bluuebunny/crossref_metadata_2025 huggingface dataset:
Morphological and mechanical properties of nascent polyethylene fibers produced via ethylene extrusion polymerization with a metallocene catalyst supported on MCM‐41 particles
Authors: Ye Zhibin , Zhu Shiping , Wang Wen‐Jun , Alsyouri Hatem , Lin Y. S.
Published: September 2003

Polyethylene (PE) fibers were prepared by ethylene extrusion polymerization with an MCM‐41‐supported titanocene catalyst. The morphological and mechanical properties of these nascent PE fibers were investigated. Three levels of fibrous morphologies were identified in the fiber samples through an extensive scanning electron microscopy study. Extended‐chain PE nanofibrils with diameters of about 60 nm were the major morphological units present in the fiber structure. The nanofibrils were parallel‐packed into individual microfibers with diameters of about 1–30 μm. The microfibers were further aggregated irregularly into fiber aggregates and bundles. In comparison with commercial PE fibers and data reported in the literature, the individual microfibers produced in situ via ethylene extrusion polymerization without posttreatment exhibited a high tensile strength (0.3–1.0 GPa), a low tensile modulus (3.0–7.0 GPa), and a high elongation at break (8.5–20%) at 35 °C. The defects in the alignment of the nanofibrils were believed to be the major reason for the low modulus values. It was also found that a slight tensile drawing could increase the microfiber strength and modulus. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2433–2443, 2003
Complete miscibility of ternary aryl polyesters demonstrating a new criterion and horizon for miscibility characterization
Authors: Woo E. M. , Kuo Yu‐Hsiao
Published: September 2003

A ternary miscible blend system comprising only crystallizable aryl polyesters [poly(ethylene terephthalate), poly(trimethylene terephthalate), and poly(butylene terephthalate)] was characterized with the criteria of thermal analyses, microscopy, and X‐ray characterizations. The reported ternary miscibility (in the quenched amorphous state of blends of the three aryl polyesters) was truly physical and under the condition of no chemical transesterifications; this justified that transesterification was not a necessary condition for miscibility in polyester blends in this case. This study further proposed and tested a novel concept of a new criterion for miscibility characterization for polymer blends of only crystallizable polymers. A single composition‐dependent cold‐crystallization‐temperature (Tcc) peak in blends of only semicrystalline polymers was taken as an indication of an intimate mixing state of miscibility. The theoretical background for establishing the single composition‐dependent Tcc peak as a valid miscibility criterion for crystallizable polymer blends was examined. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2394–2404, 2003
Mechanical properties of block copolymer vesicle and micelle modified epoxies
Authors: Dean Jennifer M. , Grubbs Robert B. , Saad Walid , Cook Robert F. , Bates Frank S.
Published: September 2003

Block copolymers with and without reactive functionalities can improve fracture resistance in brittle epoxies even when added in relatively small amounts (<5 wt %). At certain compositions, amphiphilic block copolymers spontaneously self‐assemble into vesicles, spherical micelles, or wormlike micelles in thermoset resins, and these morphologies are retained with the full curing of the resins. The addition of such block copolymers leaves the glass‐transition temperature of these blends relatively unchanged, whereas the fracture resistance increases up to a factor of 3.5 for the vesicle‐modified blends. For epoxies modified with block copolymers self‐assembled into a spherical geometry (vesicles or spherical micelles), the fracture resistance scales with the ratio of the interparticle distance to the average vesicle (or spherical micelle) diameter (Di/Dp) and increases as this quantity is reduced. Greater adhesion between the vesicle and epoxy resin improves the fracture resistance only at higher values of Di/Dp, at which the materials are more brittle. Debonding and subsequent matrix plastic deformation are identified as the toughening mechanisms in these blends. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2444–2456, 2003
Strong diffuse scattering and lamellar morphologies of syndiotactic polystyrene: Polymorphic effects
Authors: Wang Chi , Liao Wei‐Po , Cheng Yong‐Wen
Published: September 2003

Syndiotactic polystyrene (sPS) samples melt‐crystallized into neat α″‐ hexagonal and β′‐orthorhombic modifications were prepared at various temperatures thoroughly for extensive morphological studies. The lamellar morphologies of the as‐prepared sPS samples were investigated with small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM). For SAXS measured at 25°C, a barely observed scattering hump was detected for β′‐form sPS, whereas no discernible scattering feature was found for α″‐form sPS because of a small difference in the electron density between the crystalline and amorphous phases. For increased scattering contrast and strength, SAXS was carried out at 150°C so that more reliable morphological parameters would be obtained. In addition to the enhanced scattering peak relevant to the lamellar features, strong diffuse scattering near the beam stop was observed for both α″‐ and β′‐form sPS samples. The contribution of the diffuse scattering at low q regions (where q is the scattering vector) was rather prominent, obscuring the precise position of the scattering peak. On the basis of the Debye–Bueche theory, the strength and inhomogeneity length were derived to render the diffuse scattering. After the subtraction of the diffuse scattering from the observed intensities, scattering intensities exclusively associated with the lamellar features were obtained. Lamellar thicknesses were further derived from the one‐dimensional correlation function of the modified intensities, and a good agreement was reached in comparison with TEM results. From exhaustive TEM observations on the RuO4‐stained samples, long and parallel lamellae were readily observed in β′‐form sPS. However, relatively irregular packing of lamellar stacks with short lateral dimensions was detected in the as‐prepared α″‐form sPS, leading to the absence of spherulitic birefringence under polarized optical microscopy. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2457–2469, 2003
Quantification of preferential orientation in conjugated polymers using X‐ray diffraction
Authors: Breiby Dag W. , Samuelsen Emil J.
Published: September 2003

The varying degree of preferential orientation observed in films of poly(alkylthiophene)s obtained by solution casting from different solvents has been studied quantitatively with X‐ray diffraction. The (uniaxial) orientational distribution function is obtained after some important instrumental corrections that are described in detail. Orientational distribution functions of an exponential type are found to give an adequate description with only a few fitted parameters, unlike Legendre series fits, which require longer series. An attempt has been made to relate the anisotropy to the chemical and physical properties of the solvents used. The anisotropy seems to be higher for samples cast from solvents with low boiling points and, correspondingly, high vapor pressures. Also, the average crystallite size is solvent‐dependent. The characteristic dimension of the crystallites in the a‐axis direction appears to be longest for crystallites with the a axis parallel to the film normal. Along the b axis, that is, the π‐stacking direction, the crystallite dimension is shorter than along the a axis. Also, the amorphous regions of the films exhibit anisotropy. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2375–2393, 2003
Aggregation structure and mechanical properties of functionally graded polyurethane elastomers
Authors: Furukawa Mutsuhisa , Hamada Yukinori , Kojio Ken
Published: September 2003

Functionally graded polyurethane elastomers (FGPUEs) were prepared with two molds fixed at different temperatures (30 and 150 °C). The effects of the molar ratio of the curing agent (60/40, 75/25, or 97/3 1,4‐butane diol/1,1,1‐trimethylol propane) and the molecular weight of the polymer glycol (number‐average molecular weight = 2000 or 3000) on the molecular aggregation state and mechanical properties of the FGPUEs were investigated with differential scanning calorimetry, polarized optical microscopy, dynamic viscoelastic measurements, and tensile tests. The aggregation state of the FGPUEs was changed continuously from the one side (lower temperature side) to the other side (higher temperature side); for example, the glass‐transition temperature gradually increased in this direction. Also, the number of spherulites formed in the FGPUEs increased in the same manner. In the mechanical tests, the tensile strength and elongation at break of the lower temperature side were higher than those of the higher temperature side. This was correlated with the strong phase separation of the lower temperature side. The poly(oxytetramethylene glycol)‐based FGPUE with a chain extender of 75 wt % showed the largest degree of the temperature gradient. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2355–2363, 2003
Finite‐cluster approximation for deep‐level impurities in silicon carbide
Authors: Souza Sarai S. , Matos Maria
Published: September 2003

A series of 12 silicon carbide finite clusters has been investigated to determine a possible model for transition‐metal deep‐level impurities in several Si sites of the material. All electronic structure calculations were done using the extended Hückel method. By taking account of boundary condition effects upon the electronic structure of the pure and V‐substitutional systems, it was found that the cluster series could be classified into two groups: a large‐gap and a small‐gap group. The small‐gap group arises from spurious border Si‐3p levels that appear inside the forbidden gap. It was consequently suggested that large‐gap clusters are more suitable for describing the impurity. It is also suggested that possibly new V, Si, and C parameters could lead to both a good description of the pure and doped material. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 88–96, 2003
Inversion of simulated positron annihilation lifetime spectra by moving boundary subspaces
Authors: Viterbo V. C. , Neves J. L. , Braga J. P. , Monteiro R. P. G. , De Magalhães W. F.
Published: September 2003

The retrieval of the density function for the positron annihilation lifetime spectrum is obtained from simulated data using the damped singular value decomposition. Two filters factors were discussed, for noise and noiseless data, and under the L curve criterion. The obtained density function is exact in the absence of noise. When noise is considered, the predicted peaks are at positions λ1 = 0.4167 ns−1, λ2 = 2.418 ns−1, whereas the exact results are λ1 = 0.5250 ns−1 and λ2 = 2.538 ns−1. The computed areas for these peaks are also in fair agreement when compared with the theoretical results. In this case, a filter factor that takes into account all the singular values was found appropriate. The current study shows that the damped singular value decomposition approach can be used to invert real laboratory data. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 97–102, 2003
Numerically generated DVR and generator coordinate approximation for calculating rotational energies of H and H2O
Authors: Liu Chao‐Ping , Soares Neto J. J.
Published: September 2003

The numerically generated Discrete Variable Representation (DVR), a recently formulated special type of DVR, is tested for its accuracy and efficiency by applying it to calculate the rotational energies of the triatomic system H for the higher excited states of L = 2, even parity, and L = 3, even parity, where L is the angular momentum of the molecule, and of the triatomic system H2O for the first excited state L = 1, even parity. The description of the rotation of the molecule is done using the generator coordinate approximation (GCA). Comparison of the results with other results show the accuracy and efficiency of the numerically generated DVR. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 103–111, 2003
Determination of main structural compositions of nanoalloy clusters of CuxAuy (x + y ≤ 30) using a genetic algorithm approach
Authors: Lordeiro R. A. , Guimarães F. F. , Belchior J. C. , Johnston R. L.
Published: September 2003

The structures of various compositions of nanoalloy clusters CuxAuy (x + y ≤ 30) were determined using the recently developed molecular genetic algorithm. For compositions up to 18 atoms the algorithm found the global minima in all five runs. For clusters with less than 12 atoms, pure Au clusters are most stable. As the total number of atoms increases, clusters with one, two, three, or four Cu atoms become most stable. It has been observed for larger clusters that there is a tendency for structures based on Cu cores surrounded by Au atoms. The calculated second differences in binding energies are in fair agreement with experimental mass spectra for some nuclearities. Finally, the extrapolated cohesive energies for nanoalloys ((Cu3Au)m, (CuAu3)m, and (CuAu)m) gave an average relative error of 11%, compared with the bulk alloys, whereas for the nanoclusters (AuN and CuN) the error was less than 6%. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 112–125, 2003
An AM1 study on the electron‐donating and electron‐accepting character of biomolecules
Authors: Honório K. M. , Da Silva A. B. F.
Published: September 2003

Energy of the highest occupied molecular orbital (EHOMO) and energy of the lowest unoccupied molecular orbital (ELUMO) are very often related to the electron‐donating (EHOMO) and electron‐accepting (ELUMO) character of a molecule. In this work AM1 calculations for the EHOMO and ELUMO of 15 biomolecules are performed with the aim to obtain a scale for the electron‐donating and electron‐accepting character of biomolecules, as they can interact in the biological environment through a charge‐transfer mechanism. From the EHOMO and ELUMO values obtained with the AM1 semiempirical method, we were able to classify the 15 compounds studied into four groups: A (good electron‐donor molecules), B (good electron‐acceptor molecules), C (moderate electron‐donor molecules), and D (compounds that behave as both electron‐donor and electron‐acceptor molecules). © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 126–132, 2003
Theoretical study on the stereochemistry of intramolecular hetero Diels–Alder cycloaddition reactions of azoalkenes
Authors: Alves C. N. , Romero O. A. S. , Da Silva A. B. F.
Published: September 2003

The molecular mechanism for the intramolecular hetero Diels–Alder cycloaddition reactions of azoalkenes has been studied by using quantum mechanical calculations at the density functional theory (DFT) Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP) and second‐order Møller–Plesset (MP2) theory levels with the 6‐31G* basis set. The molecular mechanism corresponds to a concerted mechanism with a slightly asynchronous bond‐formation process where the formed internal bond (N4–C5) obtained with DFT is longer than the peripheral bond (C1–C6) formed in the transition state. Otherwise, the (C1–C6) bond obtained with MP2 is longer than the (N4–C5) bond formed in the transition state. Solvent (dichloromethane) effects have been considered with the polarizable continuum model (PCM). The MP2 calculations predicted an endo kinetic preference, whereas the DFT ones rendered endo/exo transition states that are practically isoenergetic. According to our analyses, the dispersion interaction (which stabilizes the endo orientation through π–π secondary contacts) and the solvent effects (which favor the exo addition) are the factors governing the stereoselectivity of the process. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 133–136, 2003
Theoretical modeling of alkynes
Authors: Rosseto Renato , Vargas Maria Domingues , Del Nero Jordan
Published: September 2003

In this work we report a semiempirical investigation of the first excited states and of the spectroscopic properties of a series of substituted alkynes. The analysis of the calculated and experimental absorption spectra and theoretical emission spectra resulting from semiempirical/configuration interaction calculations gives us some insight as to how chemical modifications of these molecules could affect the nonlinear optical properties of alkynes. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 137–143, 2003
Gaussian basis sets for polyatomic molecules containing first‐ and second‐row atoms
Authors: Pires José M. , Jorge F. E.
Published: September 2003

Contracted Gaussian basis sets plus polarization functions generated from the improved generator coordinate Hartree‐Fock (HF) method are used to calculate some physical and chemical properties of the following molecules: CH4, NH3, H2O, SiH4, PH3, and H2S. Ground‐state HF total energies and, at the HF and second‐order Møller–Plesset levels, geometrical parameters, harmonic vibrational frequencies, and electric dipole moments are evaluated and compared with the corresponding experimental values and with those calculated with the 6‐311G** basis sets. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 144–148, 2003
Quantum and classical study of vibrational states of H and H molecules
Authors: Silva Washington B. , Corrêa Eberth de A. , Acioli Paulo H. , Gargano Ricardo
Published: September 2003

In this work, we present calculations of the vibrational energy levels of the H and H systems using the correlation function quantum Monte Carlo (CFQMC) and normal model analysis. The classical results are a qualitative first approximation of the normal modes. The results of the CFQMC calculations show the importance of the quantum effects as well as anharmonicity in these systems. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 149–152, 2003
Dynamics of polarons and bipolarons with interchain coupling in conjugated polymers
Authors: Da Silva Pinheiro Cleber , E Silva Geraldo Magela
Published: September 2003

The interchain and interactions of two parallel chains are studied numerically through the dynamics of charged polaron and bipolaron in the presence of an electric field by using the Pariser–Parr–Pople model combined with the Su–Schrieffer–Heeger model. The electron–lattice coupling and the Brazovskii–Kirova‐type symmetry‐breaking interaction are introduced in the current model. The electric field is introduced in terms of a time‐dependent vector potential that is included in the Hamiltonian through a Peierls substitution of the phase factor to the transfer integral. The charge‐transfer probability between charged polarons and bipolarons belonging to neighboring chains is determined by the numerical resolution of the equations of motion within the unrestricted Hartree–Fock approximation. The effects of confinement on the polaron and bipolaron motion are determined. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 153–158, 2003
Quasiclassical trajectory calculations of isotopic reactions Na + XF → NaF + X (X = D, T and Mμ) on two different potential energy surfaces
Authors: Vilela Alessandra F. A. , Gargano Ricardo , Mundim Kleber C. , Soares Neto J. J.
Published: September 2003

Detailed reactive cross‐section calculations (J = 0) of the isotopic reactions Na + XF → NaF + X (X = D, T and Mμ) using classical trajectories at 10 collision energy values are done using two different potential energy surfaces (PES): the generalized simulated annealing (GSA) PES and the bond order 5 (BO5) PES. This study shows that the quality of the GSA PES is comparable to that of the BO5 PES and that the reactivity of the Na + HF → NaF + H reaction is a sensitive function to the mass of isotopic variants of the hydrogen atom, as was expected. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 159–163, 2003
Interaction of MSn (MRu, Rh, Pd) dimers with CH2 and CF2: A density functional study
Authors: Xavier Eder Severino , Duarte Hélio Anderson
Published: September 2003

The importance of Sn to improve the performance of noble metal‐based catalysts has been extensively discussed in the literature. However, a detailed discussion about the effect of Sn on the electronic structure of the metals has not been reported so far. In this work, density functional theory (DFT) calculations have been performed on the M2 and MSn (MRu, Rh, Pd, Sn) dimers and their interaction with CH2 and CF2 carbenes. The electronic structure, geometry, and harmonic frequencies are reported and compared with the available experimental data and to the previous published high level ab initio and DFT calculations. The calculated values of M2 are in good agreement with the experimental and theoretical results. The exception is the binding energy of the Sn dimer, which can be different from the experimental estimates as much as 16 kcal · mol−1. It has been stressed that this difficulty is related to the atomic energy reference used to estimate the binding energy. Transition metal elements are still a difficult task for DFT methods. Calculations on MSnCH2 and MSnCF2 species with different multiplicities and conformations have been performed. Bonding analyses on the MSn metal dimers showed that carbenes prefer to adsorb in bridge sites favoring a donating/backdonating mechanism of interaction that is enhanced with the presence of the Sn atom. The MSnCH2 binding energy is predicted to be 40% smaller than the MSnCF2 binding energy, following the same trends observed for the first‐row transition‐metal MCH2 and MCF2 complexes. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 164–176, 2003
Density functional theory test study on the CO ···· He dimer
Authors: Salazar Mary C. , Paz José L. , Hernández Antonio J. , Manzanares Carlos , Ludeña Eduardo V.
Published: September 2003

In the current paper we have investigated the bonding properties of the weakly bound van der Waals CO ···· He dimer by means of the Perdew–Wang91 exchange and Perdew–Wang correlation (PWPW) and Adamo and Barone's Becke‐style one‐parameter functional using the modified Perdew–Wang91 exchange and Perdew–Wang correlation (mPW1PW) exchange‐correlation functionals in the framework of the supermolecule approach, using density functional theory local‐spin‐optimized atom‐centered basis sets, complemented with bond functions optimized at the mPW1PW level of theory. These calculations show that the use of the mPW1PW functional using bond functions gives a realistic representation of the interaction‐energy potential for this van der Waals molecular complex, as compared with reference Møller–Plesset perturbation theory calculations. In contrast, the PWPW functional is unable to describe the bonding properties of the system, and all values obtained with this functional approximation are considered out‐of‐scale compared with the rest of the calculations presented in this contribution. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 177–183, 2003
Gaussian basis sets for low‐lying excited states of the atoms from He to Kr
Authors: Neto A. Canal , Rodrigues B. L. , Jorge F. E. , De Castro E. V. R.
Published: September 2003

The generator coordinate Hartree–Fock method is used to generate Gaussian basis sets for low‐lying excited states of the neutral atoms from He (Z = 2) to Kr (Z = 36). Our excitation and orbital energies are compared with the corresponding results obtained with numerical Hartree–Fock calculations. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 184–189, 2003
Highly accurate Gaussian basis sets for the atoms from H through Xe
Authors: Librelon P. R. , Jorge F. E.
Published: September 2003

An improved generator coordinate Hartree–Fock (IGCHF) method is used to generate highly accurate Gaussian basis sets (GBSs) for the atoms from H (Z = 1) through Xe (Z = 54). In this method the Griffin–Hill–Wheeler–Hartree–Fock equations are integrated using the integral discretization technique. From these basis sets the ground‐state HF total energies are calculated and compared with the corresponding values obtained with GBSs of the same size, generated with the original generator coordinate Hartree–Fock method, and with a numerical HF method. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 190–193, 2003
Investigation of the quality of standard sulfur basis sets for molecules using a new generator coordinate method approach: Application to 2,2′‐bithiophene
Authors: Duani Helena , Duarte Hélio A. , De Almeida Wagner B.
Published: September 2003

In this work we have applied the generator coordinate method (GCM) to analyze the quality of the standard 6‐311G** and 6‐311++G** Pople's basis sets for the sulfur atom at the Hartree–Fock level of theory. When using the GCM theory, it is necessary to uncontract the basis set to obtain information about the quality of the basis set. To study large molecules such as bithiophene, for which it is difficult to use uncontracted basis sets at the ab initio level, we have developed a technique that makes possible the use of contracted basis sets within the GCM theory. To test such an approach, we have studied small molecules containing the sulfur atom, such as H2S and SCl2. The results show the suitability of using contracted basis sets for these systems with the GCM theory. In the study of contracted basis sets for the bithiophene molecule, we proposed and tested a modified basis set containing additional diffuse functions on the sulfur atom in the light of the GCM approach. The current study enables for the first time the assessment of the quality of atomic basis sets inside a molecular environment. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 194–204, 2003
Investigation of the excited states of resveratrol and related molecules
Authors: Del Nero Jordan , De Melo Celso P.
Published: September 2003

Resveratrol, a natural product derived from grapes, is a phytoalexin with antioxidative activity. In this work we present initial results of an ongoing study of the geometry of the ground state and of the first two (singlet and triplet) excited states of resveratrol and the structurally related compounds diethyl‐stilbestrol‐stilbene and trans‐stilbene. The molecular geometries were optimized both at the semiempirical and ab initio (restricted Hartree–Fock/6‐31G*) levels, and the corresponding absorption spectra were calculated using the intermediate neglect of differential overlap/ spectroscopically parameterized with interaction of configurations (INDO/S‐CI) methodology. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 213–218, 2003
Insulator‐to‐metal transition on polyselenophene
Authors: Marçal Nei , Laks Bernardo
Published: September 2003

In this work we calculate the density of states (DOS) for long, finite one‐dimension polyselenophene (PSe) chains with an ordered and a disordered distribution of bipolaron defects. The theoretical model adopted is a combination of the simple Hückel model in which the compressibility of the σ framework is explicitly taken into account. The negative‐factor counting technique and the inverse iteration method were used to find the electronic DOS and the wavefunctions, respectively. Our results show the presence of extended (conducting) states at the Fermi level. This could be explained by the semiconductor–metal transition in highly doped PSe, according to the macroscopic electrical conductivity observed experimentally. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 230–236, 2003
Elastic and rotational excitation cross sections for electron–nitrous oxide collisions
Authors: Lee M.‐T. , Iga I. , Brescansin L. M. , Machado L. E.
Published: September 2003

We present a theoretical study on electron–N2O collision in the low energy range. More specifically, we report calculated elastic and rotationally inelastic differential, integral, and momentum transfer cross sections in the 1.5–30‐eV range. An optical potential is used to represent the electron–molecule interaction. The Schwinger variational method is used to solve the scattering equations. In addition, the adiabatic‐nuclei‐rotation approximation is applied to calculate rotational inelastic cross‐sections. The comparison between the calculated results and the available experimental and other theoretical data in the literature is encouraging. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 260–266, 2003
Influence of nonbonded interactions in the kinetics of formation of chalcogenol esters from chalcogenoacetylenes
Authors: Ramalho Teodorico C. , Martins Tales L. C. , Eduardo Luiz , Borges Pizarro , Figueroa‐Villar José Daniel
Published: September 2003

The influence of nonbonded interactions in the kinetics of formation of chalcogenol (thiol and selenol) esters from chalcogenoacetylenes was studied by molecular modeling. Using semiempirical and density functional theory methods it was possible to explain the differences between the reaction rates for the analogous sulfur and selenium chalcogenoacetylenes as well as evaluate the structural and electronic effects (nonbonded interactions) on the formation of the esters. The differences in the reaction rates can be explained in terms of the carbocation stabilization by the chalcogen atom. It is proposed that these differences are due to the differences in the intensity of the dominant interaction π/nY between the nonbonding orbitals of sulfur and selenium with the vacant orbital of carbon in the cationic transition state. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 267–273, 2003
Relativistic method to get analytic solutions for many‐body problems: Application to the H molecule
Authors: Nascimento D. L. , Fonseca A. L. A.
Published: September 2003

We present a method for solving many‐body problems that allows the usage of Klein‐Gordon–like equations and in which it is not necessary to take into account the spin of the particles, whose effect appears through orbital angular momentum. Hence, the method deals only with ordinary wavefunctions instead of the more complicated spinor wavefunctions of Dirac's theory. The general wave solutions are found to possess a very close resemblance to the corresponding classical solutions, which allow the usage of simple mechanic analogies to deal with the many‐body problem. The method is applied to the H molecule, yielding the correct value for the ground‐state energy and also an approximation for the associated vibrational energies. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 274–280, 2003
Relation between antioxidant activity and electronic structure of phenols
Authors: Lavarda F. C.
Published: September 2003

We describe a new physicochemical descriptor of the antioxidant activity of phenols, the energy difference between the two highest occupied molecular orbitals, which we believe will improve quantitative structure–activity relationship studies about these compounds. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 219–223, 2003
The use of neural networks for fitting potential energy surfaces: A comparative case study for the H molecule
Authors: Rocha Filho T. M. , Oliveira Z. T. , Malbouisson L. A. C. , Gargano R. , Soares Neto J. J.
Published: September 2003

The fitting of ab initio electronic energies of polyatomic molecules for different nuclear configurations is an active field in quantum chemistry and is an important step in the study of chemical reaction dynamics and for the determination of rovibrational spectra. The choice of a good‐fitting function and the decision as to which geometries are relevant for the problem remains a matter of feeling as a large number of ab initio points of good quality usually involves prohibitively large amounts of CPU times. More recently, the use of neural networks has drawn some attention for fitting potential energy surfaces (PES). Neural networks are generic function approximators for any required accuracy and are therefore well suited for fitting many‐dimensional PES. In this work we present a comparative study for the ground state PES of the H3+ molecule obtained fitting state‐of‐the‐art ab initio points. The PES is obtained using both a neural network and a polynomial function in Morse‐type symmetry‐adapted coordinates. The quality of the surfaces is asserted by computing the associated rovibrational spectra. The resulting energies are compared with known experimental results. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 281–288, 2003
A QM/MM hybrid simulation of 5‐hydroxytryptophan in solution
Authors: Amaral M. S. , Loos M. , Ito A. S.
Published: September 2003

The nonnatural amino acid 5‐hydroxytryptophan (5OHW) has been proposed as a fluorescence probe for protein structure, function, and dynamics studies. Relative to the natural fluorophore, tryptophan, 5OHW has a red‐shifted absorption edge, which permits selective photo excitation of 5OHW in the presence of W residues. In this work, we present results of quantum mechanics/molecular mechanics (QM/MM) hybrid simulation of a 5OHW zwitterionic molecule in a box of water, using the computational package THOR. The 5OHW zwitterion is described quantum chemically, applying the AM1 semiempirical Hamiltonian while the water molecules are treated classically by the extended simple‐point–charge model. The behavior of the dihedral angles χ1 and χ2 is discussed in terms of the exponential fluorescence decay in aqueous solution. Theoretical optical absorption spectra, obtained by INDO/S‐CIS semiempirical method either in vacuum or in the continuum model using an ellipsoidal cavity are discussed and compared with experimental data and recent results of INDO/S‐CIS semiempirical absorption spectra calculated from optimized geometries obtained with ab initio Hartree–Fock 4‐31G in gas phase. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 289–294, 2003
Pattern recognition and neural networks applied to structure–activity relationships of neolignans tested against Leishmania amazonensis using quantum chemical and topological descriptors
Authors: Costa Maria Cristina Andreazza , Barata Lauro Euclides Soares , Bergmann Bartira Rossi‐ , Takahata Yuji
Published: September 2003

Pattern recognition (PR) and neural networks (NN) were applied to structure–activity relationship studies of a series of neolignans tested against Leishmania amazonensis. Comparison of NN with PR methods revealed that the capability of the two methods are similar to classify the molecules in the different categories, but it was found that K‐nearest neighbors and NN are superior to SIMCA in the activity prevision of a new set of compounds. The applicability of these methods using quantum chemical descriptors and topological indices were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 295–302, 2003
Structural, electronic, and vibrational properties of BxNy(x+y= 6) clusters
Authors: Guerini Silvete , Piquini Paulo
Published: September 2003

Here we studied the geometric, electronic, and vibrational properties of the BxNy(x + y= 6) clusters through ab initio density functional theory calculations. Additional coupled‐cluster theory restricted to single, double, and noniterative triple excitation single‐point calculations are performed for clusters close in energy (≤0.5 eV). Our results show that planar ring structures (except the B6) have deemed to have lower energies as compared with three‐dimensional clusters. A greater stability of the stoichiometric cluster B3N3is predicted, followed by the B4N2and B2N4clusters. The BN5cluster is deemed to dissociate in an N2+ BN3channel. The B3N3cluster is suggested as an initial step in the growth process of the hexagonal boron nitride (BN). Some available experimental results on these six‐atom BN clusters are considered and partly explained. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 329–335, 2003
A deterministic approach for global minimization of molecular potential energy functions
Authors: Lavor Carlile
Published: September 2003

The problem of minimizing a molecular potential energy function is an example of a global optimization problem. Computing the global minimum of this function is very difficult, because it typically has a very large number of local minima that grows exponentially with the problem size. Most of the methods developed to resolve this problem are stochastic or heuristic methods. In this article we use a deterministic algorithm based on a branch‐and‐bound method that employs techniques of interval arithmetic. Using this algorithm, we can guarantee that the actual global minimum is found. The proposed approach was successfully applied to a set of test problems containing up to 22 degrees of freedom. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 336–343, 2003
Semiempirical studies of the electronic structure of polyphenylene sulfide phenyleneamine
Authors: Giro R. , Galvão D. S.
Published: September 2003

Polyphenylene sulfide (PPS) and polyaniline (PANI) are heteroatom‐containing polymers with unique properties. While PPS provides a good level of chemical and thermal stability, PANI is important due to its ability to form electrically conducting films. A combination of PPS and PANI, with alternating phenyleneamine and phenylene sulfide blocks, resulted in a new material combining the structural features of PPS and PANI. This copolymer is known as polyphenylene sulfide–phenyleneamine (PPSA). In this work we present geometric and spectroscopic studies on PPSA oligomers using the well‐known semiempirical methods PM3 (parametric method 3) and ZINDO‐S/CI (Zerner's intermediate neglect of differential overlap/spectroscopic‐configuration interaction). PM3 results show that long PPSA oligomers present alternating in‐ and out‐of‐plane rings with torsion angles about 60°. The ZINDO‐simulated spectra compare well with the available experimental data. The origin of the low PPSA conductivity is addressed in terms of electronic features presented by isolated polymeric chains. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 252–259, 2003
Ab initio potentials for weakly interacting systems: Homonuclear rare gas dimers
Authors: Laschuk Eduardo F. , Martins Márcio M. , Evangelisti Stefano
Published: September 2003

A series of high‐level ab initio interatomic potentials for the homonuclear rare gas dimers He2, Ne2, and Ar2 is presented, with predictions of rovibrational spectroscopic parameters and second virial coefficients. These potentials were created by using d‐aug‐cc‐pVnZ, n = D,T,Q basis sets, MP4 and CCSD(T) correlation energy treatments, the counterpoise correction to the basis set superposition error, and extrapolation schemes for estimating complete basis set (CBS) limits. A careful FCI correction was added to our best He2 CCSD(T) potential. The characteristic parameters De, Re, k, and σ of the ab initio potentials were compared with those of reliable empirical and ab initio potentials. Our best results for He2 recovered 99.9% of Janzen's SAPT2 well depth Janzen, A.R.; Aziz, R.A. J Chem Phys 1997, 107, 914–919. In the case of Ar2, we recovered 99.8% of Aziz's HFDID1 well depth Aziz, R.A. J Chem Phys 1993, 99, 4518–4525. For neon, second virial coefficients typically came to within 0.5–1.0 cm3mol−1 of experimental values and rovibrational energy levels exhibited errors of about 1.4 cm−1. Our best argon results exhibited second virial coefficients in agreement of 0.25 cm3mol−1 with experiment and rovibrational energy level errors around 0.2 cm−1. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 303–312, 2003
Hyperconjugation effects of hydroxyl and amine groups on chemical shifts of neighboring carbon nuclei
Authors: De M. Carneiro José Walkimar , Dias Jacques F. , Tostes José Glauco R. , Seidl Peter R. , Taft Carlton A.
Published: September 2003

Systematic investigations of conformational effects of the hydroxyl group on carbon or hydrogen chemical shifts of different types of alcohols reveal that, besides stereoelectronic effects, hyperconjugation with lone pairs may have a strong influence. In view of the growing use of chemical shifts in probing the structure of biologic molecules, we employed DFT/GIAO/NBO calculations at the B3LYP level for conformers obtained from partial optimization of structures resulting from 30° variations of the CCXH (XO, N) dihedral angles to verify if nitrogen responded in a fashion similar to oxygen. Although the hybridization and geometry of hydroxyl and amine groups are distinct, the lone pair on nitrogen reveals hyperconjugation with suitably positioned orbitals on alkyl groups. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 322–328, 2003
Shear viscosity calculated by perturbation theory and molecular dynamics for dense fluids
Authors: Da Costa Silva Fabrício , Coelho Luiz A. F. , Tavares Frederico W. , Cardoso Márcio J. E. M.
Published: September 2003

In this work, we propose a new model to calculate viscosity of dense fluids based on the reference part of the Weeks–Chandler–Andersen perturbation theory. The reference fluid intermolecular interactions are given by a repulsive soft‐sphere potential. The viscosity is calculated by means of a Chapman–Enskog equation corrected to high densities coupled with a density‐ and temperature‐dependent effective diameter. The viscosity is also calculated by molecular dynamics simulations in a wide range of temperatures and densities for fluids interacting by repulsive soft‐spheres potential. These results, obtained from molecular dynamics, are used to optimize the parameters of the effective diameter equation. To compute the contribution due to attractive intermolecular interactions, we use a temperature‐dependent term, obtained by the Stokes–Einstein relation and the results published by Straub (1992) for the self‐diffusion coefficient. The proposed model correlated experimental viscosity data from literature with absolute deviations less than 4%. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 79–87, 2003
An invariant‐preserving approach to robust finite‐element multibody simulation
Authors: Borri M. , Bottasso C.L. , Trainelli L.
Published: September 2003

We briefly describe the main concepts that have inspired a novel approach for the integration of general non‐linear structural and multibody dynamics problems within a finite‐element framework that includes geometrically exact beams. In this approach, a numerically robust family of second to fourth order accurate algorithms are devised, which retain several important qualitative features of the exact solution. Among these, we address frame indifference, invariance with respect to reference entities (such as beam axes and shell mid‐surfaces), and, for load‐free motions, preservation of the system total linear and fixed‐pole angular momenta and a rigorous bound on the system total mechanical energy. When constrained systems are analyzed, these results are obtained by additionally requiring at the discrete level the preservation of Newton's Third Law of Action and Reaction and the vanishing of the work of the reaction forces for each constrained body pair. As a result, the schemes exhibit full non‐linear unconditional stability according to the Energy Method. The underlying single‐step, two‐stage, implicit difference schemes provide tunable high‐frequency damping ranging from null (ρ∞ = 1) to asymptotic annihilation (ρ∞ = 0). Representative numerical exercises are included to illustrate the main features of the methodology.
A wavelet semidiscretisation of elastic multibody systems
Authors: Díaz J. , Führer C.
Published: September 2003

In this paper we consider a coupled system of elastic and rigid bodies. It can be mathematically formulated as a coupled system of ordinary and partial differential equations, often written in its weak form. The discretisation is performed by a Galerkin‐type ansatz in connection with a finite element approach or known eigenfunctions. Here, we demonstrate instead the use of a recently published Galerkin‐wavelet method and its application to obtain a reasonable small number of elastic modes.
Optimal control and real‐time optimization of mechanical multi‐body systems
Authors: Gerdts M.
Published: September 2003

Numerical methods for the solution and real‐time optimization of parametric optimal control problems for mechanical multi‐body systems are considered. The equations of motion are given as a differential‐algebraic equation (DAE) system of index 3 augmented by additional index 1 constraints. It turns out that the sensitivity analysis of parametric nonlinear programming problems is of central importance for the numerical solution of the optimal control problem by a direct shooting method and its real‐time optimization. On the one hand, a sensitivity analysis of the discretized optimal control problem provides a real‐time approximation of the optimal solution of a perturbed optimal control problem. On the other hand, a sensitivity analysis of the projection method used for consistent initialization yields consistent initial values of sensitivity matrices needed within the iterative solution process of the direct shooting method. Two examples conclude the article.
Dynamic modeling in the simulation, optimization, and control of bipedal and quadrupedal robots
Authors: Hardt M. , von Stryk O.
Published: September 2003

Fundamental principles and recent methods for investigating the nonlinear dynamics of legged robot motions with respect to control, stability, and design are discussed. One of them is the still challenging problem of producing dynamically stable gaits. The generation of fast walking or running motions requires methods and algorithms adept at handling the nonlinear dynamical effects and stability issues which arise. Reduced, recursive multibody algorithms, a numerical optimal control method, and new stability and energy performance indices are presented which are well‐suited for this purpose. Difficulties and open problems are discussed along with numerical investigations into the proposed gait generation scheme. Our analysis considers both bipedal and quadrupedal gaits.
Blended Lobatto methods in multibody dynamics
Authors: Schaub M. , Simeon B.
Published: September 2003

Convex combination of the L‐stable Lobatto IIIC method and the A‐stable Lobatto IIIA method leads to the new class of blended Lobatto methods. These integration schemes offer the possibility to adapt the numerical damping and asymptotic behaviour to the given tolerance and the stiffness of the multibody system. The paper introduces the method class and discusses advantages and disadvantages of numerical damping. Moreover, a parameter function is given, which optimizes the efficiency by studying the cost of accuracy. As examples, a linear equation and a stiff pendulum are simulated.
Sparsing in real time simulation
Authors: Schiela A. , Bornemann F.
Published: September 2003

Modelling of mechatronical systems often leads to large DAEs with stiff components. In real time simulation neither implicit nor explicit methods can cope with such systems in an efficient way: explicit methods have to employ too small steps and implicit methods have to solve too large systems of equations. A solution of this general problem is to use a method that allows manipulations of the Jacobian by computing only those parts that are necessary for the stability of the method. Specifically, manipulation by sparsing aims at zeroing out certain elements of the Jacobian leading to a structure that can be exploited using sparse matrix techniques. The elements to be neglected are chosen by an a priori analysis phase that can be accomplished before the real‐time simulation starts. In this article a sparsing criterion for the linearly implicit Euler method is derived that is based on block diagonalization and matrix perturbation theory.