bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synthesis of dendronized, chiral conjugated polymers with appendant Fréchet‐type dendrons
Authors: Jiang Jing , Liu Hong‐Wei , Zhao You‐Liang , Chen Chuan‐Fu , Xi Fu 
Published: October 2004

AbstractNew chiral binaphthyl‐based polyarylenes [(S)‐3a and (S)‐3b] with appendant Fréchet‐type poly(aryl ether) dendrons (first generation and second generation) were synthesized with Suzuki polycondensation from chiral (S)‐6,6′‐dibromo‐2,2′‐didendron‐substituted 1,1′‐binaphthyl derivatives and p‐phenylene diboronic acid. The polymers were studied with circular dichroism, fluorescence, and ultraviolet–visible spectra. Laser light scattering measurements of (S)‐3a and (S)‐3b showed that their weight‐average molecular weights were 2.39 × 105 and 1.09 × 104, respectively. The specific optical rotation [α]D was −59.6 for (S)‐3a and −62.7 for (S)‐3b. These dendronized conjugated polymers exhibited good thermal stability. The glass‐transition temperatures and the initial decomposition temperatures were 187.5 and 265.3 °C for (S)‐3a and 173.8 and 308.9 °C for (S)‐3b, respectively. (S)‐3a and (S)‐3b had high fluorescence quantum efficiencies, 87 and 91%, respectively, in tetrahydrofuran. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1167–1172, 2002
Structural features of crystallized poly(ethylene terephthalate) polymers
Authors: Lu Weijie , Debelak Kenneth A. , Witt Andrew R. , Yang Congkun , Collins W. Eugene , Lott Charles 
Published: October 2004

AbstractPolyethylene terephthalate (PET) is a widely used polymeric material. In this work, the microstructural features before and after the solid‐state polymerization (SSP) of several DuPont PET products were investigated by low‐voltage scanning electron microscopy (LV‐SEM) and atomic force microscopy (AFM). The microstructural features on the cross section of various PET samples included crystallites, voids, boundaries, defects, and amorphous phases. The SEM images revealed layered and stepped structural features at the micron and 10‐micron scales that are highly crystallized at the near‐edge region of the cross section for both linear and branched PET samples after the SSP process. The AFM images demonstrate that the degree of crystallization for the linear and branched PET samples increases gradually from the central area to the edge on the cross section. The linear crystallized PET has a higher degree of orientation than the branched crystallized PET in the 10‐micron to micron scales, but their crystalline structures have no significant differences in the submicron to nanometer scales. The PET crystallization process occurs when the molecular chains in the amorphous phase are aligned and folded to form straight molecular chains at the nanometer scale, and small crystallites are formed. The crystallites aggregate and align together into a polygon rod‐like‐shaped crystallites at the submicron scale. Finally, large crystallites at the micron size are formed that appear on the edge area of the cross section. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 245–254, 2002
Microphase separation and ion‐conduction mechanisms in polypropylene oxide/lithium perchlorate (LiClO4) complexes
Authors: Furukawa Takeo , Mukasa Yasuhiko , Suzuki Toshikazu , Kano Kenji 
Published: October 2004

AbstractBroadband dielectric spectroscopy was used to examine ion‐conduction mechanisms in polypropylene oxide (PPO) with a molecular weight of 4000 complexed with LiClO4. Two distinct conduction mechanisms were proposed with respect to high and low salt concentration regions. In a concentrated regime (Li/O >10%), the segmental motion of PPO molecules is significantly slowed down by enhanced cation coordination that results in a marked decrease in molar conductivity. We found a linear relationship between the ionic diffusion coefficient and the relaxation frequency of slowed segmental motion over broad temperature and salt‐concentration ranges. The use of a random walk scheme revealed that ions hop around at the same rate as slowed segmental motion for a monomer length. In a dilute regime (Li/O <0.1%), ions are temporarily localized in a limited domain. The direct current conductivity is achieved by structural renewal that releases ions from such localization and provides a diffusional character. At intermediate salt concentrations, microphase separation into ion‐depleted and ion‐rich regions was evidenced by the coexistence of fast and slow segmental processes. The molar conductivity revealed a maximum at Li/O = 3%. Its decrease at higher salt concentrations was attributed to the slowing down of segmental motion, and that at lower salt concentrations was attributed to localization of ionic motion. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 613–622, 2002; DOI 10.1002/polb.10123
Nanocomposites of polyurethane with various organoclays: Thermomechanical properties, morphology, and gas permeability*
Authors: Chang Jin‐Hae , An Yeong Uk 
Published: October 2004

AbstractThe properties of polyurethane (PU) nanocomposites with three different organoclays were compared in terms of their thermal stabilities, mechanical properties, morphologies, and gas permeabilities. Hexadecylamine–montmorillonite, dodecyltrimethyl ammonium–montmorillonite, and Cloisite 25A were used as organoclays for making PU hybrid films. The properties were examined as a function of the organoclay content in a matrix polymer. Transmission electron microscopy photographs showed that most clay layers were dispersed homogeneously into the matrix polymer on the nanoscale, although some particles of clay were agglomerated. Moreover, the addition of only a small amount of organoclay was enough to improve the thermal stabilities and mechanical properties of PU hybrid films, whereas gas permeability was reduced. Even polymers with low organoclay contents (3–4 wt %) showed much higher strength and modulus values than pure PU. Gas permeability was reduced linearly with an increasing amount of organoclay in the PU matrix. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 670–677, 2002; DOI 10.1002/polb.10124
TiO2 distribution in aged and injected isotactic polypropylene parts by synchrotron radiation X‐ray microfluorescence
Authors: Maia Dênison R. J. , Perez Carlos A. , De Paoli Marco‐A. 
Published: October 2004

AbstractWe studied the TiO2 pigment distribution along cross sections of injected isotactic polypropylene samples after they were aged by light exposure for 515 and 3000 h in accelerated test equipment. The TiO2 pigment distribution was studied so that we could understand the whitening process occurring in this type of plastic. For these studies, we used a 20‐μm X‐ray microbeam from a synchrotron light source. We observed that the aged and nonaged samples had almost homogeneous distributions of Ti in the cross sections; therefore, pigment migration could not have been responsible for the surface whitening process. There were maxima of Ti intensities that were not in the same region for all samples. This behavior could be explained by the heterogeneity of the extrusion and injection‐molding processes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 657–662, 2002; DOI 10.1002/polb.10127
Proton transport in polybenzimidazole blended with H3PO4 or H2SO4
Authors: Pu Hongting , Meyer Wolfgang H. , Wegner Gerhard 
Published: October 2004

AbstractProton transport in H3PO4‐ and H2SO4‐blended polybenzimidazoles (PBIs) has been studied with both temperature‐ and pressure‐dependent dielectric spectroscopy. The influences of the acid concentration and temperature on the relative conductance and activation volume are discussed. An Arrhenius relation is used to model the temperature‐dependent conductivity at a constant acid content. The logarithm of the relative conductance for PBI blended with H3PO4 decreases linearly with increasing pressure. As the temperature increases, the activation volume becomes smaller for PBI blended with H3PO4. It is proposed that proton transport in acid‐blended PBI is mainly controlled by proton hopping and diffusion rather than a mechanism mediated by the segmental motions in the polymer. The conductivities of PBIs blended with H3PO4 and H2SO4 are compared. At a 1.45 molar acid doping concentration, the former has the higher conductivity. With water, the conductivity of H3PO4‐blended PBI increases significantly. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 663–669, 2002; DOI 10.1002/polb.10132
Congruent qualitative behavior of complete and reconstructed phase space trajectories from biomolecular dynamics simulation
Authors: Caves Leo S.D. , Verma Chandra S. 
Published: October 2004

AbstractCentral to the study of a complex dynamical system is knowledge of its phase space behavior. Experimentally, it is rarely possible to record a system's (multidimensional) phase space variables. Rather, the system is observed via one (or few) scalar‐valued signal(s) of emission or response. In dynamical systems analysis, the multidimensional phase space of a system can be reconstructed by manipulation of a one‐dimensional signal. The trick is in the construction of a (higher‐dimensional) space through the use of a time lag (or delay) on the signal time series. The trajectory in this embedding space can then be examined using phase portraits generated in selected subspaces. By contrast, in computer simulation, one has an embarrassment of riches: direct access to the complete multidimensional phase space variables, at arbitrary time resolution and precision. Here, the problem is one of reducing the dimensionality to make analysis tractable. This can be achieved through linear or nonlinear projection of the trajectory into subspaces containing high information content. This study considers trajectories of the small protein crambin from molecular dynamics simulations. The phase space behavior is examined using principal component analysis on the Cartesian coordinate covariance matrix of 138 dimensions. In addition, the phase space is reconstructed from a one dimensional signal, representing the radius of gyration of the structure along the trajectory. Comparison of low‐dimensional phase portraits obtained from the two methods shows that the complete phase space distribution is well represented by the reconstruction. The study suggests that it may be possible to develop a deeper connection between the experimental and simulated dynamics of biomolecules via phase space reconstruction using data emerging from recent advances in single‐molecule time‐resolved biophysical techniques. Proteins 2002;47:25–30. © 2002 Wiley‐Liss, Inc.
Solution structures of a 30‐residue amino‐terminal domain of the carp granulin‐1 protein and its amino‐terminally truncated 3‐30 subfragment: Implications for the conformational stability of the stack of two β‐hairpins
Authors: Vranken Wim F. , James Susan , Bennett Hugh P.J. , Ni Feng 
Published: October 2004

AbstractCarp granulins are members of an emerging class of proteins with a sequence motif encoding a parallel stack of two to four β‐hairpins. The carp granulin‐1 protein forms a stack of four β‐hairpins, whereas its amino‐terminal fragment appears to adopt a very stable stack of two β‐hairpins in solution. Here we determined a refined three‐dimensional structure of this peptide fragment to examine potential conformational changes compared with the full‐length protein. The structures were calculated with both a traditional method and a fast semiautomated method using ambiguous NMR distance restraints. The resulting sets of structures are very similar and show that a well‐defined stack of two β‐hairpins is retained in the peptide. Conformational rearrangements compensating the loss of the carboxy‐terminal subdomain of the native protein are restricted to the carboxy‐terminal end of the peptide, the turn connecting the two β‐hairpins, and the Tyr21 and Tyr25 aromatic side chains. Further removal of the Val1 and Ile2 residues, which are part of the first β‐hairpin and components of two major hydrophobic clusters in the two β‐hairpin structure, results in the loss of the first β‐hairpin. The second β‐hairpin, which is closely associated with the first, retains a similar but somewhat less stable conformation. The invariable presence of the second β‐hairpin and the dependence of its stability on the first β‐hairpin suggest that the stack of two β‐hairpins may be an evolutionary conserved and autonomous folding unit. In addition, the high conformational stability makes the stack of two β‐hairpins an attractive scaffold for the development of peptide‐based drug candidates. Proteins 2002;47:14–24. © 2002 Wiley‐Liss, Inc.
Probabilistic sampling of protein conformations: New hope for brute force?
Authors: Feldman Howard J. , Hogue Christopher W.V. 
Published: October 2004

AbstractProtein structure prediction from sequence alone by “brute force” random methods is a computationally expensive problem. Estimates have suggested that it could take all the computers in the world longer than the age of the universe to compute the structure of a single 200‐residue protein. Here we investigate the use of a faster version of our FOLDTRAJ probabilistic all‐atom protein‐structure‐sampling algorithm. We have improved the method so that it is now over twenty times faster than originally reported, and capable of rapidly sampling conformational space without lattices. It uses geometrical constraints and a Leonard‐Jones type potential for self‐avoidance. We have also implemented a novel method to add secondary structure‐prediction information to make protein‐like amounts of secondary structure in sampled structures. In a set of 100,000 probabilistic conformers of 1VII, 1ENH, and 1PMC generated, the structures with smallest Cα RMSD from native are 3.95, 5.12, and 5.95Å, respectively. Expanding this test to a set of 17 distinct protein folds, we find that all‐helical structures are “hit” by brute force more frequently than β or mixed structures. For small helical proteins or very small non‐helical ones, this approach should have a “hit” close enough to detect with a good scoring function in a pool of several million conformers. By fitting the distribution of RMSDs from the native state of each of the 17 sets of conformers to the extreme value distribution, we are able to estimate the size of conformational space for each. With a 0.5Å RMSD cutoff, the number of conformers is roughly 2Nwhere N is the number of residues in the protein. This is smaller than previous estimates, indicating an average of only two possible conformations per residue when sterics are accounted for. Our method reduces the effective number of conformations available at each residue by probabilistic bias, without requiring any particular discretization of residue conformational space, and is the fastest method of its kind. With computer speeds doubling every 18 months and parallel and distributed computing becoming more practical, the brute force approach to protein structure prediction may yet have some hope in the near future. Proteins 2002;46:8–23. © 2001 Wiley‐Liss, Inc.
Toward minimalist models of larger proteins: A ubiquitin‐like protein
Authors: Sorenson Jon M. , Head‐Gordon Teresa 
Published: October 2004

AbstractOur recently developed off‐lattice bead model capable of simulating protein structures with mixed α/β content has been extended to model the folding of a ubiquitin‐like protein and provides a means for examining the more complex kinetics involved in the folding of larger proteins. Using trajectories generated from constant‐temperature Langevin dynamics simulations and sampling with the multiple multi‐histogram method over five‐order parameters, we are able to characterize the free energy landscape for folding and find evidence for folding through compact intermediates. Our model reproduces the observation that the C‐terminus loop structure in ubiquitin is the last to fold in the folding process and most likely plays a spectator role in the folding kinetics. The possibility of a productive metastable intermediate along the folding pathway consisting of collapsed states with no secondary structure, and of intermediates or transition structures involving secondary structural elements occurring early in the sequence, is also supported by our model. The kinetics of folding remain multi‐exponential below the folding temperature, with glass‐like kinetics appearing at T/Tf ∼ 0.86. This new physicochemical model, designed to be predictive, helps validate the value of modeling protein folding at this level of detail for genomic‐scale studies, and motivates further studies of other protein topologies and the impact of more complex energy functions, such as the addition of solvation forces. Proteins 2002;46:368–379. © 2002 Wiley‐Liss, Inc.
Hydrogen molecular ion inside penetrable prolate spheroidal boxes: Electronic and vibrational properties
Authors: Mateos‐Cortés S. , Ley‐Koo E. , Cruz S. A. 
Published: October 2004

AbstractThe exact solution of the Schrödinger equation for the hydrogen molecular ion confined inside penetrable prolate spheroidal boxes is constructed within the Born–Oppenheimer approximation.The pressure dependence of physical properties such as ground‐state energy, equilibrium bond length, vibrational force constant, polarizability and quadrupole moment are examined for different barrier heights. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001
Effective crystal field for trivalent first transition row ions
Authors: Sinitsky A. V. , Darhovskii M. B. , Tchougréeff A. L. , Misurkin I. A. 
Published: October 2004

AbstractIn the present work the semiempirical effective crystal field (ECF) method previously designed for electronic structure calculations of transition metal complexes and utilizing non‐Hartree–Fock trial wave function and parameterized for complexes of doubly charged Cr2+, V2+, Mn2+, Fe2+, Co2+, and Ni2+ cations is extended to complexes of triply charged cations of 3d‐elements. With the parameters adjusted the ECF method is applied to calculations of ground states and low‐energy spectra of the d‐shells of fluoro‐, chloro‐, aqua‐, amino‐, and cyano‐complexes of the triply charged cations. Obtained total spin and symmetry of the ground states match the experimentally observed ones. Satisfactory agreement between the calculated and experimental d‐shell electronic transition energies is achieved as well. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Ab initio quasirelativistic calculations on electronic transitions in ICl by the multireference many‐body perturbation theory
Authors: Zaitsevskii Andréi , Teichteil Christian 
Published: October 2004

AbstractA quasirelativistic perturbative method of ab initio calculations on ground and excited molecular electronic states and transition properties within the relativistic effective core potential approximation is presented and discussed. The method is based on the construction of a state‐selective many‐electron effective Hamiltonian in the model space spanned by an appropriate set of Slater determinants by means of the second‐order many‐body multireference perturbation theory. The neglect of effective spin–orbit interactions outside of the model space allows the exploitation of relatively high nonrelativistic symmetry during the evaluation of perturbative corrections and therefore dramatic reduction of the cost of computations without any contraction of the model‐space functions. One‐electron transition properties are evaluated via the perturbative construction of spin‐free transition density matrices. Illustrative calculations on the X0+ − A1, B0+, and (ii)1 transitions in the ICl molecule are reported. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Effects of electronic correlation on local properties of electronic structure of TiO2 and Ti2O3 crystals: DFT and post‐HF approaches
Authors: Evarestov R. A. , Panin A. I. 
Published: October 2004

AbstractCalculations of the electronic structure of titanium oxides both in periodic and cluster models are performed using RHF, UHF, and DFT methods. The effect of electronic correlation on local properties of electronic structure (charges, valences, covalences, and bond orders) is discussed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Hydrated electron: Nonempirical cluster approach
Authors: Novakovskaya Yulia V. , Stepanov Nikolai F. 
Published: October 2004

AbstractInterface water anions composed of several chainlike or cyclic fragments were simulated with a 6‐31++G** basis set at the unrestricted Hartree–Fock level with the second‐order Moeller–Plesset perturbation theory corrections taken into account. The estimated vertical electron detachment energies (VDEs) of (H2O) anions were approximated by a VDE‐n−1/3 dependence close to the experimental one. A hypothesis about the predominant formation of interface structures under conditions of molecular flows is put forward. The atomic population analysis, character of the highest occupied molecular orbital, and changes in the geometry of interface anions with an increase in their molecular size reveal the compact localization of the excess electron density in water clusters and allow evaluating the effective excess‐electron radius of condensed water as 2.5 Å, in good agreement with a similar empirical estimate. The scope of the data obtained shows the relatively low probability of the formation of octahedral hydration shells compared to the tetrahedral coordination of solvating water molecules. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Multiconfigurational Green's function approach with quasidegenerate perturbation theory
Authors: Khrustov V. F. , Kostychev D. E. 
Published: October 2004

AbstractThe new approach to approximation of polarization propagator (PP) for electronic states of atoms and molecules with reference state wave function (RSWF) constructed on the base of a multidimensional model space is presented. This approach exploiting the apparatus of the quasidegenerate perturbation theory (QDPT) is realized in the zeroth QDPT order and through the first one. The original complete system of excitation operators introduced in the approach is consistent with the RSWF by the perturbation order. This factor in conjunction with the flexibility of the RSWF creates the capabilities of balanced accounting of correlation and quasidegeneracy effects at different locations of nuclei in a molecule and for all the states concerned. In this way, the transition characteristics in electronic shells of molecules in a wide area of nuclear geometry parameters may be appropriately evaluated. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Quantum chemistry of ferroelectric solids: Electronic structures and peculiar behavior of zero‐dimensional K3H(SO4)2‐like materials
Authors: Dolin S. P. , Levin A. A. , Mikhailova T. Yu. , Solin M. V. , Trakhtenberg L. I. 
Published: October 2004

AbstractThe main problems of quantum chemistry of H‐bonded ferroelectrics are treated using the zero‐dimensional K3H(SO4)2‐like crystals as suitable examples. Various quantum chemical approaches and computational procedures are applied to evaluate the Ising model coupling parameters that determine different thermodynamic and dielectric properties of these materials. The calculated Ising parameters are employed to describe the peculiarities of ferroelectric behavior of the K3H(SO4)2 family crystals in the framework of mean field approximation. The problems related to the H‐bond proton (deuteron) tunneling are also discussed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002
Size dependence of Feenberg scaling
Authors: Szabados Ágnes , Surján Péter R. 
Published: October 2004

AbstractThe Feenberg scaling procedure, as applied to Rayleigh–Schrödinger perturbation theory, is not size consistent; i.e., the energy error E(AB) − E(A) − E(B) is nonzero if A and B are two different, noninteracting subsystems. The magnitude of this size inconsistency of the Feenberg scaling is determined and analyzed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005