Polynuclear manganese grids and clusters—A magnetic perspective

High nuclearity paramagnetic, spin-coupled transition metal clusters and grids are fascinating chemists and physicists partly because of their structural beauty, and the challenge of creating them, but also because of their novel physical properties. Magnetic interactions between the spin centers are a primary focus. This review will examine a selection of Mn(II) polynuclear grids and clusters, with nuclearities in the range Mn₄ to Mn₉. Theoretical treatments of the magnetic properties are discussed, and approaches to solving the exchange problem for ‘large’ spin systems related to computational difficulties. A freely available software package (MAGMUN4.1) is presented as a means of dealing simply with spin-coupled clusters in general, and symmetry reduction methods are discussed briefly as a means of dealing with ‘large’ spin systems.     

Simulations of time-dependent fluorescence in nano-confined solvents

The time-dependent fluorescence of a model diatomic molecule with a charge-transfer electronic transition in confined solvents has been simulated. The effect of confining the solvent is examined by comparing results for solutions contained within hydrophobic spherical cavities of varying size (radii of 10-20 angstroms). In previous work [J. Chem. Phys. 118, 6618 (2002)] it was found that the solute position in the cavity critically affects the absorption and fluorescence spectra and their dependence on cavity size. Here we examine the effect of cavity size on the time-dependent fluorescence, a common experimental probe of solvent dynamics. The present results confirm a prediction that motion of the solute in the cavity after excitation can be important in the time-dependent fluorescence. The effects of solvent density are also considered. The results are discussed in the context of interpreting time-dependent fluorescence measurements of confined solvent systems.     

Determination of low-level residual ethylene oxide by using solid-phase microextraction and gas chromatography

Current methods of analysis for ethylene oxide (EO) in medical devices include headspace and simulated-use extractions followed by gas chromatography with either a packed or a capillary column. The quantitation limits are about 0.5-1.0 microg/g for a packed column and about 0.1-0.2 microg/g for a capillary column. The current allowable levels of EO on medical devices sterilized with EO gas as outlined in International Organization for Standardization (ISO) 10993-7 may be significantly reduced from current levels by applying the ISO Draft International Standard 10993-17 method for establishing allowable limits. This may require EO test methods with detection and quantitation limits that are much lower than those of the currently available methods. This paper describes a new method that was developed for the determination of low-level EO by solid-phase microextraction using the direct-immersion method. Factors such as temperature and stirring were found to affect absorption efficiency and absorption time. A low extraction temperature (about 6 degrees C) was found to be more efficient than room-temperature extraction. Stirring was found to reduce absorption time by about 50%. Under these conditions, detection and quantitation limits of 0.002 and 0.009 microg/g, respectively, were obtained by using a capillary column. As a result, this method makes compliance with lower EO limits feasible.     

Ester derivatives of α-hydroxymethylacrylates: Itaconate isomers giving high molecular weight polymers

New ester derivatives of ethyl α-hydroxymethylacrylate were synthesized using acid chlorides (traditional solution reactions), sodium salts of acids (with phase transfer catalysis), and trifluoroacetic anhydride (trifluoroacetate). The interfacial process gave high yields of clean products under very mild conditions. Derivatives obtained include the formate, acetate, hexanoate, stearate, benzoate, trifluoroacetate, and adamantanoate. Bulk polymerizations with 2,2′-azobis (isobutyronitrile) gave high molecular weight polymers with intrinsic viscosities of over 2 dL/g and molecular weights of several million [based on size-exclusion chromatography (SEC) comparison to polystyrene standards]. These high molecular weights were the result of autoacceleration in the bulk as shown by monitoring molecular weight with respect to conversion. Solution polymerization in benzene gave more typical polymer, e.g., the acetate derivative showed an SEC molecular weight of 52,000. Glass transition temperatures for the n-alkyl esters decreased from the formate (77°C) to the hexanoate (15°C); the stearate showed a side-chain melting point of 40°C but no T_g. Glass transitions were observed for the trifluoroacetate, benzoate, and adamantanoate polymers at 69, 130, and 214°C, respectively. Solution ¹³C-NMR showed evidence of tacticity information for the formate and acetate derivatives with appaent preference for syndiotactic polymer formation similar to that of methyl methacrylate. FTIR and solid-state ¹³C-NMR analysis gave spectra with functional group peaks and chemical shift values expected based on composition. The stearate monomer and polymer gave solid-state ¹³C chemical shifts of 34 and 33 ppm, respectively, for the central CH₂ units consistent with monoclinic and orthorhombic crystal packing. © 1994 John Wiley & Sons, Inc.     

Determination of coumarin as an adulterant in vanilla flavoring products by high-performance liquid chromatography

A high-performance liquid chromatographic procedure was developed for the isolation and quantitation of coumarin from vanilla-based liquid flavorings of Mexican origin. Forty products representing fourteen different Mexican brands were assayed for coumarin, vanillin, and ethyl vanillin by the proposed method. The procedure has been adapted to the analysis of other products including domestic vanilla extracts and imitation vanilla flavorings for vanillin, ethyl vanillin, 4-hydroxybenzaldehyde and piperonal. Chromatographic retention data for thirty-seven compounds associated with vanillin and vanilla products employing two mobile phase systems are presented.     

磷酸三乙酯改性HZSM-5沸石的活性及理论研究

采用XRD表征了水蒸气和磷联合改性的HZSM-5沸石分子筛的结构．通过NH3-TPD和N2吸附脱附研究了样品的酸性和比表面．采用正庚烷的裂化反应研究了样品的裂化活性，研究结果表明，磷改性样品与母体样品相比，经过水蒸气处理后显示出较高的酸量和正庚烷裂化活性．上述结果首次用模型簇和计算量子化学方法进行了解释．采用Gaussian94软件包和PM3半经验量子化学方法对模型簇进行了全优化和频率分析．计算结果显示磷改性后样品的脱铝补硅反应热大于母体样品脱铝补硅反应热，从而显示出磷对骨架的稳定化作用．     

Simultaneous fermentation and separation of lactic acid in a biparticle fluidized-bed bioreactor

A bioreactor configuration has been tested for simultaneous fermentation and separation of the desired inhibitory product, lactic acid. The bioreactor is a fluidized bed of immobilized Lactobacillus delbreuckii. Another solid phase of denser sorbent particles (a poly-vinyl pyridine resin) was added to this fluidized bed. These sorbent particles fell through the bed, absorbed the product, and were removed. In test fermentations, the addition of the sorbent enhanced the fermentation and moderated the fall of the pH. The biparticle fluidized-bed bioreactor utilizing immobilized microorganisms and adsorbent particles has been shown to enhance the production of lactic acid fourfold in this nonoptimized system.

     

An investigation of some experimental parameters in atomic fluorescence spectrophotometry

Atomic fluorescence in flames is measured by an adaptation of a commercially available flame spectrophotometer. A study is reported of the effect of background radiation and source scattering on 3 flames, air-propane, air-hydrogen and air-acetylene, and of the effects of variation of fuel gas pressure, zone of measurement in the flame, analysing monochromator slit-width and wavelength of measurement. The air-propane flame appears to offer several advantages. The atomic fluorescence of 10 metals is described; those of Co, Fe and Mn have not been previously reported. Excitation of spectra is achieved by means of an a.c. xenon arc lamp or individual discharge lamps.     

Heterogeneous oxidation of pyrimidine and alkyl thioethers in ionic liquids over mesoporous Ti or Ti/Ge catalysts

Heterogeneous catalytic oxidation of a series of thioethers (2-thiomethylpyrimidine, 2-thiomethyl-4,6-dimethyl-pyrimidine, 2-thiobenzylpyrimidine, 2-thiobenzyl-4,6-dimethylpyrimidine, thioanisole, and n-heptyl methyl sulfide) was performed in ionic liquids by using MCM-41 and UVM-type mesoporous catalysts containing Ti, or Ti and Ge. A range of triflate, tetrafluoroborate, trifluoroacetate, lactate and bis(trifluoromethanesulfonyl)imide-based ionic liquids were used. The oxidations were carried out by using anhydrous hydrogen peroxide or the urea-hydrogen peroxide adduct and showed that ionic liquids are very effective solvents, achieving greater reactivity and selectivity than reactions performed in dioxane. The effects of halide and acid impurities on the reactions were also investigated. Recycling experiments on catalysts were carried out in order to evaluate Ti leaching and its effect on activity and selectivity.     

Dependence of the glass transition temperature of poly(methyl methacrylate) on tacticity and molecular weight

The preparation of five samples of poly(methyl methacrylate) covering a wide range of tacticity and their electron irradiation to produce series of varying molecular weight are described. The glass transition temperature T_g of each polymer was determined by DTA techniques. Plots of T_g and the reciprocal of the molecular weight are well fitted in every case by a straight line. The data are also fitted to the Gibbs-DiMarzio theory and the values of the energy and free-volume parameters obtained are discussed. A method of estimating T_g of pure syndiotactic poly(methyl methacrylate) by extrapolation is presented, the value obtained being 160°C.