bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of intercalating agents on clay dispersion and thermal properties in polyethylene/montmorillonite nanocomposites
Authors: Stoeffler Karen , Lafleur Pierre G. , Denault Johanne 
Published: May 2008

AbstractAlkyl pyridinium, 1‐vinyl alkyl imidazolium, 1,3‐dialkyl imidazolium, and tetraalkyl phosphonium bromides were successfully used as intercalants for the preparation of highly thermally stable organophilic montmorillonites. Nanocomposites of linear low density polyethylene (LLDPE) and linear low density polyethylene grafted with maleic anhydride (LLDPE/LLDPE‐g‐MAH) were prepared from those organoclays. The micro‐ and nano‐dispersions were analyzed through X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM): intercalation and/or partial exfoliation were found to occur only for formulations based on organoclays having an initial basal distance higher than 20 Å, suggesting the existence of a critical interfoliar distance for the delamination of silicate layers in a noninteracting polymer matrix. The properties of the nanocomposites were analyzed through differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and oscillatory rheometry. The dynamic crystallization of LLDPE was not significantly affected by the presence of clay. TGA in oxidative atmosphere proved to be very sensitive to the dispersion state of the organoclay: the thermal stability was drastically enhanced for intercalated and partially exfoliated formulations. However, the inherent thermal stability of the organoclay did not appear to influence significantly the overall thermal stability of the composite in the range of temperatures investigated (160–230°C). POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.
Synthesis of novel bis(β‐cyclodextrin)s linked with aromatic diamine and their molecular recognition with model substrates
Authors: Zhao Yan , Liu Xiao‐Qing , Gu Juan , Wang Li‐Qin , Zhu Hong‐You , Huang Rong , Wang Yu‐Fei , Yang Zi‐Ming 
Published: May 2008

AbstractNovel β‐cyclodextrin (β‐CD) dimers with aromatic diamine linkers, 1,3‐(aminomethyl)‐benzylamine‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (2), 4,4′‐diaminodiphenylmethano‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (3), and 4,4′‐ ethylenedianiline‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (4), were synthesized. The inclusion complexation behaviors of these compounds, together with 4,4′‐aminophenyl ethyl‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (5), with substrates such as acridine red (AR), neutral red (NR), ammonium 8‐anilino‐1‐naphthalenesulfonate (ANS), sodium 2‐(p‐toluidinyl) naphthalenesulfonate (TNS), rhodamine B (RhB), and brilliant green (BG), were investigated by ultraviolet, fluorescence, circular dichroism, and 2D NMR spectroscopy. The results indicated that the two linked CD units cooperatively bound to a guest, and the molecular binding affinity toward substrates, especially curved guest ANS and linear guests such as NR and AR, was increased. The linker length between two CD units played a crucial role in the molecular recognition of the hosts with guest dyes. The binding constants of the hosts for AR, TNS, ANS, and RhB decreased with increasing linker length in hosts 2‐4. Moreover, structurally similar hosts 3 and 5 exhibited very different binding behavior for the guests. Host 5 showed much higher Ks values toward positively charged guests and lower Ks toward negatively charged guests than host 3. The 2D NMR spectra of hosts 3 and 5 with RhB were acquired to understand the binding difference between 3 and 5. The molecular binding ability and selectivity of model substrates by these hosts were sufficiently investigated to reveal not only the cooperative contributions of the linker group and CD cavities upon inclusion complexation with dye guest molecules, but also the controlling factors for the molecular selective binding. Copyright © 2008 John Wiley & Sons, Ltd.
Proximity effects of oxygen atoms on the enthalpies of formation of simple diethers: a computational G3(MP2)//B3 study
Authors: Taskinen Esko , Taskinen Antti 
Published: May 2008

AbstractThe enthalpies of formation of a number of acyclic, straight‐chain ethers and diethers were determined by G3(MP2)//B3 calculations. The principal aim of the work was to study the magnitude of the O…O proximity effect on the enthalpy contents of diethers as a function of the distance (number of bonds) between the O atoms. 1,4‐Diethers and 1,5‐diethers were computed to be destabilized byca. 4.5 (±0.5) and 3.2 (±0.4) kJ mol−1, respectively, by the O…O proximity effect. The effect was calculated to be negligible in diethers with the O atoms in positions more remote than 1,5 from each other, whereas 1,3‐diethers (acetals) are stabilized byca. 22 kJ mol−1, likely on account of the anomeric effect. Calculations on simple monoethers show that the contributions to$\Delta_{\rm f} H^{\circ}_m$of CH2groups in theβandγpositions (relative to O) are reduced byca. 0.8 and 0.3 kJ mol−1, respectively, relative to those of CH2groups more remote from the O atom. The computational enthalpies of formation of the studied monoethers and diethers, both cyclic and acyclic, are generally in good agreement with experimental data, another important result of the present work. Copyright © 2008 John Wiley & Sons, Ltd.
A combined experimental and theoretical study of the alkylation of 3,5‐dithioxo‐[1,2,4]triazepines
Authors: Esseffar M. , El Messaoudi M. , Jalal R. , Domingo L. R. , Aurell M. J. 
Published: May 2008

AbstractThe chemo‐ and regioselective alkylation reactions of 3,5‐dithioxo[1,2,4]triazepine 1 in a basic medium with α,ω‐dibromoalkanes 2a–c, Br(CH2)nBr (n = 1–3), are studied experimentally and theoretically. These alkylations, which occur at the thioxo sulfur atom in position 5, afford mainly 5‐bromomethylthio‐2,7‐dimethyl‐ 2,3‐dihydro‐ 4H[1,2,4]triazepin‐3‐one 3 for n = 1, 6,8‐dimethyl‐5‐thioxo‐2,3,4,5‐tetrahydro‐6H[1,3]thiazolo[4,5‐d][1,2,4]triazepine 4 for n = 2 and 7,9‐dimethyl‐6‐thioxo‐2,3,4,5,6,7‐hexahydro[1,3]thiazino [4,5‐d][1,2,4]triazepine 5 for n = 3. Theoretical calculations have been carried out at the B3LYP/6‐31G* and B3LYP(benzene)/6‐311+G*//B3LYP/6‐31G* levels, in order to rationalize the experimental observations. Both chemo‐ and regio‐selectivities of the alkylation reactions are analyzed. Copyright © 2008 John Wiley & Sons, Ltd.
Thioenols and thioamides substituted by two β‐EWGs. Comparison with analogous amides and enols
Authors: Basheer Ahmad , Rappoport Zvi 
Published: May 2008

AbstractCondensation of organic isothiocyanates with active methylene compounds gave nine thioamides RNHCSCHYY′ or their isomeric thioenols RNHC(SH) = CYY′ for substrates in which Y and Y′ are electron‐withdrawing groups (EWG). These included derivatives of Meldrum's acid (MA) which showed 100% thioenol in all solvents. For other compounds the percentages of thioenol in CDCl3 when R = Ph are 100% when Y = CN and Y′ = CO2Me or Y′ = CO2CH2CCl3, 6% when Y = Y′ = CO2CH2CF3, and 0% when Y = Y′ = CO2Me. The chemical shift of SH (highest values 12.0–16.0 ppm) served as a probe for the thioenol structures and also for the extent of hydrogen bonding to the SH group. In contrast to simple ketones and thioketones in which thioenolization is favored over enolization by factors as large as 106, for intramolecular competition KThioenol/KEnol ratios are much lower than for systems not substituted by β‐EWGs. X‐ray crystallography of the 5‐anilido‐MA derivative shows a hydrogen‐bonded thioenol structure. δ(OH), δ(NH), KEnol, and crystallographic data for analogous thioenol and enol systems are compared. Copyright © 2008 John Wiley & Sons, Ltd.
Deuterium kinetic isotope effects and H/D exchange in dimerization of acetylene with a Nieuwland catalyst in aqueous media
Authors: Tachiyama Takashi , Yoshida Makoto , Aoyagi Tatsuhiro , Fukuzumi Shunichi 
Published: May 2008

AbstractDeuterium kinetic isotope effects were observed in the dimerization of acetylene with a Nieuwland catalyst in an aqueous media, where the copper–acetylene and copper–monovinylacetylene (MVA) π‐complexes were detected as intermediates by 1H NMR. The copper–acetylene π‐complex is responsible for an efficient H/D exchange between acetylene and proton (or deuteron) of water, which occurs rapidly with a Nieuwland catalyst. The deuterium kinetic isotope effects and the rapid H/D exchange reaction together with the detection of the copper–acetylene and copper–MVA π‐complexes provide deeper insight into the catalytic mechanism of the Nieuwland catalysis for the dimerization of acetylene. Copyright © 2008 John Wiley & Sons, Ltd.
Electrochemical glycosylation in the presence of a catalytic chemical mediator
Authors: Drouin Ludovic , Compton Richard G. , Fairbanks Antony J. 
Published: May 2008

AbstractElectrochemical glycosylation of thioglycoside donors proceeds efficiently in an undivided cell in the presence of a catalytic amount of the chemical mediator (4‐bromophenyl) ammoniumyl hexachloroantimonate (BAHA). In comparison with electrochemical glycosylation alone, the use of a catalytic amount of BAHA greatly increases the rate of reaction, reduces the potential required and inhibits ester‐protecting group migrations. Copyright © 2008 John Wiley & Sons, Ltd.
Optimal irradiation wavelength in iniferter‐based photocontrolled radical polymerization
Authors: Nemoto Y. , Nakayama Y. 
Published: May 2008

AbstractIn iniferter‐based photocontrolled radical polymerization optimal irradiation wavelength was examined for precise molecular design of star vectors, which can function as high‐performance gene carriers. Monochromatic light in the range from 330 to 400 nm with the interval of about 10 nm was irradiated to tetrafunctional iniferter, 1,2,4,5‐tetrakis(N,N‐diethyldithiocarbamylmethyl)benzene, in the presence of N,N‐dimethyaminopropylacrylamide (DMAPAAm), as a model monomer, in toluene. In all wavelengths tetrabranched polymers were produced except for 400‐nm‐irradiation. The highest conversion reaching to about 35% was observed at the wavelength of 370 nm after only 40 min of irradiation. There was no further increase in conversion by combination with another two wavelengths. HPLC analysis and NMR spectra showed that the polymerization at the optimal wavelength of 370 nm was occurred in a living manner with the ability to control the chain length (from about 5000 to 40,000) having very narrow polydispersity (about 1.6) by changing of the irradiation time, the intensity, and the monomer concentration. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4505–4512, 2008
Tris[tri(2‐thienyl)phosphine]palladium as the catalyst precursor for thiophene‐based Suzuki‐Miyaura crosscoupling and polycondensation
Authors: Li Weiwei , Han Yang , Li Binsong , Liu Caiming , Bo Zhishan 
Published: May 2008

AbstractZero‐valent palladium complex, Pd(PTh3)3, with three tri(2‐thienyl)phosphine ligands was prepared and characterized. Pd(PTh3)3 is superior to Pd(PPh3)4 in catalyzing Suzuki‐Miyaura coupling and polymerization of thiophene‐based derivatives. The Suzuki polycondensation of 3‐hexyl‐5‐iodothiophene‐2‐boronic pinacol ester with Pd(PTh3)3 as the catalyst precursor afforded high‐molecular‐weight P3HT with high regularity and yield. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4556–4563, 2008
Synthesis and encapsulation properties of dendronized polymer with Fréchet‐type dendrons peripherally decorated by carboxylic acid functionalization
Authors: Jiao Qing , Yi Zhuo , Chen Yongming , Xi Fu 
Published: May 2008

AbstractA new carboxylic acid‐terminated dendronized polymer (denpol), constructed with linear chain attaching Fréchet‐type dendrons at each repeat unit, has been designed and synthesized through a combination of macromonomer route and hydrolysis reaction. The resulting denpol exhibits excellent solubility in aqueous solution for pH ≥ 6, and significantly the denpol also encapsulates various aromatic molecules efficiently. The results of UV–vis and fluorescence spectra indicate that hydrophobic and π‐π interactions bring into effect between the water‐insoluble organic molecules and the denpol. Moreover, the photoisomerization of azobenzene solubilized in denpol aqueous solution was investigated, which indicated that trans‐to‐cis and cis‐to‐trans conversions were first‐order reactions. The enhancement of photoisomerization property may attribute to the proper microenvironment in the denpol. Therefore, the denpol is expected to be a potential candidate as amphiphilic unimolecular container. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4564–4574, 2008
Photophysical and self‐assembly behavior of poly(amidoamine) dendrons with chromophore as scaffold: The effect of dendritic architecture
Authors: Wang Bing‐Bing , Li Wu‐Song , Jia Xin‐Ru , Gao Min , Jiang Lei , Wei Yen 
Published: May 2008

AbstractTwo series of amphiphiles composed of hydrophilic poly(amidoamine) dendrons (from the first to the third generation) as the shell and hydrophobic aromatic chromophores (3,6‐di(maleimidyl)‐9‐phenyl carbazole and 9‐(4′‐maleimidyl phenyl)‐3‐maleimidyl carbazole) as the central scaffold were synthesized. The effect of dendritic architecture on the photophysical properties and the self‐assembly behavior of these amphiphiles were studied by UV–vis absorption spectroscopy, fluorescence spectroscopy, and transmission electron microscopy (TEM) measurements. Both the generation of dendritic shell and the location of dendrons at the chromophoric scaffold had great effect on the photophysical properties of these amphiphiles. In addition, different spherical aggregates were formed from these amphiphiles in the aqueous solution at different concentrations. Because of the combined effects of steric hindrance and architecture of dendritic shells, the amphiphiles from G2 dendron with central chromophore self‐organized into ordered aggregates more readily than that from G1 and G3. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4584–4593, 2008
Controlled/living radical polymerization of vinylcyclicsilazane by RAFT process and their block copolymers
Authors: Nghiem Quoc Dat , Nguyen Chi Thanh , Kim Dong‐Pyo 
Published: May 2008

AbstractHigh molecular weight poly(vinyl)silazane were synthesized successfully by reversible addition fragmentation chain transfer (RAFT) polymerization in toluene at 120 °C, using dithiocarbamate derivatives and 2,2′‐azobis‐isobutyrylnitrile (AIBN) as the RAFT agents and thermal initiator, respectively. The polymerization of a vinylcyclicsilazane oligomer with 82.5% conversion was readily controlled to increase the molecular weight from 1000 to 12,000 g/mol with a narrow polydispersity <1.5. The resulting polymer showed a high ceramic yield of 70 wt % at 1000 °C. Moreover, the approach was extended successfully to the synthesis of poly(vinyl)silazane‐block‐polystyrene as an inorganic–organic diblock copolymer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4594–4601, 2008
Remote sensing of permafrost‐related problems and hazards
Authors: Kääb Andreas 
Published: May 2008

AbstractModern remote sensing techniques can help in the assessment of permafrost hazards in high latitudes and cold mountains. Hazard development in these areas is affected by process interactions and chain reactions, the ongoing shift of cryospheric hazard zones due to atmospheric warming, the large spatial scales involved and the remoteness of many permafrost‐related threats. This paper reviews ground‐based, airborne and spaceborne remote sensing methods suitable for permafrost hazard assessment and management. A wide range of image classification and change detection techniques support permafrost hazard studies. Digital terrain models (DTMs) derived from optical stereo, synthetic aperture radar (SAR) or laser scanning data are some of the most important data sets for investigating permafrost‐related mass movements, thaw and heave processes, and hydrological hazards. Multi‐temporal optical or SAR data are used to derive surface displacements on creeping and unstable frozen slopes. Combining DTMs with results from spectral image classification, and with multi‐temporal data from change detection and displacement measurements significantly improves the detection of hazard potential. Copyright © 2008 John Wiley & Sons, Ltd.
Polysomnographic values in adolescents with ataxia telangiectasia
Authors: McGrath‐Morrow Sharon A. , Sterni Laura , McGinley Brian , Lefton‐Greif Maureen A. , Rosquist Karen , Lederman Howard 
Published: May 2008

AbstractMost adolescents with ataxia telangiectasia (A‐T) develop progressive bulbar muscle weakness and decreased pulmonary reserve. The purpose of this study was to define the patterns of sleep and respiration during sleep, and to identify sleep‐related breathing problems in subjects with A‐T. To address these issues, overnight polysomnography was performed on 12 adolescents with A‐T. Eleven of the 12 subjects completed overnight polysomnography. The median age was 16 years (range, 13–20 years). All subjects in the study were wheelchair‐bound and the median forced vital capacity (% predicted of normal) was 44% (range, 16–82%). The mean sleep efficiency was 72.6% with a mean apnea hypopnea index (AHI) of 0.7 events/hr (range, 0–2.2). The majority of apnea/hypopneas were REM related. The mean central apnea index was 0.1 events/hr (range, 0–0.2). The mean oxygen saturation nadir was 92.7% (range, 87–96) and the mean peak end‐tidal carbon dioxide ET value was 53.8 mm Hg (range, 49–60). Two of 11 subjects had ET values ≥50 mm Hg for more than 50% of total sleep time. In this study, the majority of A‐T adolescents had infrequent partial or complete upper airway obstructions during sleep and minimal nighttime hypoxemia. They did, however, have decreased sleep efficiency most likely, due in part, to their underlying neurological condition. This decrease in total sleep time may underestimate hypoventilation. Based on these findings, overnight polysomnography should be considered in adolescents with A‐T, particularly in those in which there is a clinical suspicion of sleep related breathing abnormalities. Pediatr Pulmonol. 2008; 43:674–679. © 2008 Wiley‐Liss, Inc.
Predicting helix orientation for coiled‐coil dimers
Authors: Apgar James R. , Gutwin Karl N. , Keating Amy E. 
Published: May 2008

AbstractThe α‐helical coiled coil is a structurally simple protein oligomerization or interaction motif consisting of two or more alpha helices twisted into a supercoiled bundle. Coiled coils can differ in their stoichiometry, helix orientation, and axial alignment. Because of the near degeneracy of many of these variants, coiled coils pose a challenge to fold recognition methods for structure prediction. Whereas distinctions between some protein folds can be discriminated on the basis of hydrophobic/polar patterning or secondary structure propensities, the sequence differences that encode important details of coiled‐coil structure can be subtle. This is emblematic of a larger problem in the field of protein structure and interaction prediction: that of establishing specificity between closely similar structures. We tested the behavior of different computational models on the problem of recognizing the correct orientation—parallel vs. antiparallel—of pairs of alpha helices that can form a dimeric coiled coil. For each of 131 examples of known structure, we constructed a large number of both parallel and antiparallel structural models and used these to assess the ability of five energy functions to recognize the correct fold. We also developed and tested three sequence‐based approaches that make use of varying degrees of implicit structural information. The best structural methods performed similarly to the best sequence methods, correctly categorizing ∼81% of dimers. Steric compatibility with the fold was important for some coiled coils we investigated. For many examples, the correct orientation was determined by smaller energy differences between parallel and antiparallel structures distributed over many residues and energy components. Prediction methods that used structure but incorporated varying approximations and assumptions showed quite different behaviors when used to investigate energetic contributions to orientation preference. Sequence based methods were sensitive to the choice of residue‐pair interactions scored. Proteins 2008. © 2008 Wiley‐Liss, Inc.
Early stages of decomposition in Al alloys investigated by X‐ray absorption
Authors: Klobes B. , Staab T. E. M. , Dudzik E. 
Published: May 2008

AbstractWe demonstrate the sensitivity of X‐ray absorption fine‐structure (XAFS) measurements to the earliest stages of decomposition in Al alloys, i.e. just a few minutes after quenching. XAFS is one of the few applicable experimental approaches to this regime. Three different AlCu(Mg) samples were investigated by XAFS at the Cu K edge. Significant changes of the XAFS can be detected in the course of the decomposition in these alloys during the first 15 minutes. Actually, these changes correspond to relaxations of the nearest neighbours towards the absorbing Cu atoms. The Fourier transformation of the XAFS spectra thus leads to a pseudo radial distribution function which reflects this relaxation. In addition, XAFS measurements of the S‐phase of AlCuMg are used to decide in favour of the Perlitz and Westgren model for the S‐phase. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparison of second‐order orbital‐dependent DFT correlation functionals
Authors: Grabowski Ireneusz 
Published: May 2008

AbstractThe choice of the orbital‐dependent second‐order correlation functional plays the prime role in the description of the correlation effects in orbital‐dependent DFT calculations. Using second‐order perturbation theory we were able to derive the simplest orbital‐dependent correlation functional, but even at this lowest correlation level, we had several possibilities to define it. Applications of different second‐order correlation functionals for the atomic as well as molecular systems are presented. The ab initio DFT‐type OEP2 functionals based on Møller‐Plesset or semicanonical partitioning (OEP2‐sc) are compared with those based on Epstein‐Nesbet type partitioning, showing that the latter ones can fail in more difficult molecular problems, e.g., the Be dimer potential curve. We show that currently the best performing orbital‐dependent second‐order correlation functional is the OEP2‐sc one. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
Noniterative corrections to equation‐of‐motion coupled‐cluster excited state energies based on the reduced method of moments of coupled cluster equations
Authors: Kowalski Karol , Marat Valiev  
Published: May 2008

AbstractA new formalism closely related to the Method of Moments of Coupled‐Cluster equations (MMCC) is obtained by embedding approximate coupled cluster (CC) or equation‐of‐motion CC (EOMCC) formalism into a formalism which uses cluster or excitation operators defined by excitation operators of higher rank with respect to a given approximation. Noniterative corrections akin to completely renormalized EOM‐CCSD(T) corrections reveal structural similarities to the CCSD(T) corrections for the ground state. Several benchmark calculations on the O3 and Cl2O molecules are discussed to illustrate the performance of this approach. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008