High-efficiency optical compression of Ti:sapphire laser pulses at 800 nm using a silver-coated hollow fiber

A 50 cm silver coated hollow fiber with inner diameter of 250 μm and filled with argon has been used to compress optical pulses from a Ti:sapphire laser at 800 nm. Input pulses with energy of 250 μJ and duration of 110 fs were used and compressed pulses with energy of 220 μJ and duration of 20 fs were generated by using a prism compressor. Numerical and experimental results are compared. There is good agreement between the measured beam diameters of the hollow-fiber output pulse and the calculated values obtained from propagation of the HE₁₁ mode into free space. For comparison, a similar uncoated fused-silica hollow fiber was also used to obtain 20 fs compressed pulses with an energy of 190 μJ. Received: 7 September 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-780/492-1811, E-mail: mohebbi@ee.ualberta.ca     

Vitamin B12 supported on graphene oxide: As a bio‐based catalyst for selective aerobic oxidation of alcohols

The environmental impact of chemical processes has now opened new windows of opportunity for bio‐based catalysts. In this paper a highly active bio‐based catalyst of vitamin B₁₂ supported on graphene oxide nanosheets is reported for the selective aerobic oxidation of alcohols to the corresponding carbonyl compounds. Operational simplicity, mild reaction conditions, high yield and selectivity, non‐hazardous nature, commercial availability and affordability are the main advantages of this novel catalytic system.     

High‐order difference scheme for the solution of linear time fractional klein–gordon equations

In this article, we apply a high‐order difference scheme for the solution of some time fractional partial differential equations (PDEs). The time fractional Cattaneo equation and the linear time fractional Klein–Gordon and dissipative Klein–Gordon equations will be investigated. The time fractional derivative which has been described in the Caputo's sense is approximated by a scheme of order , and the space derivative is discretized with a fourth‐order compact procedure. We will prove the solvability of the proposed method by coefficient matrix property and the unconditional stability and ‐convergence with the energy method. Numerical examples demonstrate the theoretical results and the high accuracy of the proposed scheme. © 2014 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 30: 1234–1253, 2014     

Synthesis and Characterization of Novel Stilbenophanes: the Smallest Members of this Class of Compounds

The synthesis, properties and energy-optimized structure of stilbenophanes 1a–3a are reported. ¹H NMR spectroscopy of 1a shows it contains vinylic protons (unlike larger stilbenophanes), which appear as two doublets showing its unexpected configuration. Protons of the methylene bridge of 1b are also observed at different chemical shift, showing that they are diastereomeric protons. It is also found that the stereochemical outcome of the intramolecular McMurry reaction is strongly influenced by the length of the alkyl bridge.     

Effects of graphene quantum dot (GQD) on photoluminescence,mechanical, thermal and shape memory properties of thermoplastic polyurethane nanocomposites

A group of shape memory polyurethane‐based nanocomposites containing graphene quantum dot nanoparticles (GQDs) were prepared via in‐situ polymerization method. GQD nanoparticles were synthesized by a facile and rapid microwave‐assisted method and characterized by Fourier‐transform infrared spectroscopy (FTIR), X‐ray diffraction pattern, field emission scanning microscopy, transmission electron microscopy, and fluorescence analysis. Chemical structure and hydrogen bonding index (HBI_[C=O]) of the nanocomposites were analyzed via FTIR spectra. The results show that the incorporation of GQDs in PU matrix reduces HBI_(C=O) of nanocomposites. Crystalline structure and thermal properties of the nanocomposites were investigated by differential scanning calorimetry. As results indicate, nucleation effect of GQDs raises crystallinity content of the samples. Mechanical examinations indicate that incorporation of GQDs improves Young's modulus of the nanocomposites, while their elongation at break values are reduced. In addition, shape memory analyses reveal that the presence of GQDs in PU matrix increases the shape fixity ratios in nanocomposites.     

Toward a Consistent Interpretation of the QTAIM: Tortuous Link between Chemical Bonds,Interactions, and Bond/Line Paths

Currently, bonding analysis of molecules based on the Quantum Theory of Atoms in Molecules (QTAIM) is popular; however, “misinterpretations” of the QTAIM analysis are also very frequent. In this contribution the chemical relevance of the bond path as one of the key topological entities emerging from the QTAIM’s topological analysis of the one‐electron density is reconsidered. The role of nuclear vibrations on the topological analysis is investigated demonstrating that the bond paths are not indicators of chemical bonds. Also, it is argued that the detection of the bond paths is not necessary for the “interaction” to be present between two atoms in a molecule. The conceptual disentanglement of chemical bonds/interactions from the bonds paths, which are alternatively termed “line paths” in this contribution, dismisses many superficial inconsistencies. Such inconsistencies emerge from the presence/absence of the line paths in places of a molecule in which chemical intuition or alternative bonding analysis does not support the presence/absence of a chemical bond. Moreover, computational QTAIM studies have been performed on some “problematic” molecules, which were considered previously by other authors, and the role of nuclear vibrations on presence/absence of the line paths is studied demonstrating that a bonding pattern consistent with other theoretical schemes appears after a careful QTAIM analysis and a new “interpretation” of data is performed.     

Simultaneous determination of vitamin B12 and its derivatives using some of multivariate calibration 1 (MVC1) techniques

Resolution of binary mixtures of vitamin B12, methylcobalamin and B12 coenzyme with minimum sample pre-treatment and without analyte separation has been successfully achieved by methods of partial least squares algorithm with one dependent variable (PLS1), orthogonal signal correction/partial least squares (OSC/PLS), principal component regression (PCR) and hybrid linear analysis (HLA). Data of analysis were obtained from UV-vis spectra. The UV-vis spectra of the vitamin B12, methylcobalamin and B12 coenzyme were recorded in the same spectral conditions. The method of central composite design was used in the ranges of 10-80mgL(-1) for vitamin B12 and methylcobalamin and 20-130mgL(-1) for B12 coenzyme. The models refinement procedure and validation were performed by cross-validation. The minimum root mean square error of prediction (RMSEP) was 2.26mgL(-1) for vitamin B12 with PLS1, 1.33mgL(-1) for methylcobalamin with OSC/PLS and 3.24mgL(-1) for B12 coenzyme with HLA techniques. Figures of merit such as selectivity, sensitivity, analytical sensitivity and LOD were determined for three compounds. The procedure was successfully applied to simultaneous determination of three compounds in synthetic mixtures and in a pharmaceutical formulation.     

Design and synthesis of new 2-substituted-5-[2-(2-halobenzyloxy)phenyl]-1,3,4-oxadiazoles as anticonvulsant agents

A new series of 2-substituted-5-[2-(2-halobenzyloxy)phenyl]-1,3,4-oxadiazoles was designed and synthesized as anticonvulsant agents. Electroshock and pentylenetetrazole-induced lethal convulsion tests showed that the introduction of an amino group at position 2 of 1,3,4-oxadiazole ring and a fluoro substituent at ortho position of benzyloxy moiety had the best anticonvulsant activity. Our results showed that this effect is mediated through benzodiazepine receptors mechanism.     

Prediction of thermal conductivity and viscosity of nanofluids by molecular dynamics simulation

Limitations of conventional heat transfer fluids in different industries because of their poor thermal conductivity made heat transfer improvement in working fluids was performing, as a new method of advanced heat transfer. Therefore, the dispersion solid particle idea in fluids, which has been started with mili- and micrometer particles, completed by using nanoparticles and today nanofluids have been found to provide a considerable heat transfer and viscosity enhancement in comparison to conventional fluids such as water, ethylene glycol, and engine oil. In this study, molecular dynamics simulation was used to predict thermal conductivity and viscosity of nanofluids. Water was used as a base fluid. The simple point charge-extended (SPC/E) model was used for simulation of water and Ewald sum method for electrostatic interactions. Lennard–Jones potential for Van der Waals interactions, KTS potential for water and SiO₂ and Spor and Heinzinger correlation for water and Pt were used. The results were compared with experimental data. For investigation of the effect of temperature, simulation was done for three temperatures of 20, 30, and 50?C. The results showed that the ratio of thermal conductivity of nanofluid to base fluid and viscosity will decrease as the temperature increases. The effect of the concentration of nanoparticle was studied for three different concentrations, namely, 0.45, 1.85, and 4%. The thermal conductivity of nanofluid increases with increasing the concentration. Moreover, the effect of two nanoparticle sizes (i.e., 5 and 7 nm) on the thermal conductivity of nanofluid was investigated. It was shown that an increase in the size causes a decrease in the thermal conductivity. Finally, by replacing the SiO₂nanoparticle with a Pt nanoparticle in the nanofluid, it was observed that the kind of nanoparticle had not a considerable effect on increasing the thermal conductivity of nanofluid.