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91.
Frost RL Cejka J Bostrom T Weier M Martens W 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2007,67(5):1220-1227
Raman spectra at 298 and 77K and infrared spectra of the uranyl sulfate mineral zippeite from Jáchymov (Joachimsthal), Czech Republic, K(0.6)(H(3)O)0.4[(UO(2))6(SO(4))3(OH)7].8H2O, were studied. Observed bands were tentatively attributed to the (UO(2))2+ and (SO(4))2- stretching and bending vibrations, the OH stretching vibrations of water molecules, hydroxyls and oxonium ions, and H(2)O, oxonium, and delta U-OH bending vibrations. Empirical relations were used for the calculation of U-O bond lengths in uranyl R (A)=f(nu(3) or nu(1)(UO(2))2+). Calculated U-O bond lengths are in agreement with U-O bond lengths from the single crystal structure analysis and those inferred for uranyl anion sheet topology of uranyl pentagonal dipyramidal coordination polyhedra. The number of observed bands supports the conclusion from single crystal structure analysis that at least two symmetrically distinct U6+ (in uranyls) and S6+ (in sulfates), water molecules and hydroxyls may be present in the crystal structure of the zippeite studied. Strong to very weak hydrogen bonds present in the crystal structure of zippeite studied were inferred from the IR spectra. 相似文献
92.
Shamsipur M Dastjerdi LS Haghgoo S Armspach D Matt D Aboul-Enein HY 《Analytica chimica acta》2007,601(1):130-138
19F NMR spectroscopy was applied to the quantitative determination of fluoxetine enantiomers using different chiral recognition agents in pharmaceutical formulations. Several parameters affecting the enantioresolution including the type and concentration of chiral selector, concentration of fluoxetine and temperature were studied. The chiral selectors investigated are the cyclic oligosaccharides α-, β- and γ-cyclodextrin and a diamino derivative of methylated α-cyclodextrin (DAM-α-CD), linear polysaccharides (maltodextrin with dextrose equivalents of 4.0-7.0, 13.0-17.0 and 16.5-19.5) and the macrocyclic antibiotic vancomycin. Among the chiral selectors used, DAM-α-CD turned out to give the best resolution of the 19F NMR signals of (R)- and (S)-fluoxetine. The calibration curve was linear for (R)- and (S)-fluoxetine over the range 0.10-1.35 mg mL−1, the detection limits (S/N = 3) being 5.9 and 7.5 μg mL−1 for the pure solutions of (R)- and (S)-fluoxetine, respectively. The recovery studies performed on pharmaceutical samples ranged from about 90 to 110% with relative standard deviations of <8%. The results showed that the proposed method is rapid, precise and accurate. Applying statistical Student's t-test revealed insignificant difference between the real and measured contents at the 95% confidence level. 相似文献
93.
Poorters L Armspach D Matt D Toupet L Choua S Turek P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(34):9448-9461
The C2-symmetrical diphosphane TRANSDIP was obtained in high yield by treating 6A,6B,6D,6E-tetramesylated, permethylated alpha-cyclodextrin with PPhLi2 in excess. The double cascade cyclisation thus produced is regioselective as phosphinidene capping involves only adjacent glucose units. It is also stereospecific, as both lone pairs on the phosphorus atoms are orientated towards the cyclodextrin axis. The restricted flexibility of the phosphorus atoms, which are part of nine-membered heterocyclic rings, is responsible for JP,C spin-spin couplings with the eight-bond distant CH2OMe carbon atoms of glucose units C and F. The treatment of TRANSDIP with Group 10 metal dihalides quantitatively gave square-planar chelate complexes, in which a M--X bond points towards the centre of the cavitand. The favourable P...P separation and the directional control of the lone pairs on the phosphorus atoms rule out the possibility of forming binuclear complexes or higher oligomers. Further, in all the complexes, the phosphorus atoms are in a trans arrangement. TRANSDIP may therefore be regarded as an authentic trans-spanning diphosphane. In the complex [NiBr2TRANSDIP], the cavity provides effective protection of the encapsulated M--X bond towards nucleophilic attack by MeLi. The same complex, upon activation with methylaluminoxane, efficiently dimerises ethene and propene. 相似文献
94.
Ying‐Hung So Donald L. Schmidt Matt T. Bishop Daniel R. Miller Patrick B. Smith Michael J. Radler Mindy Magyar Britton L. Kaliszewski 《Journal of polymer science. Part A, Polymer chemistry》2000,38(8):1283-1290
Upon photolysis or heating, aryl cyclic sulfonium (ACS) zwitterions polymerize by ring‐opening and loss of charge to yield nonionic polymers. These water‐soluble monomers potentially are useful for photolithography because they can be cast from and developed in water. The ACS zwitterion from bisphenol A, 1,1′‐[isopropylidenebis(6‐hydroxy‐3,1‐phenylene)] bis (tetrahydrothiophenium hydroxide) bis(inner salt) (1) is a negative‐tone, photosensitive material that after photolysis yields a crosslinked film. Unexposed regions are removed by water. The cured film has a low dielectric constant, high volume resistivity, a high degree of planarization, low residual stress, thermogravimetric stability, acceptable fracture toughness, and high hardness. These are desirable properties for a dielectric material used in microelectronic applications. However, a shortcoming of the material is its low Tg, at about 140 °C. A second ACS zwitterion, 1,1′‐[fluorenylidenebis(6‐hydroxy‐3,1‐phenylene)] bis(tetrahydrothiophenium hydroxide) bis(inner salt) (2) was prepared that yields a crosslinked polymer with a higher Tg of about 190 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1283–1290, 2000 相似文献
95.
Jeff Kay Matt Davison Henning Rasmussen 《Mathematical and Computer Modelling》2009,50(9-10):1448-1460
This paper investigates the early exercise region for Bermudan options on two underlying assets. We present a set of analytical validation results for the early exercise region which can be used as a means of validating pricing techniques. When all strike prices are identical we show the existence of an intersection point such that for any asset price pair below this point early exercise is always optimal. We develop an approximation to this point in the two asset put case. When the strike prices are not all equal, we show that three separate cases exist for the early exercise region. For a Bermudan put on two assets we present these cases and show that there exists a critical point in which the boundaries of the two asset early exercise region bifurcate. Comparisons are drawn between the Bermudan results presented and the corresponding American option results. 相似文献
96.
Xiao Y Fisher K Smith MC Newton WE Case DA George SJ Wang H Sturhahn W Alp EE Zhao J Yoda Y Cramer SP 《Journal of the American Chemical Society》2006,128(23):7608-7612
Nitrogenase catalyzes a reaction critical for life, the reduction of N(2) to 2NH(3), yet we still know relatively little about its catalytic mechanism. We have used the synchrotron technique of (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to study the dynamics of the Fe-S clusters in this enzyme. The catalytic site FeMo-cofactor exhibits a strong signal near 190 cm(-)(1), where conventional Fe-S clusters have weak NRVS. This intensity is ascribed to cluster breathing modes whose frequency is raised by an interstitial atom. A variety of Fe-S stretching modes are also observed between 250 and 400 cm(-)(1). This work is the first spectroscopic information about the vibrational modes of the intact nitrogenase FeMo-cofactor and P-cluster. 相似文献
97.
The application of theoretical methods based on density-functional theory is known to provide atomic and cell parameters in very good agreement with experimental values. Recently, construction of the exact Hartree-Fock exchange gradients with respect to atomic positions and cell parameters within the Gamma-point approximation has been introduced. In this article, the formalism is extended to the evaluation of analytical Gamma-point density-functional atomic and cell gradients. The infinite Coulomb summation is solved with an effective periodic summation of multipole tensors. While the evaluation of Coulomb and exchange-correlation gradients with respect to atomic positions are similar to those in the gas phase limit, the gradients with respect to cell parameters needs to be treated with some care. The derivative of the periodic multipole interaction tensor needs to be carefully handled in both direct and reciprocal space and the exchange-correlation energy derivative leads to a surface term that has its origin in derivatives of the integration limits that depend on the cell. As an illustration, the analytical gradients have been used in conjunction with the QUICCA algorithm to optimize one-dimensional and three-dimensional periodic systems at the density-functional theory and hybrid Hartree-Fock/density-functional theory levels. We also report the full relaxation of forsterite supercells at the B3LYP level of theory. 相似文献
98.
Corrie SR Lawrie GA Trau M 《Langmuir : the ACS journal of surfaces and colloids》2006,22(6):2731-2737
A strategy for the production and subsequent characterization of biofunctionalized silica particles is presented. The particles were engineered to produce a bifunctional material capable of both (a) the attachment of fluorescent dyes for particle encoding and (b) the sequential modification of the surface of the particles to couple oligonucleotide probes. A combination of microscopic and analytical methods is implemented to demonstrate that modification of the particles with 3-aminopropyl trimethoxysilane results in an even distribution of amine groups across the particle surface. Evidence is provided to indicate that there are negligible interactions between the bound fluorescent dyes and the attached biomolecules. A unique approach was adopted to provide direct quantification of the oligonucleotide probe loading on the particle surface through X-ray photoelectron spectroscopy, a technique which may have a major impact for current researchers and users of bead-based technologies. A simple hybridization assay showing high sequence specificity is included to demonstrate the applicability of these particles to DNA screening. 相似文献
99.
Kuhn P Sémeril D Jeunesse C Matt D Neuburger M Mota A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(20):5210-5219
Three SHOP-type catalysts, in which the C=C(O) double bond was substituted by electron-withdrawing substituents, [Ni{Ph2PC(R1)=C(R2)O}Ph(PPh3)] (2: R1,R2 = -C(Me)=NN(Ph)-; 3: R1 = CO2Et, R2 = Ph; 4: R1 = CO2Et, R2 = CF3), were assessed as ethylene-oligomerisation and -polymerisation catalysts and compared to Keim's complex, [Ni{Ph2PCH=C(Ph)O}Ph(PPh3)] (1). A rationale for the influence of the double-bond substituents of the P,O-chelate unit on the catalytic properties is proposed, on the basis of X-ray diffraction studies, spectroscopic data and DFT-B3 LYP calculations. Whatever their relative electron-withdrawing strength, the R1 and R2 substituents induce an increase in activity with respect to catalyst 1. For those systems in which the basicity of the oxygen atom is decreased relative to that of the phosphorus atom, the chain-propagation rate increases with respect to that for catalyst 1. Reduction of the basicity of the P relative to that of the O, however, induces higher chain-termination rates. 相似文献
100.
Frost RL Weier ML Clissold ME Williams PA 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2003,59(14):3313-3319
Infrared spectroscopy has proven most useful for the study of anions in the interlayer of natural hydrotalcites. A suite of naturally occurring hydrotalcites including carrboydite, hydrohonessite, reevesite, motukoreaite and takovite were analysed. Variation in the hydroxyl stretching region was observed and the band profile is a continuum of states resulting from the OH stretching of the hydroxyl and water units. Infrared spectroscopy identifies some isomorphic substitution of sulphate for carbonate through an anion exchange mechanism for the minerals carrboydite and hydrohonessite. The infrared spectra of the CO3 and SO4 stretching region of takovite is complex because of band overlap. For this mineral some sulphate has replaced the carbonate in the structure. In the spectra of takovites, a band is observed at 1346 cm(-1) and is attributed to the carbonate anion hydrogen bonded to water in the interlayer. Infrared spectroscopy has proven most useful for the study of the interlayer structure of these natural hydrotalcites. 相似文献