The Padé-Rayleigh-Ritz method for solving large hermitian eigenproblems

We make use of the Padé approximants and the Krylov sequencex, Ax,,...,A ^m–1 x in the projection methods to compute a few Ritz values of a large hermitian matrixA of ordern. This process consists in approaching the poles ofR _x()=((I–A)^–1 x,x), the mean value of the resolvant ofA, by those of [m–1/m]_Rx(), where [m–1/m]_Rx() is the Padé approximant of orderm of the functionR _x(). This is equivalent to approaching some eigenvalues ofA by the roots of the polynomial of degreem of the denominator of [m–1/m]_Rx(). This projection method, called the Padé-Rayleigh-Ritz (PRR) method, provides a simple way to determine the minimum polynomial ofx in the Krylov subspace methods for the symmetrical case. The numerical stability of the PRR method can be ensured if the projection subspacem is sufficiently small. The mainly expensive portion of this method is its projection phase, which is composed of the matrix-vector multiplications and, consequently, is well suited for parallel computing. This is also true when the matrices are sparse, as recently demonstrated, especially on massively parallel machines. This paper points out a relationship between the PRR and Lanczos methods and presents a theoretical comparison between them with regard to stability and parallelism. We then try to justify the use of this method under some assumptions.     

Intramolecular charge-transfer interactions in pi-extended tetrathiafulvalene derivatives

New electron-donor (D)-electron-acceptor (A) TTF architectures are presented in which two electron-donating 1,3-dithiole moieties are connected by a pi bridge to the weak electron-accepting quinoxaline moiety (D-pi-A compounds 9a and 9b and also two 1,3-dithiole-2-ylidene moieties are connected by a pi bridge to the electron-accepting thieno[3,4-b]quinoxaline bridge (D-pi-A-pi-D compounds 12a-c). There are through-bond intramolecular charge-transfer (ICT) interactions, predicted in theoretical calculations, and confirmed by UV-vis spectroscopy and cyclic voltammetry measurements. This work constitutes the first use of quinoxalines as electron-accepting moieties in D-pi-A compounds.     

Mixed Polymer‐Coated Magnetic Nanoparticles as Forward Osmosis Draw Agents of Tuned Hydrophilicity

We recently reported a polymer‐coated magnetic nanoparticle (MNP) draw agent for the forward osmosis (FO) water desalination process. The water flux was found to increase when the polymer poly(sodium acrylate) (PSA) was anchored to the MNP surface as compared to the polymer (or polyelectrolyte solution) alone, due to the polymer chains being stretched out and most of the hydrophilic groups on the polymer contributing to water flux. We herein report the use of a secondary polymer poly(N‐isopropylacrylamide) PNIPAM to manipulate the PSA polymer conformation and influence inter‐ and intrachain interactions to enhance the efficiency of the FO draw agent. These PSA–PNIPAM‐coated MNPs generated a much higher water flux of ～11.66 LMH when compared to the 100 % PSA‐coated MNPs featuring a value of ～5.32 LMH under identical FO conditions. The osmotic pressure and water flux driven by the mixed polymer‐coated MNPs were found to be a strong function of the net polymer coverage on MNPs, that is, net available hydrophilic groups. Our new draw agent demonstrates potential for use in the water industry due to its improved efficiency and cost effectiveness as it uses only ～0.062 % (w/v) of the draw agent solution.     

Fatty amides synthesized from vegetable oil as extractant of molybdenum(VI)

In this study, fatty amides (FAs) synthesized from palm olein were used to extract and separate Mo(VI) from acidic media. Effects of various parameters upon the separation of Mo(VI) from Co(II), Ni(II), Al(III) and Mn(II), including extractant concentration, metal ion concentration, contact time, diluent, and acidity, were investigated. It was found that Mo(VI) was successfully separated from the above commonly associated metal ions by stripping from the loaded organic phase. Different acidic and alkaline solutions were used. Ammonium hydroxide solution was an optimal. Extraction of Mo(VI) into the organic phase involved the formation of 1:3 complexes. This work presents the development of a low-cost and environmentally friendly extractant to recycle and recover molybdenum.     

Design,preparation and characterization of Cu/GA/Fe3O4@SiO2 nanoparticles as a catalyst for the synthesis of benzodiazepines and imidazoles

The synthesis and characterization of an efficient and reusable nanocatalyst, Cu/GA/Fe₃O₄@SiO₂, obtained by ultrasonic‐assisted grafting of guanidineacetic acid on modified Fe₃O₄@SiO₂ core–shell nanocomposite spheres and subsequent immobilization of Cu(II), are described. The catalyst was characterized by means of X‐ray diffraction, scanning and transmission electron microscopies, energy‐dispersive X‐ray spectroscopy, elemental analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy, vibrating sample magnetometry and inductively coupled plasma optical emission spectrometry. The prepared nanocatalyst facilitated an efficient and straightforward friendly procedure for the synthesis of benzodiazepines and imidazoles in ethanol and under solvent‐free conditions, respectively. The nanocatalyst can be easily recovered using a magnet and reused several times without any significant loss of activity. Copyright © 2016 John Wiley & Sons, Ltd.     

Novel and sensitive UPLC–MS/MS method for quantification of sofosbuvir in human plasma: application to a bioequivalence study

A novel and sensitive LC–MS/MS method was developed and validated for determination of sofosbuvir (SF) using eplerenone as an internal standard. The Xevo TQD LC–MS/MS was operated under the multiple‐reaction monitoring mode using electrospray ionization. Extraction with tert‐butyl methyl ether was used in sample preparation. The prepared samples were chromatographed on Acquity UPLC BEH C₁₈ (50 × 2.1 mm, 1.7 μm) column by pumping 0.1% formic acid and acetonitrile in an isocratic mode at a flow rate of 0.35 mL/min. Method validation was performed as per the US Food and Drug Administration guidelines and the standard curves were found to be linear in the range of 0.25–3500 ng/mL for SF. The intra‐ and inter‐day precision and accuracy results were within the acceptable limits. A very short run time of 1 min made it possible to analyze more than 500 human plasma samples per day. A very low quantification limit of SF allowed the applicability of the developed method for determination of SF in a bioequivalence study in human volunteers. Copyright © 2016 John Wiley & Sons, Ltd.     

Novel palladium nanoparticles supported on β‐cyclodextrin@graphene oxide as magnetically recyclable catalyst for Suzuki–Miyaura cross‐coupling reaction with two different approaches in bio‐based solvents

A novel nanocatalyst was designed and prepared. Initially, the surface of magnetic graphene oxide (M‐GO) was modified using thionyl chloride, tris(hydroxymethyl)aminomethane and acryloyl chloride as linkers which provide reactive C═C bonds for the polymerization of vinylic monomers. Separately, β‐cyclodextrin (β‐CD) was treated with acryloyl chloride to provide a modified β‐CD. Then, in the presence methylenebisacrylamide as a cross‐linker, monomers of modified β‐CD and acrylamide were polymerized on the surface of the pre‐prepared M‐GO. Finally, palladium acetate and sodium borohydride were added to this composite to afford supported palladium nanoparticles. This fabricated nanocomposite was fully characterized using various techniques. The efficiency of this easily separable and reusable heterogeneous catalyst was successfully examined in Suzuki–Miyaura cross‐coupling reactions of aryl halides and boronic acid as well as in modified Suzuki–Miyaura cross‐coupling reactions of N‐acylsuccinimides and boronic acid in green media. The results showed that the nanocatalyst was efficient in coupling reactions for direct formation of the corresponding biphenyl as well as benzophenone derivatives in green media based on bio‐based solvents. In addition, the nanocatalyst was easily separable, using an external magnet, and could be reused several times without significant loss of activity under the optimum reaction conditions.     

Rapid parameterization of small molecules using the force field toolkit

The inability to rapidly generate accurate and robust parameters for novel chemical matter continues to severely limit the application of molecular dynamics simulations to many biological systems of interest, especially in fields such as drug discovery. Although the release of generalized versions of common classical force fields, for example, General Amber Force Field and CHARMM General Force Field, have posited guidelines for parameterization of small molecules, many technical challenges remain that have hampered their wide‐scale extension. The Force Field Toolkit (ffTK), described herein, minimizes common barriers to ligand parameterization through algorithm and method development, automation of tedious and error‐prone tasks, and graphical user interface design. Distributed as a VMD plugin, ffTK facilitates the traversal of a clear and organized workflow resulting in a complete set of CHARMM‐compatible parameters. A variety of tools are provided to generate quantum mechanical target data, setup multidimensional optimization routines, and analyze parameter performance. Parameters developed for a small test set of molecules using ffTK were comparable to existing CGenFF parameters in their ability to reproduce experimentally measured values for pure‐solvent properties (<15% error from experiment) and free energy of solvation (±0.5 kcal/mol from experiment). © 2013 Wiley Periodicals, Inc.