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971.
Norman L. Allinger Fanbing Li Liqun Yan Julia C. Tai 《Journal of computational chemistry》1990,11(7):868-895
The MM3 molecular mechanics program has been extended to conjugated systems. A VESCF method is applied to the pi-system to calculate bond orders, from which various stretching and torsional parameters are obtained. The procedure gives somewhat better results than the analogous MM2 calculations. It has been applied to a study of 81 compounds of aromatic and other conjugated hydrocarbons, as well as 45 alkenes and unconjugated polyenes. The structures calculated are generally in good agreement with experiment, and the heats of formation of these compounds can be calculated with a rms value of 0.62 kcal/mol, which may be compared with the average experimental error of 0.61 kcal/mol. In addition, vibrational frequencies for five representative conjugated model structures are calculated, with an rms value of 46 cm?1, and from these, other properties such as entropy can be calculated. 相似文献
972.
Shashkina JA Philippova OE Zaroslov YD Khokhlov AR Pryakhina TA Blagodatskikh IV 《Langmuir : the ACS journal of surfaces and colloids》2005,21(4):1524-1530
Rheological studies were performed with aqueous salt solutions of viscoelastic cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) and its mixtures with hydrophobically modified polyacrylamide. The solutions of surfactant itself above the concentration of crossover of wormlike micelles exhibit two regions of rheological response. In the first region, they behave like polymer solutions in semidilute regime characterized by viscoelastic behavior with a spectrum of relaxation times. In the second region, unlike polymer solutions their relaxation after shear is dominated by a single relaxation time. Being composed of "living" micelles, the EHAC solutions easily lose their viscosity at the variation of the external conditions. For instance, heating from 20 to 60 degrees C reduces viscosity by up to 2 orders of magnitude, while added hydrocarbons induce a sudden drop of viscosity by 3-6 orders of magnitude. Polymer profoundly affects the rheological properties of EHAC solutions. The polymer/surfactant system demonstrates a 10,000-fold increase in viscosity as compared to pure-component solutions, the effect being more pronounced for polymer with less blocky distribution of hydrophobic units. A synergistic enhancement of viscosity was attributed to the formation of common network, in which some subchains are made up of elongated surfactant micelles, while others are composed of polymer. At cross-links the hydrophobic side groups of polymer anchor EHAC micelles. In contrast to surfactant itself, the polymer/surfactant system retains high viscosity at elevated temperature; at the same time it keeps a high responsiveness to hydrocarbon medium inherent to EHAC. 相似文献
973.
974.
Julia Y ChanFrank R Fronczek David P YoungJ.F DiTusa P.W Adams 《Journal of solid state chemistry》2002,163(2):385-389
The compound Be1.09B3 was prepared by arc-melting of the elemental constituents. The structure of single crystals taken from the arc-melted boule was determined from single-crystal X-ray data (T=120 K) and is hexagonal, having space group P6/mmm, and lattice parameters a=9.7738(7) Å and c=9.5467(6) Å, R=0.047. The structure consists of a hexagonal array of boronicosahedra, nonicosahedral B12 cages, and B18 cages. Stacked hexagonal layers of boron atoms, hexagons formed by B and Be, and equilateral triangles of boron atoms disordered by a 60° rotation exist along a 6-fold axis down the [001] direction. A superconducting transition at 0.72 K is clearly indicated by resistivity measurements. 相似文献
975.
Glutamate oxidase (E.C. 1.4.3.7) was immobilized at a platinized activated carbon electrode and the enzyme electrodes were used for the amperometric determination of L-glutamate in a stirred aqueous solution by the electrochemical detection of enzymically produced hydrogen peroxide at + 320 mV vs. Ag/AgCl. A linear calibration graph was obtained between 2 μM and 2 mM with a steady-state response time of 1 min. The glutamate oxidase electrode was subsequently applied to the measurement of aspartate aminotransferase (AST) (E.C. 2.6.1.1) and alanine aminotransferase (ALT) (E.C. 2.6.1.2) in serum. The performance of the electrode was compared with that of techniques used in the hospital diagnostic laboratory. The responses of the enzyme electrode to AST and ALT activities were linear over the clinically relevant range (5-500 U l ?1), and correlated well (r=0.99) with the methods used for routine clinical analysis. 相似文献
976.
Julia?A.?Bahteeva Ilia?A.?Leonidov Mikhail?V.?Patrakeev Edward?B.?Mitberg Victor?L.?KozhevnikovEmail author Kenneth?R.?Poeppelmeier 《Journal of Solid State Electrochemistry》2004,8(9):578-584
The conductivity of the entire solid solution La1–xSrxFeO3–, where x=0.2, 0.4, 0.5, 0.7 and 0.9, in the oxygen partial pressure range 10–19 to 0.5 atm and temperatures between 750 and 950 °C is reported. The analysis of the isothermal pressure dependences of the conductivity reveal that the lanthanum-strontium ferrites can be characterized as mixed ion-electron conductors in the low-oxygen pressure/high-oxygen deficiency limit. The partial contribution to conductivity from oxygen ions increases with strontium content and attains a maximal value at x=0.5. Further increase in doping results in the development of oxygen vacancy ordering phenomena and deterioration of the conducting properties.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003 相似文献
977.
New approaches to the characterization of resonances in the solid-state NMR spectroscopy of half-integer quadrupolar nuclei are explored, on the basis of the acquisition of heteronuclear separate-local-field spectra on rotating solids. In their two-dimensional version, these experiments correlate for each chemical site a second-order quadrupolar MAS powder pattern with the dipolar MAS sideband pattern to nearby heteronuclei. As 3D NMR sequences, such 2D anisotropic correlation spectra become separated for inequivalent chemical sites along a third, isotropic dimension. Extending in such manner separate-local-field NMR approaches to quadrupoles facilitates the assignment of inequivalent resonances to specific structural environments, and provides new tools for the investigation of dynamics in solids. Details about these 2D and 3D NMR experiments are given, and their application is illustrated with 1H-23Na recoupling experiments on mononucleotides possessing multiple bound cations. 相似文献
978.
A robust, accurate and sensitive analytical procedure for the determination of Se in plant and peat samples by hydride generation-atomic fluorescence spectrometry (HG-AFS) was developed. Aliquots (200 mg) of dried samples were digested with 3 mL nitric acid and 0.5 mL hydrogen peroxide in closed, pressurized PTFE vessels in a microwave oven at 220 °C. Addition of HBF4 or HF to the digestion mixture was not required because experiments demonstrated that Se was not hosted in the silicate fraction of the investigated sample matrices. Selenium(VI) was directly reduced to Se(IV) in the undiluted digestion solutions after addition of 3.8 mL of 4 M HCl in a microwave oven at 103 °C for 3 min. Other reduction reagents, such as hydroxylamine hydrochloride or urea, were not necessary to cope with potential interferences from nitrogen oxides that could hamper the reliable determination of Se by HG-AFS. Optimum hydride generation of Se was achieved by using 0.9% NaBH4 and 4.5 M HCl. A solution detection limit of 11 ng L−1 was obtained under the optimized experimental conditions which corresponds to a method detection limit of 2.8 ng g−1 in solid peat and plant materials. The precision of replicate measurements was better than 3% at Se concentrations of 50 ng L−1. The analytical procedure was critically evaluated by analysing two certified plant reference materials (SRM 1515 Apple Leaves and SRM 1547 Peach Leaves) as well as three peat reference materials. Excellent agreement between the experimental values ranging from 50 ng g−1 to ∼2 μg g−1 and the certified concentrations was obtained. 相似文献
979.
Julia S. Lock Bruce L. May Philip Clements Stephen F. Lincoln Christopher J. Easton 《Journal of inclusion phenomena and macrocyclic chemistry》2004,50(1-2):13-18
E-4-(2-(4- tert - butylphenyl) ethen-1- yl)benzoate, E-1–, photoisomerizes to the Z-1– isomer and vice versa in the free state and in the binary complexes 2·E-1–, 2·Z-1–, 3·E-1– and 3·Z-1– where 2 is the urea-linked cyclodextrin N-(6
A
-deoxy--cyclodextrin-6
A
- yl)-N-(6
A
-deoxy--cyclodextrin-6
A
- yl)urea and 3 is N,N- bis(6
A
-deoxy--cyclodextrin-6
A
- yl)urea. In 2·E-1–and 3·E-1– the stilbene occupies both cyclodextrin (CD) components of 2 and 3, whereas in 2·Z-1– and 3·Z-1– it only occupies one CD component while the other CD component is unoccupied. 4- tert - Butylphenolate, 4–, and its carboxylate, 5–, and sulfonate, 6–, analogues form the ternary complex 2·Z-1–·4– and its analogues and also 3·Z-1–·4– and its analogues. These photoisomerize to 2·E-1–and 3·E-1– and either free 4–, 57– or 6– and thereby function as molecular devices. 相似文献
980.
Julia Hitzbleck 《Journal of organometallic chemistry》2007,692(21):4702-4707
Half-sandwich dibenzyl complexes of scandium have been prepared by stepwise treatment of scandium trichloride with lithium derivatives of silyl-functionalized tetramethylcyclopentadienes (C5Me4H)SiMe2R (R = Me, Ph) and benzyl magnesium chloride. The resulting complexes [Sc(η5-C5Me4SiMe3)(CH2Ph)2(THF)] and [Sc(η5-C5Me4SiMe2Ph)(CH2Ph)2(1,4-dioxane)] show structure related to that of the corresponding bis(trimethylsilylmethyl) compounds [Sc(η5-C5Me4SiMe2R)(CH2SiMe3)2(THF)]. The four-coordinate complexes display η1-coordinated benzyl ligands without significant interaction of the ipso-carbon of the phenyl moiety. Conversion of [Sc(η5-C5Me4SiMe3)(CH2Ph)2(THF)] into the cationic species by treatment with triphenylborane in THF led to the formation of a stable charge separated complex [Sc(η5-C5Me4SiMe3)(CH2Ph)(THF)x][BPh3(CH2Ph)]. Benzyl cation formed using [Ph3C][B(C6F5)4] in toluene resulted in a moderately active syndiospecific styrene polymerization catalyst. 相似文献