Search results for "four"
showing 10 items of 1678 documents
The reluctant polymorph: investigation into the effect of self-association on the solvent mediated phase transformation and nucleation of theophylline
2015
Little is known concerning the pathway of the crystallization of the thermodynamically stable polymorph of theophylline, form IV. Here we study the reasons why the thermodynamically stable theophylline form IV can be obtained only by slow, solvent mediated phase transformation (SMPT) in specific solvents, and whether the presence of prenucleation aggregates affect the polymorphic outcome. Solution concentration, polymorphic composition and morphology were monitored over time during the transformation from form II to form IV in several solvents. NMR and FTIR spectroscopy were used to detect prenucleation molecular aggregates present in the solutions. It was determined that theophylline self-…
Structural study of ethyl 3-arylcarbamoyl-2,3-diazabicyclo[2.2.1]hept-5-ene-2-carboxylates: conformation and transmission of substituent effects acro…
2001
Nine new ethyl 3-arylcarbamoyl-2,3-diazabicyclo[2.2.1]hept-5-ene-2-carboxylates were prepared by a [4 + 2] cycloaddition and their FTIR, 1H, 13C and 15N NMR spectra were measured and assigned. Single crystals were grown for five compounds and their X-ray data were obtained. The electronic structure and the conformations were calculated by the semi-empirical AM1 method. Using correlations between the spectral, empirical and theoretical structural data, the transmission of substituent effects and the preferential conformation connected with the consecutive double nitrogen inversion and regarding the mutual orientation of NH and CO bonds were investigated. The results are compared with those f…
Low-cost set-up for Fourier-transform infrared spectroscopy in diamond anvil cell from 4000 to 400 cm−1
2011
An experimental set-up for Fourier-transform infrared (FTIR) studies at high pressure in the mid-IR region (400–4000 cm−1) is constructed using a compact TEO-400 FTIR interferometer module and an external microscopic optical bench with cassegrain focusing objectives. Cassegrain-type reflective objectives act as an excellent beam condenser that facilitates the interfacing between FTIR spectrometer and diamond anvil cell. This set-up is capable of recording transmission and reflection infrared spectra at high pressure. Preliminary results are reported both in the reflection (pressure dependence of polar phonons in CuWO4) and transmission configuration (polarized light absorption of polar phon…
Microwave emission related to cyclotron instabilities in a minimum-Belectron cyclotron resonance ion source plasma
2015
Electron cyclotron resonance ion sources (ECRIS) have been essential in the research and applications of nuclear physics over the past 40 years. They are extensively used in a wide range of large-scale accelerator facilities for the production of highly charged heavy ion beams of stable and radioactive elements. ECRISs are susceptible to kinetic instabilities due to resonance heating mechanism leading to anisotropic electron velocity distribution function. Instabilities of cyclotron type are a proven cause of frequently observed periodic bursts of 'hot' electrons and bremsstrahlung, accompanied with emission of microwave radiation and followed by considerable drop of multiply charged ions c…
On-line gradient liquid chromatography–Fourier transform infrared spectrometry determination of sugars in beverages using chemometric background corr…
2008
Abstract An on-line gradient reversed phase liquid chromatography–Fourier transform infrared spectrometry (LC–FTIR) method was developed for the determination of fructose, glucose, sucrose and maltose in beverages. Improved chromatographic resolution was obtained using a linear gradient from 75 to 55% (v/v) acetonitrile in water in 15 min. Changes in the background spectra were corrected employing “univariate background correction based on the use of a reference spectra matrix” (UBC-RSM) and using the ratio of absorbance (AR) at 2256 and 2253 cm −1 for the identification of the eluent spectra within the RSM. The method provided limits of detection in the order of 0.75 mg ml −1 . The precisi…
Quantitative Analysis, Infrared
2000
In this article, after reviewing the different mathematical methods used in quantification we will describe and discuss their respective advantages and inconveniences. First of all, different examples of applications of infrared (IR) spectroscopy, used with or without other analytical chemistry methods, will be presented. Some examples of direct quantitative analysis in food analysis will be reviewed: lipids (measurement of unsaturation degrees, lipids determination), carbohydrates, proteins (secondary structures, quantitative analysis). We will also focus on original examples of the use of IR spectroscopy combined with enzymes. Secondly, some examples of Fourier transform infrared (FTIR) a…
Using ATR-FTIR Spectroscopy To Design Active Antimicrobial Food Packaging Structures Based on High Molecular Weight Chitosan Polysaccharide
2007
ATR-FTIR spectroscopy has been used in this study to characterize the molecular mechanisms and kinetic processes that take place when a chitosonium acetate thin coating is put in contact with water solutions, Staphylococcus aureus solutions, microbial nutrient solutions, and with a high water activity TSA hydrogel medium to simulate the effect of direct contact with high moisture foods such as fresh meats, fish, and seafood products or beverages. The results of this work suggest that the biocide carboxylate groups that form when chitosan is cast from acetic acid solutions are being continuously evaporated from the formed film in the form of acetic acid (mechanism I) in the presence of envir…
Influence of synthesis conditions on the performance of chitosan–Heteropolyacid complexes as membranes for low temperature H2–O2 fuel cell
2015
Flat, free-standing chitosan/phosphotungstic acid (PTA) polyelectrolyte membranes were prepared by in-situ ionotropic gelation process at room temperature on porous alumina support firstly impregnated by H3PW12O40. Scanning electron microscopy revealed the formation of compact and homogeneous membranes, whose thickness resulted to be dependent on chitosan concentration and reticulation time. X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) evidenced the formation of almost amorphous membrane without appreciable concentration of not protonated NH2 groups and PTA3- ions with preserved Keggin structure. Membranes were tested as proton conductor in low temperature H2-O2 fuel…
Ab initio calculations and vibrational spectroscopy on the phenylenediamine isomers
1998
Molecular orbital calculations at HF and MP2 levels have been performed using the 6-3IG** basis set for full geometry optimization of the phenylenediamine isomers. Our results show that only a transoid conformer is found for o-phenylenediamine, whereas cis and trans conformers exist for m- and p-phenylenediamine. Vibrational normal modes have been also analyzed for the gas phase and in chloroform solution, and compared with experimental data we have obtained using FTIR spectroscopy. © 1998 Elsevier Science B.V.
Biogenic synthesis of palladium nanoparticles using Pulicaria glutinosa extract and their catalytic activity towards the Suzuki coupling reaction.
2014
Green synthesis of nanomaterials finds the edge over chemical methods due to its environmental compatibility. Herein, we report a facile and eco-friendly method for the synthesis of palladium (Pd) nanoparticles (NPs) using an aqueous solution of Pulicaria glutinosa, a plant widely found in a large region of Saudi Arabia, as a bioreductant. The as-prepared Pd NPs were characterized using ultraviolet-visible (UV-vis) spectroscopy, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform-infrared spectroscopy (FT-IR). The hydroxyl groups of the plant extract (PE) molecules were found mainly responsible for the red…