Synthesis and crystallization kinetics of the novel ion exchanger Na<Subscript>4</Subscript>Ti<Subscript>4</Subscript>Si<Subscript>3</Subscript>O<Subscript>10</Subscript> for cesium removal

Thermodynamics and crystallization kinetics of the hydrothermal synthesis of Na₄Ti₄Si₃O₁₀ (NaTS) were systemically studied by both experiments and model simulation. Experimental results showed that the curve of crystallinity with time was a characteristic signmoid in the shape that indicated the crystallization of Na₄Ti₄Si₃O₁₀ was a typical spontaneous nucleation process on the laboratory scale. Crystallization of NaTS belongs to the liquid-liquid transformation mechanism and the reaction is endothermic (ΔH = 15.3 kJ/mol). A mathematic model of crystallization kinetics was developed to simulate the synthesis of NaTS. Runge-Kutta and simplex methods were adopted to solve the partial differential equations. Model results fitted well with the experimental data and showed that the synthesis process belongs to spontaneous nucleation and crystal growth. Moreover, the very small crystal growth constant (5.6·10⁻⁷) and gel dissolution constant (7.0·10⁻⁷) indicate they are the rate-limiting steps of the whole synthesis process.     

Groups of paramagnetic centres in coal samples with different carbon contents

Complex systems of paramagnetic centres existing in demineralised flame coal (71.4 wt% C), medium-rank coal (85.6 wt% C) and anthracite (94.9 wt% C) were analysed by electron paramagnetic resonance spectroscopy (EPR). Different groups of paramagnetic centres were searched in macerals (exinite, vitrinite, inertinite) of coals with carbon contents of 73.8 wt% C and 85.6 wt% C. Experimental EPR spectra were fitted by different superposition of theoretical lines. Total concentration of paramagnetic centres both in coal and macerals increases with carbon content in the sample. Total concentration of paramagnetic centres in macerals increases from exinite to inertinite. Four groups of paramagnetic centres were found in medium-rank coal, and only two groups exist in flame coal and anthracite. The EPR spectrum of medium-rank coal is a superposition of two broad and two narrow lines. Both broad and narrow, and two narrow lines were measured in EPR spectra of flame coal and anthracite, respectively. Two types of paramagnetic centres with broad lines and one group with narrow lines exist in exinite and vitrinite from medium-rank coal. Two narrow components were observed in EPR spectra of its inertinite. Two different broad electron paramagnetic resonance signals were detected for macerals from low-coalificated coal. Paramagnetic centres systems of medium-rank coal samples reveal the most complex character. Broad EPR lines were not observed for the higher coalificated samples, i.e., anthracite and inertinite from medium-rank coal. Narrow lines do not occur in EPR spectra of low-coalificated macerals. Strong dipolar interactions and fast spin-lattice relaxation processes are characteristic for paramagnetic centres with broad lines. Strong exchange interactions and short spin-lattice relaxation time were measured for paramagnetic centres with the narrowest EPR signals.     

Oxidation of some catecholamines by sodium N-chloro-p-toluenesulfonamide in acid medium: A kinetic and mechanistic approach

The kinetics of the oxidation of five catecholamines viz., dopamine (A), L-dopa (B), methyldopa (C), epinephrine (D) and norepinephrine (E) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in presence of HClO₄ was studied at 30±0.1 °C. The five reactions followed identical kinetics with a first-order dependence on [CAT] _o , fractional-order in [substrate] _o , and inverse fractional-order in [H⁺]. Under comparable experimental conditions, the rate of oxidation of catecholamines increases in the order D>E>A>B>C. The variation of ionic strength of the medium and the addition of p-toluenesulfonamide or halide ions had no significant effect on the reaction rate. The rate increased with decreasing dielectric constant of the medium. The solvent isotope effect was studied using D₂O. A Michaelis-Menten type mechanism has been suggested to explain the results. Equilibrium and decomposition constants for CAT-catecholamine complexes have been evaluated. CH₃C₆H₄SO₂NHCl of the oxidant has been postulated as the reactive oxidizing species and oxidation products were identified. An isokinetic relationship is observed with β=361 K, indicating that enthalpy factors control the reaction rate. The mechanism proposed and the derived rate law are consistent with the observed kinetics.     

Decarboxylierung cyclischer und acyclischer dicarboxylato-komplexe des platins

The interaction of (Ph₃P)₂PtO₂ (I) with the dicarboxylic acids HO₂C(CH₂)_nCO₂H (n = 1–3), phthalic acid and maleic acid gives the dicarboxylato complexes (Ph₃P)₂$\text{PtO(O)C(CH}{}_2\text{)}{}_{\mathrm{<mtext>n</mtext>}}\text{C(O)O}$ (II) (n = 1–3), (Ph₃P)₂$\text{PtO(O)CC}{}_6\text{H}{}_4\text{C(O)O}$ (III) and cis-[(Ph₃P)₂Pt(O(O)CCHCHC(O)OH)₂] (IV) in nearly quantitative yield. Thermal and photoinduced decarboxylation of III and IV yields the platina heterocycles (Ph₃P)₂$\text{PtC}{}_6\text{H}{}_4\text{C(O)O}$ (V) and (Ph₃P)₂$\text{PtCHCHC(O)O}$ (VI) with a carbon-platinum σ-bond. Complex VI has been characterized by an X-ray crystal structure determination.     

Kinetic determination of magnesium and calcium by stopped-flow injection analysis

A kinetic method for the simultaneous determination of magnesium and calcium ions in solution is described. The method is based on the dissociation reactions of cryptand (2.2.2) complexes; a stopped-flow injection technique is used with spectrophotometric monitoring of the phthalein complexone complexes of the released metal ions. Analyses were done at a rate of 80 h^?1, with injected sample volumes of 80 μl. A microcomputer system for data acquisition and control of the system is described.     

Aqueous p-nitrotoluene oxidation induced with direct glow discharge plasma

A plasma induced degradation process has been studied to treat 4-nitrotoluene (4-NT) present as an aqueous pollutant. The plasma was locally generated from a glow discharge around a tip of a platinum anode in an electrolytic solution. The influence of initial pH and Fe²⁺ on the degradation was examined. Major intermediates resulting from the degradation process were identified. Amongst the aromatic intermediates, p-hydroxybenzoic acid was the predominant degradation product. The formation of oxalic acid, malic acid was also observed. The final products of degradation were NH ₄ ⁺ , NO ₃ ⁻ and CO₂. Based on the analysis of intermediates and the kinetic considerations, the degradation was shown to follow a pseudo-first order reaction hence, a possible reaction pathway was proposed.     

Preparation of a set of selectively protected disaccharides for modular synthesis of heparan sulfate fragments: toward the synthesis of several <Emphasis Type="Italic">O</Emphasis>-sulfonated [β-D-GlcUA-(1→4)-β-D-GlcNAc]OPr types

A concise method for a stereocontrolled synthesis of a set of selectively protected disaccharides is reported. Coupling of the donor 11 onto acceptors 23 and 24, promoted by trimethylsilyl triflate-N-iodosuccinimide (TMSOTf-NIS), generated the disaccharides 25 and 26. Under typical conditions, condensation of the fully protected donor 12 onto acceptors 23 and 24 produced the disaccharides 27 and 28. The building blocks 25–28 were prepared in moderate yields having exclusive β-stereoselectivity. A unique pattern of protecting groups distinguished clearly between positions to be sulfated and functional groups remaining as free hydroxyl groups. Acetyl and/or levulinoyl esters temporarily protected the positions to be sulfated, while benzyl ethers were used for permanent protection. The anomeric positions were protected as allyl ethers, whereas the 4′-positions were masked as p-methoxybenzyl (PMB) ethers. The orthogonality of the PMB and allyl groups can then be used for further elongation of the chain by recurrent deprotection and activation steps. The hydroxyl group, OH-6, of glucosamine moieties was protected as a TBDPS ether to avoid oxidation. A five-step deprotection/sulfonation sequence was applied to the disaccharide 27 to generate the corresponding sulfated [β-D-GlcUA-2-OSO₃Na-(1→4)-β-D-Glc pNAc]-(1→O-Pro) 34.     

Optimization of the design and operation of FAIMS analyzers

Field asymmetric waveform ion mobility spectrometry (FAIMS) holds significant promise for post-ionization separations in conjunction with mass-spectrometric analyses. However, a limited understanding of fundamentals of FAIMS analyzers has made their design and operation largely an empirical exercise. Recently, we developed an a priori simulation of FAIMS that accounts for both ion diffusion (including anisotropic components) and Coulomb repulsion, and validated it by extensive comparisons with FAIMS/MS data. Here it is corroborated further by FAIMS-only measurements, and applied to explore how key instrumental parameters (analytical gap width and length, waveform frequency and profile, the identity and flow speed of buffer gas) affect FAIMS response. We find that the trade-off between resolution and sensitivity can be managed by varying gap width, RF frequency, and (in certain cases) buffer gas, with equivalent outcome. In particular, the resolving power can be approximately doubled compared to "typical" conditions. Throughput may be increased by either accelerating the gas flow (preferable) or shortening the device, but below certain minimum residence times performance deteriorates. Bisinusoidal and clipped-sinusoidal waveforms have comparable merit, but switching to rectangular waveforms would improve resolution and/or sensitivity. For any waveform profile, the ratio of two between voltages in high and low portions of the cycle produces the best performance.     

A combined theoretical and experimental study of efficient and fast titanocene-catalyzed 3-exo cyclizations

The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.     

Application of a universal force field to mixed Fe/Mo-S/Se cubane and heterocubane clusters. 1. Substitution of sulfur by selenium in the series [Fe4X4(YCH3)4]2-; X = S/Se and Y = S/Se

A series of Fe-S and Fe-Se cubane clusters containing all four combinations of the general formula [Fe(4)X(4)(Y-CH(3))(4)](2)(-) (X = S/Se, Y = S/Se) is investigated with FTIR and Raman spectroscopy. The terminally selenolate coordinated clusters (Y = Se) are prepared by a new synthetic route. All four cluster compounds are structurally characterized by X-ray single-crystal structure determination. Infrared and Raman spectra of all compounds are presented and interpreted with normal coordinate analysis. The corresponding force fields are based on that developed for the Fe(4)S(4)-benzyl cluster (Czernuszewicz, R. S.; Macor, K. A.; Johnson, M. K.; Gewirth, A.; Spiro, T. G. J. Am.Chem. Soc. 1987, 109, 7178-7187). An empirical procedure is presented to convert Fe-S into Fe-Se force constants. Only minor changes in force constants are found upon S --> Se exchange, reflecting the similarity of the Fe-S and Fe-Se bonds. The drastic frequency shifts in the metal-ligand region observed upon substitution of sulfur by selenium are, therefore, primarily due to the corresponding mass changes.