Heterogeneous Wheel‐Shaped Cu20‐Polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25− Catalyst for Solvent‐Free Aerobic Oxidation of n‐Hexadecane

The selective oxidation of alkanes as a green process remains a challenging task because partial oxidation is easier to achieve with sacrificial oxidants, such as hydrogen peroxide, alkyl hydroperoxides or iodosylbenzene, than with molecular oxygen or air. Here, we report on a heterogeneous catalyst for n‐hexadecane oxidation comprised of the wheel shaped Cu₂₀‐polyoxotungstate [Cu₂₀Cl(OH)₂₄(H₂O)₁₂(P₈W₄₈O₁₈₄)]^25? anchored on 3‐aminopropyltriethoxysilane (apts)‐modified SBA‐15. The catalysts were characterized by powder X‐ray diffraction (XRD), N₂‐adsorption measurements and Fourier transform infrared reflectance (FT‐IR) spectroscopy. The heterogeneous Cu₂₀‐polyanion system catalyzed the solvent‐free aerobic oxidation of n‐hexadecane to alcohols and ketones by using air as the oxidant under ambient conditions. The catalyst exhibits an exceptionally high turn over frequency (TOF) of 20 000 h^?1 at 150 °C and is resistant to poisoning by CS₂. Moreover, it can be easily recovered and reused by filtration without loss of its catalytic activity. Possible homogeneous contributions also have been examined and eliminated. Thus, this system can use air as oxidant, which, in combination with its good overall performance and poison tolerance, raises the prospect of this type of heterogeneous catalyst for practical applications.     

Mathematical modeling of pulsatile flow of non-Newtonian fluid in stenosed arteries

The pulsatile flow of blood through mild stenosed artery is studied. The effects of pulsatility, stenosis and non-Newtonian behavior of blood, treating the blood as Herschel–Bulkley fluid, are simultaneously considered. A perturbation method is used to analyze the flow. The expressions for the shear stress, velocity, flow rate, wall shear stress, longitudinal impedance and the plug core radius have been obtained. The variations of these flow quantities with different parameters of the fluid have been analyzed. It is found that, the plug core radius, pressure drop and wall shear stress increase with the increase of yield stress or the stenosis height. The velocity and the wall shear stress increase considerably with the increase in the amplitude of the pressure drop. It is clear that for a given value of stenosis height and for the increasing values of the stenosis shape parameter from 3 to 6, there is a sharp increase in the impedance of the flow and also the plots are skewed to the right-hand side. It is observed that the estimates of the increase in the longitudinal impedance increase with the increase of the axial distance or with the increase of the stenosis height. The present study also brings out the effects of asymmetric of the stenosis on the flow quantities.     

Ferromagnetic to spin glass cross over in (La,Tb)2/3Ca1/3MnO3

In the series La_2/3?xTb_xCa_1/3MnO₃, it is known that the compositions are ferromagnetic for smaller values of x and show spin glass characteristics at larger values of x. Our studies on the magnetic properties of various compositions in the La_2/3?xTb_xCa_1/3MnO₃ series show that the cross over from ferromagnetic to spin glass region takes place above x ≈ 1/8. Also, a low temperature anomaly at 30 K, observed in the ac susceptibility curves, disappears for compositions above this critical value of x. A mixed phase region coexists in the narrow compositional range 0.1 ≤ x ≤ 0.125, indicating that the ferromagnetic to spin glass cross over is not abrupt.     

Structural,mechanical and thermal properties of some Holmium Pnictides under pressure: A theoretical approach

The high pressure structural, elastic and thermal properties of holmium pnictides HoX (X=N, P, As and Bi) were investigated theoretically by using an inter-ionic potential theory with modified ionic charge parameter. We have predicted a structural phase transition from NaCl (B₁) to CsCl (B₂)-type structure at pressure of 139 GPa for HoN, 52 GPa for HoP, 44 GPa for HoAs and 26 GPa for HoBi. Other properties, such as lattice constant, bulk modulus, cohesive energy, second and third-order elastic constants were calculated and compared with the available experimental and theoretical data. In order to gain further information the brittle behaviour of these compounds was observed. Some other properties like Shear modulus (G), Young's modulus (E), Poisson's ratio (ν), anisotropy factor (A), sound velocities, Debye temperature (θ_D) were calculated. The variation of elastic constants (C₁₁ and C₄₄) and Debye temperature (θ_D) with pressure was also presented.     

A Rhodamine-Based Dual Chemosensor for Cu(II) and Fe(III)

An “off-on” rhodamine-based fluorescence probe for the selective signaling of Cu(II) and Fe(III) has been designed and synthesized. The optical properties of this compound have been investigated in acetonitrile-water (1:1) binary solution. Very interestingly, this compound showed sensitivity and selectivity towards Cu(II) during absorption process and towards Fe(III) during emission process. So this is a nice example of an excellent dual chemosensor for two biologically/physiologically very important transition metal ions using only the two very different techniques (absorption and emission); both cases displayed only intensity enhancement.     

Gel growth of α and γ glycine and their characterization

The microbial free single crystals of α and γ glycine were grown from gel at room temperature in a new chemical route. These crystals showed a superior quality than the solution grown crystals. The metastable α-form and the stable γ-form of glycine were crystallized in silica gel by solubility reduction method. The form of crystallization is confirmed by single crystal and powder X-ray diffraction analyses. The crystals of α and γ glycine were found to crystallize in monoclinic and hexagonal crystal systems, respectively. For analyzing the functional group and thermal stability of α and γ glycine crystals, spectroscopic and thermal analyses have been carried out. The dielectric studies were performed to find the dielectric constant of the grown crystals and the results are discussed. Second harmonic generation efficiency of the crystal was measured by Kurtz’s powder method using Nd:YAG laser and it was found to be 2.68 times that of potassium dihydrogen phosphate crystals.     

Single crystal EPR study of Mn(II) doped MgRb2(SO4)2·6H2O: A case of interstitial site

Single crystal electron paramagnetic resonance studies on magnesium rubidium sulphate hexahydrate doped with Mn(II) impurity, in the form of 0.1% of manganous chloride, are carried out at room temperature. The angular variation spectra of the crystal in the three orthogonal planes show that the paramagnetic impurity Mn(II) has entered the host lattice interstitially. The spin Hamiltonian parameters calculated are: g_xx = 1.974, g_yy = 1.928, g_zz = 1.980; A_xx = 9.28, A_yy = 8.58, A_zz = 9.87 mT and D_xx = ‐18.71, D_yy = ‐5.59, D_zz = 24.30 mT. The room temperature and liquid nitrogen temperature study for polycrystalline sample indicate no phase transition. The percentage covalency of Mn–O bond has been estimated to be 11. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fuzzy handoff algorithms for wireless communication

In order to manage the high call density expected of future cellular systems, microcells must be used. A migration to microcells will increase the number of handoffs, and require faster handoff algorithms – in terms of decision making. In the case of line-of-sight transmission, it is important that the handoff algorithm detects the cell boundary early enough, otherwise this will lead to channel dragging into the new cell subsequently increasing the chance of co-channel interference. In the case of non-line-of-sight transmission, a mobile station on turning a street corner will experience a phenomenon known as the Manhattan corner effect that causes the received signal level to drop by 20–30 dB in 20–30 m. This corner effect problem can lead to a loss of communication if not identified early enough. This paper presents two new handoff techniques using fuzzy logic as possible solutions to microcellular handoff. The first algorithm uses an adaptive fuzzy predictor, while the second uses a fuzzy averaging technique. The results of the simulation show that fuzzy is a viable option for microcellular handoff.     

Photopolymers (I): photoinitiating role of monochloroacetic acid in the synthesis of poly(methyl methacrylate)

Photopolymerization of methyl methacrylate in bulk and in solution at 40 °C using monochloroacetic acid –dimethyl aniline (MCAA–DMA) combination as photoinitiator was studied kinetically. The apparent activation energy was found to be 4.39 kcal/mol (18.37 kJ/mol) while the kinetic parameter k_p²/k_t was 1.27 × 10⁻² 1/mol/sec. The kinetic data indicated that polymerization followed a radical mechanism. The initiator order was found to be 0.25, indicating significant deviation from the square root dependence for normal free radical kinetics. The non‐ideality in the kinetics can be explained on the basis of significant initiator‐dependent termination through primary radicals or degradative initiator transfer. The observed monomer order was significantly less than unity (i.e. nonideal behavior) for use of carbon tetrachloride, chloroform, methylethyl ketone and acetic acid as diluents, but it was unity (i.e. ideal behavior) for use of benzene as the diluent. Solvents other than benzene contributed to enhancement of rate of polymerization by influencing the radical generation step. End‐group analysis indicated the incorporation of DMA and MCAA moieties as end‐groups in the polymers. Copyright © 1999 John Wiley & Sons, Ltd.