Synthesis,crystal structure and reactivity of trans-diaquo N,N′-ethylenebis-(3-methoxysalicylideneiminato)chromium(III)

A new chromium(III) Schiff base complex, [Cr(3-methoxysalen)(H₂O)₂]ClO₄, where salen=N,N-ethylenebis- (salicylideneimine), has been synthesized and characterized by spectroscopic techniques. Single crystal X-ray data reveals that the complex assumes a trans-diaquo structure with formulation [Cr(C₁₈H₂₂N₂O₆)]ClO₄ but, unlike in Cr (salen)(H₂O)₂ ⁺, the two trans-water molecules are equidistant from Cr^III. The effect of the substituent on the phenyl ring in respect of redox reactivity has been investigated. The kinetics of the oxidation of [Cr(Schiff base)- (H₂O)₂]ClO₄, where Schiff base=salen, (1) and 3-OMe-salen, (2) by PhOI has been studied. The bimolecular rate constant for the formation of the O=Cr^v Schiff base in case of (2) was four times faster than that of (1). The introduction of the OMe group substituent on the phenyl ring influences not only the structure and crystal packing, but also the reactivity of the complex and the electronic environment around the metal ion.     

Brownian dynamics simulation and experimental study of colloidal particle deposition in a microchannel flow

This paper reports an analysis of the irreversible deposition of colloidal particles from the pressure-driven flow in a microchannel within the framework of DLVO theory. A theoretical model is presented on the basis of the stochastic Langevin equation, incorporating the random Brownian motion of colloidal particles. Brownian dynamics simulation is used to compute the particle deposition in terms of the surface coverage. To validate the theoretical model, experiments are carried out using the parallel-plate flow cell technique, enabling direct videomicroscopic observation of the deposition kinetics of polystyrene latex particles in NaCl electrolytes. The theoretical predictions are compared with the experimental results, and good agreement is found.     

A new zirconium inorganic building brick forming metal organic frameworks with exceptional stability

Porous crystals are strategic materials with industrial applications within petrochemistry, catalysis, gas storage, and selective separation. Their unique properties are based on the molecular-scale porous character. However, a principal limitation of zeolites and similar oxide-based materials is the relatively small size of the pores, typically in the range of medium-sized molecules, limiting their use in pharmaceutical and fine chemical applications. Metal organic frameworks (MOFs) provided a breakthrough in this respect. New MOFs appear at a high and an increasing pace, but the appearances of new, stable inorganic building bricks are rare. Here we present a new zirconium-based inorganic building brick that allows the synthesis of very high surface area MOFs with unprecedented stability. The high stability is based on the combination of strong Zr-O bonds and the ability of the inner Zr6-cluster to rearrange reversibly upon removal or addition of mu3-OH groups, without any changes in the connecting carboxylates. The weak thermal, chemical, and mechanical stability of most MOFs is probably the most important property that limits their use in large scale industrial applications. The Zr-MOFs presented in this work have the toughness needed for industrial applications; decomposition temperature above 500 degrees C and resistance to most chemicals, and they remain crystalline even after exposure to 10 tons/cm2 of external pressure.     

Line shape parameters measurement and computations for self-broadened carbon dioxide transitions in the 30012 ← 00001 and 30013 ← 00001 bands, line mixing, and speed dependence

Transitions of pure carbon dioxide have been measured using a Fourier transform spectrometer in the 30012 ← 00001 and 30013 ← 00001 vibrational bands. The room temperature spectra, recorded at a resolution of 0.008 cm⁻¹, were analyzed using the Voigt model and a Speed Dependent Voigt line shape model that includes a pressure dependent narrowing parameter. Intensities, self-induced pressure broadening, shifts, and weak line mixing coefficients are determined. The results obtained are consistent with other studies in addition to the theoretically calculated values. Exponential Power Gap (EPG) and Energy Corrected Sudden (ECS) scaling laws were used to calculate the relaxation matrix elements.     

Coke formation during the methanol-to-olefin conversion: in situ microspectroscopy on individual H-ZSM-5 crystals with different Brønsted acidity

Coke formation during the methanol‐to‐olefin (MTO) conversion has been studied at the single‐particle level with in situ UV/Vis and confocal fluorescence microscopy. For this purpose, large H‐ZSM‐5 crystals differing in their Si/Al molar ratio have been investigated. During MTO, performed at 623 and 773 K, three major UV/Vis bands assigned to different carbonaceous deposits and their precursors are observed. The absorption at 420 nm, assigned to methyl‐substituted aromatic compounds, initiates the buildup of the optically active coke species. With time‐on‐stream, these carbonaceous compounds expand in size, resulting in the gradual development of a second absorption band at around 500 nm. An additional broad absorption band in the 600 nm region indicates the enhanced formation of extended carbonaceous compounds that form as the reaction temperature is raised. Overall, the rate of coke formation decreases with decreasing aluminum content. Analysis of the reaction kinetics indicates that an increased Brønsted acid site density facilitates the formation of larger coke species and enhances their formation rate. The use of multiple excitation wavelengths in confocal fluorescence microscopy enables the localization of coke compounds with different molecular dimensions in an individual H‐ZSM‐5 crystal. It demonstrates that small coke species evenly spread throughout the entire H‐ZSM‐5 crystal, whereas extended coke deposits primarily form near the crystal edges and surfaces. Polarization‐dependent UV/Vis spectroscopy measurements illustrate that extended coke species are predominantly formed in the straight channels of H‐ZSM‐5. In addition, at higher temperatures, fast deactivation leads to the formation of large aromatic compounds within channel intersections and at the external zeolite surface, where the lack of spatial restrictions allows the formation of graphite‐like coke.     

Enhanced electrocatalytic performance of functionalized carbon nanotube electrodes for oxygen reduction in proton exchange membrane fuel cells

Although nitrogen doped CNTs (N-CNTs) are considered as a promising alternative to platinized carbon for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs), the origin of the enhanced ORR activity with N-CNTs is not clear at present. Among several plausible reasons, the exposure of edge plane and creation of impurity band/surface states near the Fermi level are considered as major causes behind the catalytic activity. However, CNTs without nitrogen doping are not known to catalyze the ORR. In this work, we study the ORR activity of functionalized carbon nanotubes with different functional groups, such as sulfonic acid and phosphonic acid, in order to understand the role of surface functionalities in catalyzing the reaction. Functionalized CNTs show significantly enhanced activity towards the ORR, while CNTs without such surface functional groups do not reveal any such special ORR activity. Linear sweep voltammetry experiments with different rotation rates show diffusion controlled limiting current values for functionalized CNTs, and the 'n' values derived from Koutecky-Levich plots are 3.3 and 1.7 for S-MWCNTs and P-MWCNTs, respectively. This work demonstrates the ORR activity of functionalized MWCNTs, which opens up new strategies for electrocatalyst design in PEMFCs.     

High-performance liquid chromatographic method using ultraviolet detection for measuring metrifonate and dichlorvos levels in human plasma.

We report high-performance liquid chromatographic methods using ultraviolet detection, developed for the first time in our laboratory with sensitivity to detect clinically significant concentrations of metrifonate (MTF), an experimental drug for Alzheimer disease, and its active anticholinesterase metabolite, dichlorvos (DDVP). The determination limit of the method for MTF and DDVP was 1 microgram/ml and 40 ng/ml, respectively. Stability of MTF and DDVP at various temperatures in water, buffered solutions and in human plasma were also studied.     

Studies on optical absorption and photoluminescence of thioglycerol-stabilized CdS quantum dots

Nanoparticles of CdS were prepared at 303 K by aqueous precipitation method using CdSO4 and (NH4)2S in presence of the stabilizing agent thioglycerol. Adjustment of the thioglycerol (T) to ammonium sulphide (A) ratio (T:A) from 1:25 to 1:3.3 was done during synthesis and nanoparticles of different size were obtained. The prepared colloids were characterized by UV-vis and photoluminescence (PL) spectroscopic studies. Prominent first and second excitonic transitions are observed in the UV-vis spectrum of the colloid prepared with a T:A ratio of 1:3.3. Particle size analysis was done using XRD, high resolution TEM and dynamic light scattering and found to be approximately 3 nm. UV-vis and PL spectral features also agree with this particle size in colloid with T:A of 1:3.3. The band gap of CdS quantum dots has increased from the bulk value 2.4-2.9 eV. PL spectra show quantum size effect and the peak is shifted from 628 to 556 nm when the ratio of T:A was changed from 1:25 to 1:3.3. Doping of CdS with Zn2+ and Cu2+ is found to enhance the PL intensity. PL band shows blue-shift and red-shift on doping with Zn2+ and Cu2+, respectively. UV and PL spectral features of the CdS/Au hybrid nanoparticles obtained by a physical mixing of CdS and Au nanoclusters in various volume ratios is also discussed. Au red-shifts and rapidly quenches the PL of CdS. An additional low energy band approximately 650 nm is observed in the UV visible spectrum of the hybrid nanoparticles.