bluuebunny/crossref_metadata_2025_split huggingface dataset:
Conformational studies of octalene and its benzo‐derivatives
Authors: Favini Giorgio , Moro Giorgio , Todeschini Roberto , Simonetta Massimo 
Published: December 2004

AbstractTheoretical calculations by the combined empirical force‐field (EFF)‐extended Hückel molecular orbital (EHMO) approach confirm that octalene and benzo‐|c|‐octalene present π‐bond fixation with a common single bond between the cyclooctatetraene and cyclooctatriene fragments, whereas the structure of dibenzo‐|c, j|‐octalene is characterized by a central double bond. The dynamic behavior of these compounds is discussed and the interconversion energy barriers are calculated; the inversion of the cyclooctatriene ring is faster than that of the cyclooctatetraene ring in octalene, but it is slower in benzo‐|c|‐octalene.
Ab initioMO calculations on the acidities of water and methanol, and hydrogen bond energies of the conjugate ions with a water molecule
Authors: Ikuta Shigeru 
Published: December 2004

AbstractThe acidities, deprotonation energies, of water and methanol were calculated by the use of the ab initio self‐consistent‐field (SCF) molecular orbital (MO) method with electron correlation computed by the thirdorder Møller–Plesset perturbation method and configuration interaction with double excitations. Zero‐point vibrational energy correction translational energy change, and the PV work term were included to evaluate the accurate acidities. The calculated acidity difference including these corrections was 7 kcal/mol, which is somewhat smaller than the experimental ones (9.5–12.5 kcal/mol) recently determined. The hydrogen bond energies of the conjugate ions (OH− and CH3O−) with a water molecule were calculated to be 2.3 kcal/mol near the Hartree–Fock limit; this energy only amounts to 25% of the (total) hydration energy difference between the two negative ions. The aqueous solvation effect on the acidity scale was discussed.
Ab initio calculations of the electronic structure of helical polymers
Authors: André J. M. , Vercauteren D. P. , Bodart V. P. , Fripiat J. G. 
Published: December 2004

AbstractAn ab initio self‐consistent‐field (SCF) algorithm taking into account all the features of the one‐dimensional translational periodicity and the helical symmetry is presented. This algorithm includes the long‐range correction to the Coulomb potential and is designed to calculate the band structure of periodic one‐dimensional polymers (planar or helical). Its efficiency in terms of computing time and numerical accuracy is tested via applications on a (LiH)n chain, polyethylene, and four conformers of polypropylene.
Classical conformational analysis of strained organic molecules. I. [l,m,n]Propellanes with l,m,n equal to 2, 3, and 4
Authors: Dodziuk Helena 
Published: December 2004

AbstractClassical conformational analysis was applied to the study of the variation of heats of formation (HOF) and geometrical parameters of small‐ring propellanes. As expected, the calculated HOF is the smallest for [4,4,4] propellane I and the largest for [2,2,2]propellane X. In spite of the different trends exhibited by the strain energy components, the calculated HOF values may be reproduced within a simple additivity scheme. The calculated bridgehead–bridgehead bond length is the longest for [2,2,2]propellane (1.655 Å), lowers with increasing molecular size, and is equal to 1.558 Å for [4,4,4]propellane. In agreement with experimental findings the latter value is significantly larger than the standard value of 1.54 Å typical for unstrained hydrocarbons. Other calculated geometrical parameters as well as HOF values cannot be compared with experimental and/or theoretical results since there is a lack of reliable experimental data. Surprisingly, [2,2,2]propellane was found to be more flexible than the less strained bicyclo[2,2,2]octane.
Unique description of chemical structures based on hierarchically ordered extended connectivities (HOC procedures). V. New topological indices, ordering of graphs, and recognition of graph similarity
Authors: Mekenyan Ovanes , Bonchev Danail , Balaban Alexandru T. 
Published: December 2004

AbstractThe vertex numbering obtained by application of the HOC algorithm can be converted into two sequences of numbers: If each vertex starting with vertex 1 is only counted once, the sums of numberings of adjacent vertices form sequence Si (i = 1−N), while the sums of Si values form sequence Mi (i = 1−N). These two sequences can be used for (i) two new topological indices, ℳ︁ and 𝒩, the latter being of extremely low degeneracy, and the former correlating with boiling points of alkanes; (ii) a criterion based on sequence Si for ordering graphs which possess the same number N of vertices; and (iii) a quantitative measure, also based on sequence Si, for appreciating the similarity or dissimilarity of pairs of graphs. Comparisons with other topological indices, ordering criteria, and similarity measures for graphs show that the newly devised procedures compare favorably with those known previously.
Application of an ion‐fitted pseudopotential to HF, H2O, NH3, BeO, and HCl in a Gaussian lobe basis
Authors: Nguyen T. T. , Raychowdhury P. N. , Shillady D. D. 
Published: December 2004

AbstractThe core potentials for atoms of atomic numer 1–18 fitted to ion spectra by Chang, Habitz, Pittel, and Schwarz have been extended to the molecular case in a Gaussian lobe basis by using a six‐Gaussian (6G‐POT) representation for the exponential factors of the atomic core potentials. In a (9s/5p/1d) basis the 6G‐POT one‐electron energies, dipole moments, and Mulliken charges are improved over a one‐Gaussian potential form for HF, NH3, and H2O; BeO also yields good agreement within 2.6% of the experimental bond length. For HCl, the core potential shows larger errors in the dipole moment (7%) and one‐electron eigenvalues (2%), but a 75% saving in computer time is realized for HCl compared with only about 35% for first‐row systems using the 6G‐POT core potentials. Analytical expressions are given to extend the 6G‐POT method up to s, p, d, f, and g valence shells.
Studies of a proposed mechanism for the reaction between 2H‐pyran‐2‐ones and organomagnesium compounds
Authors: Dreux J. , Lhoste P. , Moreau M. , Royer J. 
Published: December 2004

AbstractPart of the reaction between 2H‐pyran‐2‐ones and organomagnesium compounds has been investigated by means of MNDO and ab initio calculations. Criteria for the mechanism of reaction are provided by the stereoselectivity observed and calculations are consistent with this stereoselectivity. Conformations of the reaction intermediates are given.
Comparison between 6‐31 + bond functions and 6‐31 G* basis sets in UMP calculations of dissociation energies of the first row element compounds. I
Authors: Mach P. , Kysel O. 
Published: December 2004

AbstractA basis set with bond functions (6‐31G + BF) has been tested for its applicability to calculation of dissociation energies of single and multiple bonds by Moeller‐Plesset perturbation theory at the second and third orders. Results have been compared with those calculated in the 6‐31G* basis set. The 6‐31G + BF basis at the MP2 and MP3 levels yields better results than 6‐31G* basis and the time consumption is less as well. Consideration of the bond functions on the bonds neighboring the bond being broken has no significant influence on the dissociation energies either at the SCF or at the MP2 levels. If both reactants and products can be characterized by two‐center bonds, the 6‐31G + BF basis and UMP2 variant of perturbation theory can be recommended for practical calculation of De values, especially for the systems where the use of more exact bases is rather difficult.
Geometrically feasible binding modes of a flexible ligand molecule at the receptor site
Authors: Ghose Arup K. , Crippen Gordon M. 
Published: December 2004

AbstractA very efficient algorithm for determining the geometrically feasible binding modes of a flexible ligand molecule at the receptor site is presented. It is based on distance geometry but maintains the requirements of three dimensions. The distance geometry manipulation can superimpose two bodies without explicitly calculating the necessary rigid rotation and translation. The whole conformation space of a flexible molecule can be efficiently examined by considering only a finite number of conformational points. The method is suitable only when the criterion for superposition is some minimum distance limit. It cannot, however, give the exact distance between two points in two different bodies.
Unique description of chemical structures based on hierarchically ordered extended connectivities (HOC procedures). III. Topological, chemical, and stereochemical coding of molecular structure
Authors: Balaban Alexandru T. , Mekenyan Ovanes , Bonchev Danail 
Published: December 2004

AbstractA topological code is devised on the basis of the unique topological representation of the molecule described in the preceding two parts of this series.1 By adding to the topological code additional chemical information on atoms and/or bonds, as well as stereochemical information, a chemical and respectively stereochemical code (SHOC) are also constructed. The advantages of the new linear codes are that they are convention‐free codes, preserving the symmetry of molecular graph, and easily implemented either manually or by means of computer programs. By concentrating all topological, chemical, and stereochemical information, our code (SHOC) is more compact and more general than the codes based on several separate lists.
Unique description of chemical structures based on hierarchically ordered extended connectivities (HOC procedures). VIII. General principles for computer implementation
Authors: Ralev Nikolai , Karabunarliev Stoyan , Mekenyan Ovanes , Bonchev Danail , Balaban Alexandru T. 
Published: December 2004

AbstractThe computer implementation of the HOC procedures presented in Parts I, II and III of this series is based on the module principle. It contains a main program and seventeen subroutines in Fortran IV. The system is able to provide unique atom numbering and code for all kinds of chemical compounds in their tremendous variety of structures.
Systematic errors in the total energy of molecular wave functions calculated within the PRDDO approximations
Authors: Throckmorton Linda , Marynick Dennis S. 
Published: December 2004

AbstractWe compare calculated total energies for 150 open‐chain molecules using ab initio methodology and the PRDDO approximations. The bulk of the errors implicit in the PRDDO approximations are apparently of a one‐center nature, i.e., they are due to the number and type of atoms in the molecule, and not the details of the molecular geometry. Atomic correction factors are developed which reduce the errors in the calculated total energy of PRDDO wave functions by a factor of eight relative to the ab initio reference calculations. PRDDO calculations on ring and cage compounds are shown to have additional systematic errors in the total energy.
The effect of including polarization functions on the geometrical parameters calculated for pyridine
Authors: Bock Charles W. , Trachtman Mendel , George Philip 
Published: December 2004

AbstractGeometrical parameters for pyridine have been calculated using the 6‐31G, 6‐31G*(5D), 6‐31G**(6D), and 6‐31G(2 × 6D) basis sets. Comparisons are made with a microwave substitution structure and with results of other ab initio calculations reported in the literature. Particular attention is paid to the influence of polarization functions on the magnitude of the ring angle, 〈C6N1C2, which is analogous to the ipso angle in monosubstituted benzene derivatives.
A minimal multiconfigurational technique
Authors: Fernández Rico J. , Paniagua M. , GarcíA De La Vega J. M. , Fernández‐alonso J. I. , Fantucci P. 
Published: December 2004

AbstractA direct minimization method previously presented by the authors is applied here to biconfigurational wave functions. A very moderate increasing in the time by iteration with respect to the one‐determinant calculation and good convergence properties have been found. So qualitatively correct studies on singlet systems with strong biradical character can be performed with a cost similar to that required by Hartree‐Fock calculations.
An all atom force field for simulations of proteins and nucleic acids
Authors: Weiner Scott J. , Kollman Peter A. , Nguyen Dzung T. , Case David A. 
Published: December 2004

AbstractWe present an all atom potential energy function for the simulation of proteins and nucleic acids. This work is an extension of the CH united atom function recently presented by S.J. Weiner et al. J. Amer. Chem. Soc., 106, 765 (1984). The parameters of our function are based on calculations on ethane, propane, n−butane, dimethyl ether, methyl ethyl ether, tetrahydrofuran, imidazole, indole, deoxyadenosine, base paired dinucleoside phosphates, adenine, guanine, uracil, cytosine, thymine, insulin, and myoglobin. We have also used these parameters to carry out the first general vibrational analysis of all five nucleic acid bases with a molecular mechanics potential approach.
Kinetics of particle growth VII. NH4NO3from the NH3‐O3 reaction in the presence of air and water vapor
Authors: Olszyna Kenneth J. , De Pena Rosa G. , Heicklen Julian 
Published: December 2004

AbstractOur earlier work on the formation of particulate NH4NO3 in the NH3O3 reaction at 25°C is extended to include air as a diluent and H2O vapor as an additive. More extensive data at different values of [NH3]/[O3]0 were obtained also, where [O3]0 is the initial O3 concentration. In our earlier work we concluded that NH4NO3 vapor was dissociated to NH3 + HNO3 and that the HNO3 was removed by diffusion to the walls with a rate coefficient kdiff = 0.4 min−1 or by condensation on the suspended particles. Particles were nucleated by 8 NH3HNO3 pairs when their concentration product reached 5.8 × 1027 molec2/cm6 with a rate coefficient knucl of 6.2 × 10−224 cm45/min and removed by coagulation with a rate coefficient kcoag of 1.3 × 10−7 cm3/min. A corrected calculation modifies the number of pairs required to 6–7 with a correspondingly changed value of knucl.With the more extensive data of the present study the indications are that the vapor‐phase NH4NO3 monomer is not dissociated and that its diffusion constant for loss to the walls varies between 0.3 and 0.9 min−1 for different reaction conditions. Nucleation occurs when the NH4NO3 vapor concentration reaches 1.0 × 1012 molec/cm3 via.

where r is 9 and the nucleation rate coefficient knucl is 3 × 10−108 cm24/min. With 5.0 or 9.5 torr of H2O vapor present, there is an excess of particles produced over that expected from this rate coefficient, indicating an additional nucleation step in which H2O vapor participates directly to produce a hydrated salt. The coagulation coefficient of (1.87 ± 0.14) × 10−7 cm3/min found here is in good agreement with that found previously.
Recommendations for measuring second‐order rate constants and kinetic solvent isotope effects on acid‐catalyzed reactions
Authors: Jensen James L. , Carr Michael D. , Yamaguchi Kenneth S. 
Published: December 2004

AbstractKinetic solvent isotope effects (KSIE) were measured for the hydrolyses of acetals of benzaldehydes in aqueous solutions covering the pH (pD) range of 1–6. For p‐methoxybenzaldehyde diethyl acetal, k/k = 1.8–3.1, depending on the procedure used to calculate the KSIE and on the pH (pD) range used as the basis for k(k). It is shown that this variation is an experimental artifact, and is a characteristic of KSIE measurements in general. It is recommended that k be calculated from a least‐squares fit of data to the equation kobs = k[L+], and that the KSIE be reported as k/k. The limitation remains, however, that the KSIE measured for a variety of substances over quite different pH (pD) ranges may not be comparable to more than ℜ20%. The source of these observations is discussed in terms of small changes in the activity coefficient ratios (a specific salt effect), including the solvent isotope effect on the activity coefficient ratio [eq. (3)].
Rate constants for the reactions of alkyl radicals with 1,4‐cyclohexadiene
Authors: Hawari J. A. , Engel P. S. , Griller D. 
Published: December 2004

AbstractAn electron paramagnetic resonance (EPR) technique was used to show that simple alkyl radicals readily abstract hydrogen from 1,4‐cyclohexadiene. Rate constants for the reaction were ca. 104–105 M−1 s−1 at 300 K and activation energies 5–7 kcal mol−1. For the stabilized radicals, allyl and benzyl, the rate constants were <102 M−1 s−1 at 300 K. The data suggest that 1,4‐cyclohexadiene could be used as an effective trap to probe rearrangement reactions of carbon centered radicals and biradicals.
Kinetic studies on the oxidation of carbon monoxide over pure, reduced, and doped indium sesquioxides
Authors: Lee Sung Han , Heo Gweon , Kim Keu Hong , Choi Jae Shi 
Published: December 2004

AbstractKinetic studies on the oxidation of carbon monoxide have been carried out between 413 and 473 K at different partial pressures of carbon monoxide and oxygen by means of the static method using vacuum‐activated, hydrogen‐reduced, and NiO‐doped In2O3 semiconductors as catalysts. A strong carbon dioxide inhibiting effect is observed. The experimental data satisfactorily fit an equation derived by assuming the controlling step to be the adsorption of gaseous oxygen on the surface of catalyst. CO and CO2 adsorb on the lattice oxygens (O), while O2 adsorbs on the oxygen vacancies (V) formed by vacuum‐activation, H2‐reduction, and NiO‐doping of In2O3. When CO2 formed during the reaction is removed by means of liquid nitrogen trap, the oxidation is found to be first‐order with respect to CO and to be half‐order with respect to O2. The concentration of oxygen vacancy in the solid catalyst is shown to be the controlling factor for the oxidation of carbon monoxide. A possible reaction mechanism can be explained by the n‐type character of In2O3 and proposed from the experimentally obtained kinetic data and conductivities.
Kinetics of the acyl migration in some adducts of acyl‐1, 4‐benzoquinones: Part I
Authors: Dabbagh Abdul Majeed , Al‐Hamdany Raad 
Published: December 2004

AbstractThe rates of migration of acetyl, propionyl, isobutyryl, and benzoyl substituent in Diels‐Alder adducts of acyl‐1, 4‐benzoquinones, (enediones), (I), to form the corresponding 2‐acyl hydroquinones, (II), in 50% (v/v) ethanolic pyridine have been studied in detail. Rate dependence on pyridine concentration has been established and maximum rate was found at pyridine a volume percent of 25 in an ethanolic reaction mixture. The study indicated first‐order dependence on reactants and fractional dependence on pyridine concentration. The effect of substituents of the acyl group on the rate showed that electronic factors play an important role on the stability of the transition state. A plausible mechanism, which is consistent with the results has been postulated and suitable rate law in consonance with the postulated mechanism have been derived.
Reinigung von Metalloberflächen
Authors: Schultze G. 
Published: December 2004

AbstractFür die Entfernung von Oberflächenverunreinigungen von Metallen stehen heute verschiedenartige Behandlungsmethoden zur Verfügung, die kurz umrissen wurden. Die chemische Reinigung dürfte an Bedeutung gewinne, und zwar um so mehr, je allgemeiner die Bedeutung der sauberen Metalloberfläche für den weiteren Einsatz erkannt wird. In immer höherem Maße wird der sauren Beizung der Vorzug gegeben und trotz der bisher erreichten überraschenden Ergebnisse dürfte bei systematischer Bearbeitung der Schutzeffekt noch wesentlich umfassender gestaltet werden können.
Effects of orientation on dynamic mechanical properties of ABS
Authors: Daane J. H. , Matsuoka S. 
Published: December 2004

AbstractMechanical properties of a composite of glassy and rubbery polymers, ABS, were studied. Orientation induces two effects: (1) the intensity of the loss dispersion attributed to the rubbery component initially increases, then is reduced, and (2) a new loss peak at a temperature approximately 40°C below the original loss peak for the rubbery component appears. Assuming that this new peak is due to the dilated rubber particles embedded in the glassy matrix under strain, it is judged from the theory of Newman and Strella that a built‐in strain of 6% would be sufficient to cause this shift in Tg. Above the Tg of the rubbery component, the strain energy required to break increases with temperature. The reinforcement through incorporation of rubber particles is accomplished by enhancing the ability of the glassy matrix to draw on a local, microscopic scale. Such a local cold drawing may well be identified with crazing of glassy polymers.
Viscous dissipation in capillary flow of rigid PVC and PVC degradation
Authors: Morrette R. A. , Gogos C. G. 
Published: December 2004

AbstractThe steady state, non‐isothermal behavior of rigid polyvinyl chloride melt, flowing in capillaries of circular cross‐section, was investigated by solving, with the aid of a digital computer, the momentum and energy balance equations. It was assumed that the polymer melt can be described by the “Power Law” constitutive equation. The shear rate, temperature and pressure dependent properties of the fluid were obtained experimentally. The effects of the thermal degradation of PVC on its viscosity, were also introduced in the equations of momentum and energy.The velocity, temperature and pressure profiles, obtained for both adiabatic flow and flow through a tube of constant wall temperature, indicate that considerable heating of the melt, due to viscous dissipation, can be achieved at moderate flow rates. Thermal degradation occurs in the capillary under certain conditions of temperature history and residence time of the fluid. The results of this work are in fair agreement with experimental results in this area.
The interfacial stresses in particulate composite systems
Authors: Matonis V. A. 
Published: December 2004

AbstractIt is postulated that property gradients exist at the interface forming an “interphase” between the insert and the matrix. Such gradients might be caused by partial solubility and diffusivity of the two phases comprising a composite. For simplicity, the “interphase” is replaced by a shell exhibiting averaged properties (Eshell = 1/2 Einsert + 1/2 Ematrix, etc.). An exact linear elastic solution was employed to evaluate stress fields throughout all regions. The possible effect of the “interphase” and its thickness on several physical properties of the composite are evaluated in terms of the particular stress fields.
Folded chain model of highly drawn polyethylene
Authors: Peterlin A. 
Published: December 2004

AbstractNeither the fringed‐micelle, fringed‐fibril or extreme folded‐chain models explain the morphological and mechanical properties of highly drawn polyethylene. The modified folded‐chain model, assuming that a substantial fraction (between 5 and 30 percent) of the molecules do not fold back at the crystal surface but go through the “amorphous” surface layer and enter the next crystal, avoids the insufficiencies of the above mentioned models.The elastic modulus and tensile strength of drawn polyethylene, both increasing with draw ratio, are to a large extent the consequence of the molecules bridging the quasi‐amorphous layers and interconnecting the folded‐chain lamellae oriented more‐or‐less perpendicular to the machine direction. The folds create enough space for the accommodation of more‐or‐less extended tie molecules in the quasi‐amorphous layers between the lamellae. Electron microscopy and calorimetry of samples as drawn, annealed and/or etched with fuming nitric acid support the model.