Capillary electrochromatographic separation of enantiomers on chemically bonded type of cellulose derivative chiral stationary phases with a positively charged spacer

Positively charged chiral stationary phases (CSPs) were prepared for capillary electrochromatography (CEC) separation of enantiomers by chemically immobilizing cellulose derivatives onto diethylenetriaminopropylated silica (DEAPS) with tolylene-2,4-diisocyanate (TDI) as a spacer reagent. Anodic electroosmotic mobility was observed in both nonaqueous and aqueous mobile phases due to the positively charged amines on the surface of the prepared CSPs. For comparison, the traditionally used 3-aminopropyl silica (APS) was also adopted as the base material instead of DEAPS to prepare CSP. It was observed that the EOF on the DEAPS-based CSP was 18%-60% higher than that on the APS-based CSP under nonaqueous mobile phase conditions. Separation of enantiomers in CEC was performed on the positively charged CSPs with the nonaqueous mobile phases of pure ethanol or mixture of hexane-alcohol and the aqueous phases of acetonitrile-water or 95% ethanol. Fast separation of enantiomers was achieved on the newly prepared CSPs.

AN ALGEBRAIC METHOD TO ESTIMATE GENERAL TYPE OF PHASES OF STRUCTURE FACTORS

The algebraic formulas of 1.5 and 2.5 rank are given for four space groups P2(1), Pn, Pna2(1), P2(1)2(1)2(1). It is better that the results of applying them to estimating general type of phases for four correspondent crystal structures. And a method of transforming algebraic formulas from 1.5(2.5) rank is proposed.

THE ALGEBRAIC-METHOD ESTIMATING +/-PI/2 TYPE OF PHASES OF STRUCTURE FACTORS

The algebraic formulas of 1.5 and 2.5 rank which can be applied to estimating +/- pi/2 type of phases for P2(1)2(1)2(1) space group were derived using the method of structure factor algebra. Both types of the formulas are satisfactory for two known crystal structures in estimating their +/- pi/2 type of phases.

Investigation of Mn-Implanted n-Type Ge

Mn+ irons were implanted to n-type Ge(1 1 1) single crystal at room temperature with an energy of 100 keV and a dose of 3 x 10(16) cm(-2). Subsequently annealing was performed at 400degreesC for 1 h under flowing nitrogen gas. X-ray diffraction measurements show that as-implanted sample is amorphous and the structure of crystal is restored after annealing. Polycrystalline germanium is formed in annealed sample. There are no new phases found except germanium. The samples surface morphologies indicate that annealed sample has island-like feature while there is no such kind of characteristic in as-implanted sample. The elemental composition of annealed sample was analyzed by Auger electron spectroscopy. It shows that manganese ions are deeply implanted into germanium substrate and the highest manganese atomic concentration is 8% at the depth of 120 nm. The magnetic properties of samples were investigated by an alternating gradient magnetometer. The annealed sample shows ferromagnetic behavior at room temperature.

Investigation of Mn-implanted n-type Ge

Mn+ irons were implanted to n-type Ge(1 1 1) single crystal at room temperature with an energy of 100 keV and a dose of 3 x 10(16) cm(-2). Subsequently annealing was performed at 400degreesC for 1 h under flowing nitrogen gas. X-ray diffraction measurements show that as-implanted sample is amorphous and the structure of crystal is restored after annealing. Polycrystalline germanium is formed in annealed sample. There are no new phases found except germanium. The samples surface morphologies indicate that annealed sample has island-like feature while there is no such kind of characteristic in as-implanted sample. The elemental composition of annealed sample was analyzed by Auger electron spectroscopy. It shows that manganese ions are deeply implanted into germanium substrate and the highest manganese atomic concentration is 8% at the depth of 120 nm. The magnetic properties of samples were investigated by an alternating gradient magnetometer. The annealed sample shows ferromagnetic behavior at room temperature. (C) 2004 Elsevier B.V. All rights reserved.

A new structure of In-based ohmic contacts to n-type GaAs

Electrical, structural and reaction characteristics of In-based ohmic contacts to n-GaAs were studied. Attempts were made to form a low-band-gap interfacial phase of InGaAs to reduce the barrier height at the metal/semiconductor junction, thus yielding low-resistance, highly reliable contacts. The contacts were fabricated by e-beam sputtering Ni, NiIn and Ge targets on VPE-grown n(+)-GaAs film (approximate to 1 mu m, 2 x 10(18) cm(-3)) in ultrahigh vacuum as the structure of Ni(200 Angstrom)/NiIn(100 Angstrom)/Ge(40 Angstrom)/n(+)-GaAs/SI-GaAs, followed by rapid thermal annealing at various temperatures (500-900 degrees C). In this structure, a very thin layer of Ge was employed to play the role of heavily doping donors and diffusion limiters between In and the GaAs substrate. Indium was deposited by sputtering NiIn alloy instead of pure In in order to ensure In atoms to be distributed uniformly in the substrate; nickel was chosen to consume the excess indium and form a high-temperature alloy of Ni3In. The lowest specific contact resistivity (rho(c)) of (1.5 +/- 0.5)x 10(-6) cm(2) measured by the Transmission Line Method (TLM) was obtained after annealing at 700 degrees C for 10 s. Auger sputtering depth profile and Transmission Electron Microscopy (TEM) were used to analyze the interfacial microstructure. By correlating the interfacial microstructure to the electronical properties, InxGa1-xAs phases with a large fractional area grown epitaxially on GaAs were found to be essential for reduction of the contact resistance.

Synthesis of chiral stationary phases with radical polymerization reaction of cellulose phenylcarbamate derivatives and vinylized silica gel

Cellulose phenylcarbamate derivatives having methacrylate groups were synthesized with regioselective and non-regioselective procedures. These derivatives were chemically immobilized onto a vinylized silica gel, respectively, via a radical co-polymerization reaction. The immobilization was efficiently attained using a small amount of AIBN. The chiral recognition abilities of the prepared chiral stationary phases (CSPs) were evaluated by HPLC resolution of test enantiomers. It was observed that most of the enantiomers were completely resolved with markedly high column efficiency of 30,000-40,000 plates per metre for the eluted peaks. The effect of the amount of methacrylolyl chloride used for preparation on resolution was investigated. A direct comparison of the chiral recognition ability was made on the regioselectively and non-regioselectively prepared CSPs. In addition, the chemically bonded-type of CSPs were found to be relatively stable with addition of solvents such as tetrahydrofuran (THF) and chloroform into the mobile phase, which can lead to the dissolution of cellulose derivatives on the coated CSPs. Thus the choice of solvents used as the mobile phase is greatly extended and better resolution of several test enantiomers was observed on the prepared CSPs with THF and chloroform as a composition in the mobile phase. The batch-to-batch and run-to-run reproducibility was also discussed on the newly prepared CSPs. (C) 2004 Elsevier B.V. All rights reserved.

Synthesis and characteristics of composite chiral stationary phases based on cellulose derivatives

Composite chiral stationary phases (CSPs) were prepared on the basis of cellulose derivatives coated or bonded onto silica. "Molecular exterior" type CSPs were prepared by mixing together two different cellulose tris-derivatives before or after being coated or bonded onto silica, and the "molecular interior" type was obtained by synthesizing non-regioselectively heterosubstituted cellulose derivatives coated or bonded onto silica. For the sake of comparison, the individual phases were also prepared with corresponding cellulose derivatives by coating or bonding approaches, respectively. All of the prepared CSPs were characterized and their chiral recognition properties were evaluated by HPLC with several test racemates. The experimental results demonstrated that the "molecular exterior" CSPs generally exhibit chiral recognition capacities intermediate between those of the two individual phases. However, in the separation of some racemates higher enantioselectivity may be achieved on the "molecular interior" phases than on individual phases, thus broadening the application range of a single cellulose-based CSP.

Self-assembly of rod-coil-rod ABA-type triblock copolymers

Self-assembled behavior of symmetric ABA rod-coil-rod triblock copolymer melts is studied by applying self-consistent-field lattice techniques in three-dimensional space. The phase diagram is constructed to understand the effects of the chain architecture on the self-assembled behavior. Four stable structures are observed for the ABA rod-coil-rod triblock, i.e., spherelike, lamellar, gyroidlike, and cylindrical structures. Different from AB rod-coil diblock and BAB coil-rod-coil triblock copolymers, the lamellar structure observed in ABA rod-coil-rod triblock copolymer melts is not stable for high volume fraction of the rod component (f(rod)=0.8), which is attributed to the intramolecular interactions between the two rod blocks of the polymer chain.

Study of self-assembly of symmetric coil-rod-coil ABA-type triblock copolymers by self-consistent field lattice method

The self-assembly of symmetric coil-rod-coil ABA-type triblock copolymer melts is studied by applying self-consistent field lattice techniques in a three-dimensional space. The self-assembled ordered structures differ significantly with the variation of the volume fraction of the rod component, which include lamellar, wave lamellar, gyroid, perforated lamellar, cylindrical, and spherical-like phases. To understand the physical essence of these phases and the regimes of occurrence, we construct the phase diagram, which matches qualitatively with the existing experimental results. Compared with the coil-rod AB diblock copolymer, our results revealed that the interfacial grafting density of the separating rod and coil segments shows important influence on the self-assembly behaviors of symmetric coil-rod-coil ABA triblock copolymer melts. We found that the order-disorder transition point changes from f(rod)=0.5 for AB diblock copolymers to f(rod)=0.6 for ABA triblock copolymers. Our results also show that the spherical-like and cylindrical phases occupy most of the region in the phase diagram, and the lamellar phase is found stable only at the high volume fraction of the rod.

Synthesis of 2-benzylthio-5-phenyl-3,4-disubstituted thiophenes by intramolecular condensation of alpha-oxo ketene dibenzylthioacetals

A facile route to 2-benzylthio-5-phenyl-3,4-disubstituted thiophenes was described. Catalyzed by sodium hydroxide, the title compounds were synthesized in moderate to good yields simply from the intramolecular aldol type condensation of alpha-oxo ketene dibenzylthioacetals. The chemical selectivity for this annulation reaction was studied and discussed.

Estimation of individual phases from the four-phase structure invariants in the single isomorphous replacement case

The probability distribution of the four-phase invariants in the case of single isomorphous replacement has been developed to estimate some individual phases. An example of its application to obtain the phases having special values of 0, pi or +/-pi /2 is given for a known protein structure in space group P2(1)2(1)2(1). The phasing procedure includes the determination of starting phases and an iterative calculation. The initial values of starting phases, which are required by the formula, can be obtained from the estimate of one-phase seminvariants and by specifying the origin and enantiomorph. In addition, the calculations lead to two sets of possible phases for each type of reflection by assigning arbitrarily an initial phase value. The present method provides a possibility for the multisolution technique to increase greatly the number of known phases while keeping the number of the trials quite small.

STUDIES ON THE PEROVSKITE-TYPE MIXED CATALYSTS OF TITANIUM SYSTEM FOR OXIDATIVE COUPLING OF METHANE - THE INFLUENCE OF CALCINATION TEMPERATURE

The correlations of the calcination temperature, structure and catalytic activity for the oxidative coupling of methane on the LiLa0.5Ti0.5O2+lambda catalysts whose main phase and major active phase is Perovskite-type ternary complex oxide LaTi1-yLiyO3-lambda have been studied. The surface and bulk structures of the catalysts were characterized by means of XRD, XPS, IR, BET and so on, The results cleary indicated that the effect of calcination temperature on the activity for the oxidative coupling of methane is twofold. On one hand, it is favorable for Li+ substitution for Ti3+ to enter into the lattice of LaTiO3 and produce more oxygen vacancies in which active oxygens are formed; however, excessively high calcination temperature make the amount of Li+ substitution for Ti3+ lower, due to a little change of structure or phases for the catalyst. On the other hand, the conversion of CH4 drops because of the decrease of surface area, when the calcination temperature is raised.