Synthesis,structure, and electrochemistry of pyridinecarboxamide cobalt(III) complexes; the effect of bridge substituents on the redox properties

Two series of complexes of the types trans-[Co^III(Mebpb)(amine)₂]ClO₄ {Mebpb²⁻ = N,N-bis(pyridine-2-carboxamido)-4-methylbenzene dianion, and amine = pyrrolidine (prldn) (1a), piperidine (pprdn) (2a), morpholine (mrpln) (3a), benzylamine (bzlan) (4a)}, and trans-[Co^III(cbpb)(amine)₂]X {cbpb²⁻ = N,N-bis(pyridine-2-carboxamido)-4-chlorobenzene dianion, and amine = pyrrolidine (prldn), X = PF₆ (1b), piperidine (pprdn), X = PF₆ (2b), morpholine (mrpln), X = ClO₄ (3b), benzylamine (bzlan), X = PF₆ (4b)} have been synthesized and characterized by elemental analyses, IR, UV–Vis, and ¹H NMR spectroscopy. The crystal structure of 1a has been determined by X-ray diffraction. The electrochemical behavior of these complexes, with the goal of evaluating the effect of axial ligation and equatorial substitution on the redox properties, is also reported. The reduction potential of Co^III, ranging from −0.53 V for (1a) to −0.31 V for (3a) and from −0.48 V for (1b) to −0.22 V for (3b) show a relatively good correlation with the σ-donor ability of the axial ligands. The methyl and chloro substituents of the equatorial ligand have a considerable effect on the redox potentials of the central cobalt ion and the ligand-centered redox processes.     

Photooxidation of Ethylbenzene with TiO 2 and Metal Coated TiO 2 and its Kinetics

Photooxidation of ethylbenzene with oxygen to give ethylbenzene hydroperoxide has been achieved in a stirred photochemical reactor that was cooled by a water system by irradiation with a 400W high-pressure mercury lamp and using TiO₂ powder and metal coated TiO₂. The effects of the amount of copper or silver coated on TiO₂ and of the temperature on the rate of oxidation have been investigated. It is suggested that thermal cleavage of the O–O bond and photochemically generated singlet oxygen should be considered as the initiating step in a radical chain mechanism. An optimum loading of 6% Ag or 4–5% Cu was observed for photooxidation of ethylbenzene.     

Dual‐parameter correlations on rate of an aromatic nucleophilic substitution reaction in aqueous solutions of methanol,ethanol, and propan‐2‐ol

Reaction kinetics of 1‐chloro‐2,4‐dinitrobenzene with piperidine was studied spectrophotometrically in aqueous solutions of methanol, ethanol, and propan‐2‐ol at 25°C. The reaction in these solutions is not catalyzed by piperidine. The plots of second‐order rate constants of the reaction vs. mole fraction of water show maxima in the all‐aqueous solutions. Single‐parameter correlations of log k₂ vs. π* (dipolarity/polarizability), α (hydrogen‐bond donor acidity), and E_T^N (normalized polarity parameter) are very poor in the all solutions (for example, in aqueous solutions of ethanol, regression coefficients are 0.814, 0.113, and 0.486, respectively). Dual‐parameter correlations of log k₂ vs. π* and α in all cases represent significant improvement with regard to the single‐parameter models (in aqueous solutions of ethanol: n = 11, r = 0.980, and s = 0.034). Dipolarity/polarizability and hydrogen‐bond donor acidity (HBD) of media have opposite effects on the reaction rate. The activated complex leading to the zwitterionic intermediate is expected to be favored by increasing the solvent dipolarity/polarizability parameter. Increasing the hydrogen‐bond donor acidity of solvent stabilizes piperidine and hence the reaction rate decreases. A dual‐parameter equation of log k₂ vs. π* and α was obtained in the all‐aqueous solutions (n = 31, r = 0.956, s = 0.055) in which π* and α have approximately equal and opposite effects on the reaction rate. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 118–123, 2001     

Kinetics study of a Diels‐Alder reaction in mixtures of an ionic liquid with molecular solvents

The second‐order rate constants for cycloaddition reaction of cyclopentadiene with naphthoquinone were determined spectrophotometrically in various compositions of 1‐(1‐butyl)‐3‐methylimidazolium terafluoroborate ([bmim]BF₄) with water and methanol at 25 °C. Rate constants of the reaction in pure solvents are in the order of water > [bmim]BF₄ > methanol. Rate constants of the reaction decrease sharply with mole fraction of the ionic liquid in aqueous solutions and increase slightly to a maximum in alcoholic mixtures. Multi‐parameter correlation of logk₂ versus solute–solvent interaction parameters demonstrated that solvophobicity parameter (Sp), hydrogen‐bond donor acidity (α) and hydrogen‐bond acceptor basicity (β) of media are the main factors influencing the reaction rate constant. The proposed three‐parameter model shows that the reaction rate constant increases with Sp, α and β parameters. Copyright © 2008 John Wiley & Sons, Ltd.     

Estimates on the dimension of an exponential attractor for a delay differential equation

We study the long time behavior of delay differential equation, considered in a bounded domain in ? ^d . Using the short trajectory method to prove the existence of the exponential attractor. Also we have estimates on the fractal dimension of an exponential attractor.     

Synthesis and properties of polyimides derived from bis-(Aminophenoxy) containing naphthalene, [Phenyl] propane and [Methyl] cyclohexane segment and 4, 4′-carbonyldiphthalic anhydride

ABSTRACT

Three wholly, semi aromatic and aliphatic-aromatic polyimides containing bis(phenoxy) naphthalene, bis[(phenoxy) phenyl] propane and bis(phenoxy-methyl) cyclohexane segments by the two-step procedure from 2, 7-bis(4-aminophenoxy) naphthalene (BAPON), 2, 2-bis[4-(4-aminophenoxy)phenyl]propane (BAPOP), 1, 4-bis (4-aminophenoxy methyl) cyclohexane (BAPMC) as a diamine and 4,4′-carbonyldiphthalic anhydride (CDPA) were prepared. The first step of this procedure including ring-opening polyaddition in a polar solvent to give poly(amic-acid)s, second step containing cyclodehydration reaction to form polyimides. Synthesized monomer and polyimides were characterized by FT-IR, ¹H NMR spectroscopy and elemental analyses (CHN) that obtained results gave the most powerful evidence. The polyimide synthesized from BAPON was characterized as semi-crystalline, whereas the other polyimides showed amorphous patterns by the x-ray diffraction studies. The inherent viscosity was ranging between 0.87–1.01 dL/g. Tensile strength, initial moduli, and elongation at break of the polyimide films ranged from 88–117 MPa, 1.98–2.32 GPa, and 5–8%, respectively. Thermogravimetric analysis in nitrogen atmosphere shows that these polymers having good stability, so 10% weight will be lost in the range of 500–630°C. The point of polyimide with BAPMC segment, is “adding of good thermal stability and processability” lower moisture absorption and dielectric constant (0.75% and 2.90).     

Thermal properties of undoped, S-doped, Nb-doped, and S, Nb co-doped titania nanoparticles prepared by sol–gel method

One of the classic methods to extend the spectral response of a wide band gap titania semiconductor to visible light is impurity doping. This study has studied undoped, S-doped, Nb-doped, and S, Nb co-doped titania nanopowders prepared by sol–gel. The relationship between the doping conditions and the thermal stability, phase composition, crystallinity, morphology of particles, and the influence of dopant in similar conditions was investigated. Also the relationship between the dopant and the properties of titania nanosized powders, such as thermal stability, phase composition, crystallinity, morphology and size of particles, was investigated. Thermogravimetry coupled with differential thermogravimetry, X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning electron microscope was used for the characterization of the materials. During heating of the air dried samples, their chemical degradation took place giving rise to anatase phase.     

Efficient Synthesis of Highly Functionalized,S -Alkyl 1-Alkyl-4,4-dimethyl-2,5-dioxo-pyrrolidine-3-carbothioates

5-Isopropylidene-2,2-dimethyl-1,3-dioxane-4,6-dione (the condensation product of Meldrum's acid and acetone) reacts with alkyl isocyanides in the presence of this to produce S -alkyl 1-alkyl-4,4-dimethyl-2,5-dioxo-pyridine-3-carbothioates in good yield.     

Thermal and structural studies of zinc zirconate nanoscale composite derived from sol–gel process

In the present investigation, a novel synthetic zinc zirconate nanocomposites were prepared by a sol–gel technique using a very stable sol containing zirconium acetylacetonate, zinc acetate, monoethanolamine, and 1,3-propanediol as chelating agent. Thermal analysis results indicated that the decomposition of zinc zirconate precursors occurred at 225 and 234 °C. The influence of thermal annealing (temperatures and duration) on their structural properties has been studied by means of X-ray diffraction and Fourier transform-infrared spectroscopy techniques. The results indicated that a higher percent of zinc zirconate is formed at 800 °C and reached maximum at 1,000 °C in 120 min. The morphology, composition, and bandgap properties of zinc zirconate nanoparticles were characterized by transmission electron microscope, scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy, and ultraviolet diffiuse reflectance. The SEM observation showed that average grain size of zinc zirconate nanopowders was 58 nm. The optical results revealed maximum absorbances at 394, 413, and 438 nm for ZnZrO₃ sample annealed at 400, 800, and 900 °C for 30 min, respectively. This is an indication that the nanopowder can absorb lights in the higher wavelength.     

Effect of Nanofluid Treatment on Water Sensitive Formation to Investigate Water Shock Phenomenon,An Experimental Study

Permeability reduction in porous media as a result of frail and tenuous fine particles migration would decrease the productivity index in the subterranean reservoirs. During reservoir stimulation by injecting fluids into the reservoir, as the salinity condition of the formation brine changes, fine particles initiate the triggering process. In this study, MgO-based nanofluid as a fines fixation agent was stably prepared based on the particle size distribution and characterized through transmission electron microscopy analysis. Afterward, several core flooding tests were performed using Berea sandstone cores to study the effect of nanofluid injection on fines fixation in the water shock phenomenon. Permeability reduction occurred up to 95% of inchoate permeability for no treatment case, which was also confirmed by scanning electron microscopy analysis. Finally, MgO nanofluid with 0.03 wt% concentration and 120 minutes soaking time in the core mitigated the fine particles release and fixed them on the pore walls' surfaces critically reducing the formation damage. The analysis shows that outweighing the attraction potentials compared to repulsions was the main mechanism after nanofluids treatment.