐fitting program to generate force field parameters from raw data is given in Appendix I.
The ground state potential energy surface of methyl fluoride dimer
Authors: Hasanein Ahmed A.
Published: December 2004

Potential energy surface for methyl fluoride dimer has been studied theoretically with ab initio molecular orbital method, using a 4‐31G basis set. Dimer dissociation energies, Mulliken electronic populations, and dipole moments were obtained.
MNDO Study of reaction pathways for SN2 reactions. Menschutkin reaction potential energy surfaces
Authors: Viers Jimmy W. , Schug John C. , Stovall Michael D. , Seeman Jeffrey I.
Published: December 2004

MNDO molecular orbital calculations have been employed to investigate limited reaction pathways and potential energy surfaces for a series of SN2 reactions. Model calculations for X− + CH3X (X = H, F, OH, OCH3, and CN) indicate that the MNDO method gives qualitative agreement with ab initio studies except for the hydride–CH4 exchange. Studies involving alkylation of pyridine (Menschutkin reaction) were also carried out. For the reaction of pyridine with CH3Cl, which involves charge separation, our MNDO studies (which do not include solvation effects) do not produce a characteristic SN2 pathway. For the reaction of pyridine with trimethyloxonium cation [(CH3)3O+] as the alkylating agent, a well defined SN2 reaction pathway was obtained; this reaction involves charge transfer. A potential energy surface for the pyridine–trimethyloxonium cation reaction shows the presence of a saddle point transition state that resembles starting materials, in agreement with the Hammond postulate for this exothermic reaction.
Towards the convergence of molecular‐mechanical force fields
Authors: Meyer A. Y. , Forrest F. R. F.
Published: December 2004

It is noticed that inclusion of an electrostatic term in the molecular‐mechanical treatment of hydrocarbons would compel the nonbonding parameters of different force fields to become more alike than they are at present. Apart from removing the discontinuity in passing from the calculation of an unfunctionalized parent compound to the calculation of its functionalized derivatives, it is expected that the inclusion would improve results for the hydrocarbons themselves.
The α and α parameters for spin polarized Hartree–Fock–Slater calculations
Authors: Lees W. , Manoli S. , Whitehead M. A.
Published: December 2004

α and α are derived for use in spin polarized Hartree–Fock–Slater programs. They are assumed to depend only on the number of up and down spin electrons in the atom. The calculated eigenvalues show a slight improvement only for carbon, nitrogen, and oxygen.
Highly accurate vibration–rotation Franck–Condon factors for high levels
Authors: Dagher Mounzer , Kobeissi Hafez
Published: December 2004

Highly accurate vibration–rotation Franck–Condon factors qab, for a transition between two diatomic electronic states (a) and (b), are sought. When the potentials of states (a) and (b) are of the RKR type, the computation of qab is reduced to that of Franck–Condon integral ℐab(i) = ∫ ψa(r)ψb(r) dr in an interval ri, ri+1. By using convenient interpolations for the potentials Ua and Ub in the considered interval, this integral becomes ℐab(i) = ∑ δ (ri+1 – ri)n+1/(n + 1), where the “coupling constants” δ depend uniquely on the eigenvalues Ea and Eb of the considered transition and on the potentials Ua and Ub (the number N of terms depends on the desired accuracy). The method used computes the Franck–Condon factors qab without the explicit use of the wave function and by replacing the integrals by simple summations. To test the values of qab obtained by this method, the orthogonality rule ∫ ψv′ψv″ dr = 0 (for v′ ≠ v″) is used for one state or the other. This test, along with other tests, show that the Franck–Condon factors computed by the present method are accurate to nine significant figures for high and low levels.
A note on torsional force constants in molecular mechanics for a methyl group attached to a conjugated system
Authors: Liljefors Tommy , Allinger Norman L.
Published: December 2004

The molecular mechanics (MMP2) description of the torsion of a methyl group attached to a conjugated system has been improved by using bond order dependent torsional force constants.
Theoretical studies on the hydrogen atom transfer reaction
Authors: Lee Bon‐Su , Lee Ikchoon , Song Chang Hyun , Choi Jae Young
Published: December 2004

The hydrogen atom transfer reaction between substituted methanes (substituents; H, F, CH3, OH, and CN) and methyl radicals was studied by 4‐31G (UHF) calculations using the MINDO/3 geometries. The transition state structures and energy barriers were determined, and variations of the transition state and of the reactivity due to the change of substituent were analyzed based on the potential energy surface characteristics. It was concluded that the reaction is of the SH2 type with a backside attack, and transition state variations are controlled by the vector sum of the component parallel to (Hammond rule) and one perpendicular to the reaction coordinate (anti‐Hammond rule). It was also concluded that the most important factor influencing the reactivity is bond dissociation energy effect directly related to the spin transfer of the radical species, and the polar effect need not be overemphasized.
Computational schemes for modeling proton transfer in biological systems: Calculations on the hydrogen bonded complex [CH3OH · H · NH3]+
Authors: Topiol S. , Mercier G. , Osman R. , Weinstein H.
Published: December 2004

Different schemes are explored for the calculation of the proton transfer process in the hydrogen bonded cation [CH3OH · H · NH3]+. Results from ab‐initio calculations with the STO‐3G, 3‐21G and 4‐31G basis sets, are compared in search for an efficient reliable scheme to study the potential energy curves for the proton transfer. The curve constructed from the lowest energies calculated with the frozen optimized geometries of the two possible pairs of proton donor and acceptor fragments, (i.e., CH3OH/NH3 and CH3OH/NH) is in good agreement with that obtained when all the fragments of the hydrogen bonded complex are completely optimized simultaneously.
Plasminogen activator inhibitor type‐1 gene expression and induced migration in TGF‐β1‐stimulated smooth muscle cells is pp60c‐src/MEK‐dependent
Authors: Samarakoon Rohan , Higgins Craig E. , Higgins Stephen P. , Kutz Stacie M. , Higgins Paul J.
Published: December 2004

Transforming growth factor‐β1 (TGF‐β1) stimulates expression of plasminogen activator inhibitor type‐1 (PAI‐1), a serine protease inhibitor (SERPIN) important in the control of stromal barrier proteolysis and cell‐to‐matrix adhesion. Pharmacologic agents that target MEK (PD98059, U0126) or src family (PP1) kinases attenuated TGF‐β1‐dependent PAI‐1 transcription in R22 aortic smooth muscle cells. Pretreatment with PP1 at concentrations that inhibited TGF‐β1‐dependent PAI‐1 expression also blocked ERK1/2 activation/nuclear accumulation suggesting that the required src kinase activity is upstream of ERK1/2 in the TGF‐β1‐initiated signaling cascade. The IC50 of the PP1‐sensitive kinase, furthermore, specifically implied involvement of pp60c‐src in PAI‐1 induction. Indeed, addition of TGF‐β1 to quiescent R22 cells resulted in a 3‐fold increase in pp60c‐src autophosphorylation and kinase activity. Transfection of a dominant‐negative pp60c‐src construct, moreover, reduced TGF‐β1‐induced PAI‐1 expression levels to that of unstimulated controls or PP1‐pretreated cells. A ≥170 kDa protein that co‐immunoprecipitated with TGF‐β1‐activated pp60c‐src was also phosphorylated transiently in response to TGF‐β1. TGF‐β1 is known to transactivate the 170 kDa EGF receptor (EGFR) by autocrine HB‐EGF or TGF‐α mechanisms suggesting involvement of EGFR activation in certain TGF‐β1‐initiated responses. Incubation of quiescent R22 cells with the EGFR‐specific inhibitor AG1478 prior to growth factor (EGF or TGF‐β1) addition effectively blocked EGFR activation as determined by direct visualization of receptor internalization. AG1478 suppressed (in a dose‐dependent fashion) EGF‐induced PAI‐1 protein levels and, at a final concentration of 2.5 μM, virtually eliminated EGF‐dependent PAI‐1 synthesis. More importantly, AG1478 similarly repressed inducible PAI‐1 levels in TGF‐β1‐stimulated R22 cultures. PP1, PD98059, and U0126 also inhibited TGF‐β1‐dependent cell motility at concentrations that significantly attenuated PAI‐1 expression. Consistent with the AG1478‐associated reductions in EGF‐ and TGF‐β1‐stimulated PAI‐1 expression, pretreatment of R22 cell cultures with AG1478 effectively suppressed growth factor‐stimulated cell motility. These data indicate that two major phenotypic characteristics of TGF‐β1‐exposure (i.e., transcription of specific target genes [e.g., PAI‐1], increased cell motility) are linked in the R22 vascular smooth muscle cell system, require pp60c‐src kinase activity and MEK signaling and involve activation of an AG1478‐sensitive (likely EGFR‐dependent) pathway. © 2004 Wiley‐Liss, Inc.
The rate of destruction of O2 (1Δg) by atomic hydrogen
Authors: Schofield Keith
Published: December 2004

An analysis of the atmospheric observations of O2(1δg) in the dayglow and twilight confirms the crude experimental assessments of Brown [1] and the conclusions that can be made from recent work of Westenberg, Roscoe, and DeHaas [2] that the reaction

is relatively slow, much slower than was expected or can be explained easily in theoretical terms. Using a value for the concentration of atomic hydrogen at 85 km, an upper limit rate of 3×10−13 cm3 molecule−1 sec−1 would be compatible with current atmospheric models. An evaluation of the available data for the rates of several reactions involving ground state or electronically excited species, for which the values are reasonably well established, is included to better analyze the general effects of an electronically excited reactant. This further illustrates the unusual slowness of the H + O2(1Δg) reaction.
Secondary hydrogen isotope effect for the transition from olefin to free radical
Authors: Henderson Richard W. , Pryor William A.
Published: December 2004

Secondary kinetic isotope effects occur in radical additions to deuterated olefins. Substitution of a deuterium at a carbon undergoing an sp2 to sp3 hybridization‐change during reaction, Cβ in eq. (1), leads to an
equation image
inverse isotope effect of 3–5% (kH/kD = 0.95–0.97). The effect at a carbon going from an olefinic to a radical center, Cα in eq. (1), generally has been assumed to be negligible, since a nominal sp2 hybridization is maintained throughout reaction. Using new, sensitive instrumentation for radioactivity determination and a recently developed quench correction technique, we now find that there is a small, normal isotope effect (kH/kT 1) associated with a change from olefin to radical. Specifically, when R· is the polystyryl radical, X is phenyl, and the α‐C bears a tritium, kH/kT = 1.04. This result is discussed in relation to recent data on cycloaddition reactions.
Gas‐phase elimination kinetics of 1‐substituted ethyl acetates. Effect of polar substituents at the α carbon of secondary acetates
Authors: Hernández A. José A. , Chuchani Gabriel
Published: December 2004

The gas‐phase elimination of several polar substituents at the α carbon of ethyl acetates has been studied in a static system over the temperature range of 310–410°C and the pressure range of 39–313 torr. These reactions are homogeneous in both clean and seasoned vessels, follow a first‐order rate law, and are unimolecular. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: 2‐acetoxypropionitrile, log k1 (s−1) = (12.88 ± 0.29) – (203.3 ± 2.6) kJ/mol (2.303RT)−1; for 3‐acetoxy‐2‐butanone, log ±1(s−1) = (13.40 ± 0.20) – (202.8 ± 2.4) kJ/mol (2.303RT)−1; for 1,1,1‐trichloro‐2‐acetoxypropane, log ℜ1 (s−1) = (12.12 ± 0.50) – (193.7 ± 6.0) kJ/mol (2.303RT)−; for methyl 2‐acetoxypropionate, log ℜ1 (s−1) = (13.45 ± 0.05) – (209.5 ± 0.5) kJ/mol (2.303RT)−1; for 1‐chloro‐2‐acetoxypropane, log ℜ1 (s−1) = (12.95 ± 0.15) – (197.5 ± 1.8) kJ/mol (2.303RT)−1; for 1‐fluoro‐2‐acetoxypropane, log ℜ1 (s−1) = (12.83 ± 0.15)– (197.8 ± 1.8) kJ/mol (2.303RT)−1; for 1‐dimethylamino‐2‐acetoxypropane, log ℜ1 (s−1) = (12.66 ± 0.22) –(185.9 ± 2.5) kJ/mol (2.303RT)−1; for 1‐phenyl‐2‐acetoxypropane, log ℜ1 (s−1) = (12.53 ± 0.20) – (180.1 ± 2.3) kJ/mol (2.303RT)−1; and for 1‐phenyl−3−acetoxybutane, log ℜ1 (s−1) = (12.33 ± 0.25) – (179.8 ± 2.9) kJ/mol (2.303RT)−1. The CαO bond polarization appears to be the rate‐determining process in the transmition state of these pyrolysis reactions. Linear correlations of electron‐releasing and electron‐withdrawing groups along strong σ bonds have been projected and discussed. The present work may provide a general view on the effect of alkyl and polar substituents at the CαO bond in the gas‐phase elimination of secondary acetates.
Kinetics of dialin thermolysis
Authors: Allen D. T. , Gavalas G. R.
Published: December 2004

The liquid‐phase thermolysis of 1,2‐dihydronaphthalene was studied in a batch reactor in the range of 350–400°C. The measured product distributions were in good agreement with calculations based on a free‐radical scheme with rate constants estimated by thermochemical methods. The kinetic calculations were carried out by numerical integration and by the long‐chain approximation (LCA), which yielded a closed‐form solution.
High temperature measurements of the rate of the reaction of OH with NH3
Authors: Zabielski M. F. , Seery D. J.
Published: December 2004

Low pressure (4.67 kPa) CH4/O2/Ar flames were seeded with approximately 5300 ppm NH3. The concentration profiles of stable and radical species in lean (ϕ = 0.92) and rich (ϕ = 1.13) flames were determined by molecular beam sampling mass spectrometry. Temperature profiles in these flames were measured with thermocouples whose readings were corrected for radiative losses by the Na‐line reversal method. Regions of the flames were selected where the principal reaction leading to the destruction of NH3 was
equation image
By correcting the measured concentrations for diffusion, the net rate of NH3 loss rate was determined in the temperature range 2080–2360 K. The rate constant k1 was determined from the net loss rate with correction for the reaction
equation image
using measured values of (O) and k2 values given by Salimian, Hanson, and Kruger [1]. The best‐fit Arrhenius expression for k1 in the temperature range 2080–2360 K is 1013.88 exp(−4539/T) cm3/mol‐s. The results of this study combined with previous lower temperature data confirm the non‐Arrhenius behavior of k1 suggested by Salimian, Hanson, and Kruger [1]. The best‐fit modified three parameter expression for the range 300–2360 K is 106.33±0.2 T2 exp(−169/T) cm3/mol‐s.
Arrhenius parameters for the hydrogen abstraction from ammonia by CF3 radicals
Authors: Pasteris L. , Staricco E. H.
Published: December 2004

The reaction of CF3 radicals with NH3 has been studied over a wide temperature range 298–673 K, using the photolysis and the thermal decomposition of CF3I as the free radical source.

It was found that the reaction could not be explained in terms of a simple mechanism
equation image
in the whole temperature range because a marked pressure dependence on the rate of products formation and the presence of a dark reaction complicate the system at low temperatures. Thus, Arrhenius parameters for reaction (1) have been calculated relative to the CF3 recombination from data in the range 523–673 K where pure hydrogen transfer occurs. The rate constant expression is given by

where kH/k is in units of cm3/2/mol1/2 s1/2 and θ = 2.303 RT/kJ/mol.
Das Rhenium und seine heutigen Anwendungen in der Technik
Authors: Noddack Walter , Noddack Ida
Published: December 2004

Das Rhenium wurde vor 26 Jahren entdeckt. Es dürfte das seltenste aller stabilen Metalle sein. Nur in wenigen Hüttenprodukten und Erzen ist es soweit angereichert, daß seine Darstellung lohnt. Die physikalischen und chemischen Eigenschaften des Elementes, das den Platinmetallen nahesteht, ließen voraussehen, daß es sich für zahlreiche technische Anwendungen eignen würde. Das geschmolzene Re‐Metall besitzt eine große Härte, seine Legierungen sind ebenfalls hart und ohne Sprödigkeit, meist auch korrosionsbeständig. Diese technisch wertvollen Eigenschaften haben in vielen Fällen bereits praktische Verwertung gefunden. In den Thermoketten mit Platin‐metallen besitzt Rhenium singuläre Eigenschaften. Von besonderem Interesse sind die katalytischen Eigenschaften des Rheniums.
Korrosions‐ und Hitzebeständigkeit von Kohlenstoffstahl und von Sonderstahl nach chemisch‐technischer Oberflächenbehandlung
Authors: Köhler Walther
Published: December 2004

Kritischer Überblick über das einschlägige neuere, sowjetrussische Schrifttum. Im einzelnen werden behandelt: Schichtbildung, Schichtaufbau, Korrosions‐ und Hitzebeständigkeit von Kohlenstoffstahl und Sonderstahl nach Zementieren, Nitrieren, Cyanieren, Alitieren, Chromieren, Silizieren, Sherardisieren, Beryllieren, Molybdänisieren, Wolframieren und Vanadisieren.
Spannungskorrosionsartige Erscheinungen an mechanisch beanspruchtem Stahl durch diffundierenden Wasserstoff
Authors: Class I.
Published: December 2004

Die Abhandlung beschreibt das Auftreten von Rissen mit vorwiegen transkristallinem Charakter an unlegierten und legierten Vergütungsstählen im Kondensationsteil von Hochdruckanlagen, sowie Versuche zur Erzielung gleichartiger Erscheinungen im Laboratorium. Die Ursache der Risse wird darin gesehen, daß der durch einen schwachen Korrosionsvorgang an der Stahloberfläche entstehende Wasserstoff durch die Gegenwart von spezifisch wirkenden Substanzen, und zwar insbesondere von Schwefelwasserstoff, in atomarer Form in das Metall eindringt und dor durch Rekombination an ausgezeichneten Stellen hohe Drücke entwickelt. Auf entsprechende Erscheinungen in der Erdölindustrie wird unter Erwähnung der umfangreichen Forschungstätigkeit auf diesem Gebiet in US hingewiesen.

Es werden die Faktoren beleuchtet, welche die Entstehung der Risse und den Rißverlauf beeinflussen, wobei auch die Bedeutung der Zähigkeit des Werkstoffes erwähnt wird. Betriebliche Kontroll‐ und Schutzmaßnahmen werden beschrieben.

Da die Grenzen des Begriffs der Spannungskorrosion, deren Wesensmerkmale die geschilderten Erscheinungen tragen, beim heutigen Stand der Erkenntnisse noch nicht scharf abgesteckt sind, wird vorgeschlagen, diese hier — etwa unter der Bezeichnung kathodische Spannungskorrosion — einzuordnen.
Strukturmechanische Grundlagen der Werkstoffermüdung
Authors: Macherauch E. , Mayr P.
Published: December 2004

Der gegenwärtige Stand der Ermüdungsforschung ist durch eine Fülle sowohl grundlagen‐ als auch anwendungsorientierter Einzelarbeiten gekennzeichnet. Trotzdem erscheint es zur Zeit nicht möglich, eine geschlossene Theorie des Ermüdungsverhaltens metallischer Werkstoffe in nächster Zukunft zu entwickeln. Die vorliegende Arbeit soll einen Überblick geben über die hinreichend gesicherten Erkenntnisse auf dem Gebiet der Metallermüdung. Die Forschungstätigkeit der letzten zwei Jahrzehnte führte zur Entwicklung einiger Ordnungskriterien für die vielfältigen experimentellen Ergebnisse sowie zu einigen quantitativen Abschätzungsverfahren. Für die einzelnen Ermüdungsstadien, den Anfangsbereich, den Rißbildungsbereich, die Rißausbreitung und den Restbruch werden einige wichtige Ergebnisse vorgestellt und diskutiert. Plastische Verformung ist die Voraussetzung für das Auftreten eines Ermüdungsbruchs bei metallischen Werkstoffen. Sie führt in der anrißfreien Phase der Ermüdung zu einer Veränderung der mechanischen Eigenschaften des Werkstoffs sowie zur Bildung besonderer Oberflächenverformungsstrukturen, die bevorzugt Ausgangspunkte für Ermüdungsrisse sind. Auch die Rißausbreitungsphase ist durch das Auftreten von plastischen Deformationen charakterisiert und erfordert eine hinreichend große Zugspannungskomponente. Strukturabhängige Werkstoffkenngrößen wirken sich insbesondere in der anrißfreien Phase des Ermüdungsvorgangs aus.
Der Einfluß von Festigkeit und Druckeigenspannungen auf die Wechselfestigkeit von Stählen bei höheren Temperaturen
Authors: Bahre K.
Published: December 2004

Das Wechselfestigkeitsverhalten krz Stähle ist bei Raumtemperatur durch zwei Endzustände gekennzeichnet, die sich unterhalb zweier verschiedener Grenzwechselspannungen stationär einstellen: Unterhalb der einen können beliebig viele Lastwechsel ohne Anrißbildung, unterhalb der anderen ohne Rißfortschritt ertragen werden. Bei glatten Bauteilen bestimmt die erstere die Dauerfestigkeit und nimmt vor allem mit der Werkstoffhärte zu. Bei gekerbten Proben dagegen wird die Dauerhaltbarkeit durch die zum Rißfortschritt erforderliche Mindestwechselspannung bestimmt, die durch Druckeigenspannungen erheblich heraufgesetzt werden kann. – Bei höheren Temperaturen entstehen keine stabilen Endzustände mehr und es gibt keine Wechselamplituden, die nicht sowohl zum Anriß als auch zum Rißfortschritt bis zum Bruch führen. Beide Einflußgrößen Werkstoffestigkeit und Druckeigenspannungen sind abhängig von der Beanspruchungstemperatur, ihr Temperaturverlauf kann mit den Verfestigungsmechanismen nach Snoek und nach Cottrell bzw. aus dem unterschiedlichen Eigenspannungsabbau infolge temperaturveränderlicher Streckgrenzen erklärt werden.
Warmzug‐ und Zug‐Kriechversuche an reaktionsgesintertem Siliziumnitrid
Authors: Gebhard W.
Published: December 2004

Es wird die Konstruktion einer Zugprüfeinrichtung zur Durchführung von Zug‐ und Zug‐Kriechversuchen an keramischen Werkstoffen von Raumtemperatur bis 1500 °C an Luft beschrieben. Besondere Bedeutung wird der Vermeidung überlagerter Biegebeanspruchung und deren Verminderung beigemessen. Ergebnisse der Zugfestigkeit von isostatisch gepreßtem RB‐SN verschiedener Dichte in Abhängigkeit von der Prüftemperatur werden mitgeteilt und das Kriechverhalten bei 1200 °C, 1250 °C und 1400 °C an Luft im Zug‐ und Biegeversuch verglichen.
Rheology of fiber‐ or flake‐filled plastics
Authors: Utracki L. A. , Fisa B.
Published: December 2004

Thermoplastics and thermoset materials filled with inexpensive, mainly inorganic, particulates are an important class of engineering plastics. Here, the short fiber and flake fillers play a major role. In this paper, the melt rheology and processability of these resins is reviewed, and the flow behavior of mica‐filled polyethylene is discussed in greater details.
Hot‐rolling and forging of mica‐filled polyolefins
Authors: Lee I. , Turner S. , Woodhams R. T.
Published: December 2004

As in the case of fibers, the mechanical properties of plastic composites containing mica flakes are extremely sensitive to flake orientation in the direction of an applied stress, so that even a small angular displacement can cause major reductions in the strength, modulus, and fracture toughness. In order to encourage parallel alignment of mica flakes in a thermoplastic composite, two methods of flow orientation were examined. In the first series, rectangular billets of mica‐filled, high‐density polyethylene were hot‐pressed in order to cause longitudinal melt flow in a narrow channel. A parallel series of experiments was also carried out with mica‐filled polypropylene in which the composite was extruded and calendered into a thin, continuous strip. In both processing techniques, the resulting extensional flow produced large increases in the tensile and flexural properties. The performance of the mica‐filled poly‐propylene was limited by its tendency to fibrillate during rolling. Drop‐impact measurements recorded a four‐fold increase in fracture toughness. The increased tensile and flexural properties were attributed to both the greater degree of parallel alignment of the mica flakes and the increased molecular orientation in the direction of flow. Such flow orientation methods appear necessary if the full benefit of mica‐flake reinforcement is to be achieved.
Experimental measurement of temperature profiles of molten flowing polymers in a heat exchanger
Authors: Forsyth T. H. , Murphy N. F.
Published: December 2004

Molten polymer was pumped through an isothermal‐wall heat exchanger and the temperature at four radial positions was measured with a hypodermic thermocouple. Temperature profiles were obtained for heating, cooling, and isothermal flow of polyethylene, polystyrene and polypropylene at Graetz numbers from 1.7 to 9.2. The results show large viscous heating effects, particularly for cooling.
Photopolymerization studies: I. Polymers from new photoredox catalyst system
Authors: Rust J. B. , Miller L. J. , Margerum J. D.
Published: December 2004

A new photoredox catalyst is described wherein a phenothiazine dye in its excited state oxidizes a catalyst to a free radical. This free radical efficiently initiates the polymerization of metal acrylates giving rise to polymers which are insoluble in the monomer‐containing medium. The polymers precipitate as insoluble colloidal particles. Optical transmission measurements are used to follow the polymerization and to calculate the number and size of the polymer particles. The effect of various factors such as the nature of the catalyst and the photooxidant, the pH and the temperature of the medium on photopolymerization are described. It is indicated that this new system may be useful for rapid photographic imaging and display applications.
The melting of a polymer on a hot surface
Authors: Stammers E. , Beek W. J.
Published: December 2004

Polymer pellets are frequently melted on a melting grid. The melting capacity of the gird depends on the force with which the bed of pellets is pressed against the grid bars and on the difference between the temperature of the grid bars and the melt temperature. In this investigation, this dependence has been examined, theortically as well as experimentally, for the case where the pellets melt on the top of the grid. Agreement with the theory was found.
Rheological behavior of poly(vinyl chloride) mixture. II. Elastic behavior
Authors: Sieglaff C. L. , Vinson C. G.
Published: December 2004

This study was undertaken in order to understand the effect of low molecular weight additives on the visco‐elastic properties of polymers and, in particular, Diamond Shamrock Corporation's PVC‐40 resin. The viscous response has been reported in Part I of this series (11). This report describes the elastic properties of these mixtures.

A new theoretical analysis of elastic response has been formulated. The direct calculation of stored elastic energy from capillary rheometer measurements is now possible.

The conclusion drawn from both the viscous and elastic response study is that a change in viscosity is not a necessary nor sufficient condition for improved processability. Increased flow is obtained by lowering the viscous losses, but the melt fracture phenomena may be unchanged. In order to completely characterize the compound, both viscous and elastic response must be measured.
The effect of temperature on the delayed yield and failure of “plasticized” epoxy resin
Authors: Ishai O.
Published: December 2004

Epoxy‐Versamid specimens were loaded in tension up to failure at different constant strain‐rates and temperatures. Results revealed three modes of behavior prevailing at different temperature‐strain‐rate regions and associated with brittle, ductile and rubbery failure modes. The ductile region was found to be confined within a narrow band on the temperature‐strain‐rate plane, and is characterized by a yield plateau in the stress‐strain curve and by linear dependence of yield stress on log strain rate and temperature. Yield strain seems to be almost unaffected by strain‐rate, but decreases slightly with temperature rise.

Analysis indicated that experimental data within the ductile region are consistent with Eyring's formulation for non‐Newtonian viscoplastic flows. It leads to the evaluation of the “apparent activation energy” and activation volume for the two epoxy systems tested.

Comparison with previous work indicates that the above parameters as well as yield stress and elastic modulus tend to increase with the decrease of the Versamid content in the resin.
Some physical organic chemistry of aging processes
Authors: Mayo F. R. , Irwin K. C.
Published: December 2004

Work at Stanford Research Institute on the deterioration of polymers is based on the assumption that aging in the light is the most serious problem and that it has at least three components that must be separated if we are to understand the overall process. We have started by measuring the production of free radicals on the polymer when pairs of t‐butoxy radicals are generated from thermal decomposition of di‐t‐butylperoxy oxalate (DBPO) in bulk atactic polypropylene at temperatures approximating service conditions, 35–50°C. Our preliminary experiments have brought out experimental difficulties and have raised more theoretical questions than they have answered. Nevertheless, they show that the efficiency of production of polymer radicals is low and sensitive to the mobilities of the polymer chains. They also suggest that two kinetically different oxidation processes, with different dependencies on rates of initiation and with different susceptibilities to inhibitors, are proceeding simultaneously. The implications of these findings for results of some previous work and for design of improved accelerated tests is considered.
An izod specimen for the simulation of conditions of plastics use
Authors: Vinson C. G.
Published: December 2004

The stress concentration and effective flexure stiffness of an ASTM V‐notch Izod specimen is excessive for most cases of plastics use. A more typical stiffness is that of a thin plate in two dimensional bending. Such can be approached with a larger radius and deeper V‐notch. Instead of a V‐notch, however, a double edge notch with the same radius and thickness at the root is recommended. The special advantage of the double edge notch is that the notch at the back edge reduces the energy loss due to the clamp.
Oxidation of ether linked thermoplastics
Authors: Kelleher P. G. , Gesner B. D.
Published: December 2004

Studies have been made of the heat and light induced oxidation of ether linked commercial thermoplastics using a variety of analytical methods. Our studies on polyoxymethylene, polyphenylene oxide, and its blends with polystyrene are discussed here. Comparisons of heat and light stability are made with thermoplastics previously reported.

Heat aging of polyoxymethylene causes a decrease in elongation and weight as well as volatile product formation and changes in the infrared spectrum indicative of chain scission. Exposure to ultraviolet light accelerates the oxidative reaction. Polyoxymethylene copolymer is more thermally stable but both polymers are rapidly degraded during light aging.

Investigation of the heat and light induced oxidation of polyphenylene oxide shows that oxidation, in either case, leads to a crosslinking reaction resulting in embrittlement of the polymer. The process is difficult to retard. However, oxygen uptake results indicate that blends of polyphenylene oxide and polystyrene can be suitably stabilized against thermal oxidation. Preliminary results indicate that oxidation of these blends proceeds by a chain scission mechanism.
Temperature profiles for polymer melts in tube flow. Part II. Conduction and shear heating corrections
Authors: Kim H. T. , Collins E. A.
Published: December 2004

A temperature probe system to measure radial temperature profile of polymer flowing in a rod die and a method to systematically correct the conduction and the frictional shear effects were developed. Experimental data obtained on a 1‐1/2‐inch extruder using a blow molding compound show that both conduction and frictional shear heating effects are significant in melt temperature measurement and that the radial temperature profiles of the melt in the rod die are influenced by the RPM of the screw and the die‐wall temperature.

The reliability of the temperature data obtained was compared with the solution obtained from the equations of motion and energy. A good agreement between the predicted versus experimental temperature profile exists. For this polymer system, the relationship between local Nusselt number and the velocity parameter could be adequately described with the theory of Van LeeuWen.
The mechanical properties of textile materials as influenced by complexity and rate of testing
Authors: Figucia Frank , Weiner Louis , Laible Roy
Published: December 2004

Fibrous structures of high complexity, as introduced by ply and twist, have been subjected to high and low speed tensile tests. The ratio of strength at high speed to low speed is always greater than one, but decreases with increasing complexity of fibrous structure. The ratio of elongation to break at high speed to that at low speed is always less than one and also decreases with increasing structure complexity. The ability to absorb energy markedly increases with increasing complexity of geometry at low strain rates, characteristic of an Instron. The reasons for this additional energy absorption are discussed. Tests at high rates of strain point out the inability of many complex textile structures to translate their superior energy absorption characteristics to high strain rates. The results obtained and the principles demonstrated are applied to the development of improved materials for use in aerial delivery and ballistic applications.
Factors influencing the thermal oxidation of polyethylene
Authors: Hawkins W. L. , Chan M. G. , Link G. L.
Published: December 2004

The thermal stability of polyethylene containing conventional antioxidants is adversely affected by contact with copper, by certain pigments, and by the addition of a few percent of polypropylene as a processing aid. Polyethylene inhibited with 0.1% of phenolic antioxidants has approximately the same oxidative stability when in contact with a copper surface as the unprotected polymer. A mechanism is suggested to account for the loss of stability in the presence of copper. Pigments vary in the extent to which they adversely affect the oxidative stability of polyethylene. Since several factors may combine to decrease the stability of protected polyethylene compositions, as determined by accelerated tests, it is essential that their contribution under service conditions be determined. The predicted life at temperatures encountered in service is determined by extrapolating accelerated test data to that constant temperature which is calculated to cause the same degree of degradation as would occur during the daily and seasonal temperature cycles encountered in use.
Prospective photosensitization of photopolymerization systems
Authors: Bulloff Jack J.
Published: December 2004

The current art for image‐wise photopolymerization is reviewed and the prospective future developments are briefly assessed. Photomechanics, photoengraving, gravure, lithography, chromophores, dye sensitization, cyanine photopolymerization, bathochromy, and phototanning are covered. Primary photoprocesses, their sensitization by cyanines, Oster photopolymerization, photovinyls, diazo and diazido‐sensitized systems, and secondary photoprocesses are discussed. A bibliography is provided.
Ultraviolet sources for photochemical application
Authors: Wright A. N.
Published: December 2004

This article reviews recent developments in laser and UV source technologies that are potentially useful for photochemical processes and photopolymerization processes in particular. Methods for measurement and calibration of light energy are described. A few publications on newer photochemical processes are also briefly reviewed.
Electrical dissipation factor: Guide to molding quality graphite/resin composites
Authors: Pike R. A. , Douglas F. C. , Wisner G. R.
Published: December 2004

Determination of the change in electrical dissipation factor and capacitance of epoxy/graphite and polyimide/graphite composites during compression molding provides a reproducible means of obtaining low void content composites by permitting accurate application of low pressure at the resin gel point. Of the two parameters, change in dissipation factor was found to be the more sensitive to molecular changes in the resin matrix as well as to variations in solvent content, particularly with the polyimide systems. The simple laboratory equipment required to carry out these electrical measurements is described.
Weathering of fibre reinforced plastics. Progress of dbonding detected in model systems by using fibres as light pipes
Authors: Field S. Y. , Ashbee K. H. G.
Published: December 2004

The progress of interfacial debonding caused by the diffusion of boiling water to glass fibre/polyester resin interfaces has been followed by shining a He‐Ne laser beam along fibres and recording the changes in intensity of the emergent beam. Compared with measurements of electical conductivity arising from the presence of diffused water at the interface, it is concluded that the laser beam method provides more certain evidence of debonding.

The glass/resin interface has also been investigated in partially cured composites and in filled composites, during exposure to boiling water.