Summary. Cross polarisation is extensively used in solid state NMR for enhancing signals of nuclei with low gyromagnetic ratio. However,
the use of the method for providing quantitative structural and dynamics information is limited. This arises due to the fact
that the mechanism which is responsible for cross polarisation namely, the dipolar interaction, has a long range and is also
anisotropic. In nematic liquid crystals these limitations are easily overcome since molecules orient in a magnetic field.
The uniaxial ordering of the molecules essentially removes problems associated with the angular dependence of the interactions
encountered in powdered solids. The molecular motion averages out intermolecular dipolar interaction, while retaining partially
averaged intramolecular interaction. In this article the use of cross polarisation for obtaining heteronuclear dipolar couplings
and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment were considered
and their utility illustrated. A method for obtaining proton–proton dipolar couplings, by utilizing cross polarisation from
the dipolar reservoir, is also presented.
Corresponding author. E-mail: kvr@sif.iisc.ernet.in
Received May 28, 2002; accepted June 19, 2002 相似文献
Abstract The paper reports results of the X-ray diffraction structural studies of liquid bicyclohexyl, C6H11—C6H11 at room temperature, using CuKα radiation. Angular distribution of X-ray scattered intensity was measured, and the electron-density radial-distribution function was calculated. Computer techniques were used to minimize the effects of experimental errors, uncertainties in the scattering factors, and termination errors. Good agreement between the theoretical and experimental functions was obtained on assuming the statistically most probable model of this molecule. The structural data obtainable by X-ray analysis for liquid bicyclohexyl were discussed. The mean distances between the neighbouring molecules and the radii of coordination spheres were found. The maxima in the differential radial distribution function were at: r1 = 4.68, r2 = 5.57, r3 = 6.62, r4 = 8.05 Å. X-ray structural analysis was applied to determine the packing coefficient of bicyclohexyl molecules. A simple model of short-range arrangement of the molecules in liquid bicyclohexyl was proposed. 相似文献
The interaction of the ionic liquid [C4C1Im][BF4] with anatase TiO2, a model photoanode material, has been studied using a combination of synchrotron radiation photoelectron spectroscopy and near‐edge X‐ray absorption fine structure spectroscopy. The system is of interest as a model for fundamental electrolyte–electrode and dye‐sensitized solar cells. The initial interaction involves degradation of the [BF4]? anion, resulting in incorporation of F into O vacancies in the anatase surface. At low coverages, [C4C1Im][BF4] is found to order at the anatase(101) surface via electrostatic attraction, with the imidazolium ring oriented 32±4° from the anatase TiO2 surface. As the coverage of ionic liquid increases, the influence of the oxide surface on the topmost layers is reduced and the ordering is lost. 相似文献
Silica gels retaining double pore structure in the size ranges of micrometer and nanometer have been applied to the rod-shaped
monolithic column for liquid chromatography. The macropore structure was designed by controlling the phase separation process
induced by the hydrolysis and polycondensation of alkoxysilane, whereas the mesopore structure was tailored by the solvent
exchange treatments on wet gels. The size exclusion chromatograms on polystyrene standards exhibited almost similar features
for octadecyl-modified rod and conventional packed beads columns. The dependence of plate height on the velocity of mobile
phase determined for amylbenzene was by far weaker in the rod column than in the packed beads column, suggesting that additional
geometrical factors should be considered in describing the separation mechanism in the rod column. 相似文献
Control of ion arrangements in ionic liquids represents a major challenge owing to the presence of the predominant coulombic interactions between cationic and anionic ion species that forms the coulombic ordering. Here, water-induced ion rearrangement in a room-temperature ionic liquid (RT-IL) metal complex, (1-ethyl-3-methylimidazolium)2[MnN(CN)4], is demonstrated through coordinative interactions between anions. Solidification occurred, which was associated with the formation of a “separated” structure consisting of cation columns and anionic cyanide-bridged one-dimensional coordination polymers. The energy diagram is in accord with the resultant RT-IL incorporating mononuclear [MnN(CN)4]2− molecules being a kinetic phase stabilized by inter-ion repulsions of the anionic divalent metal complex moieties. Water acts to decrease the coulombic interactions, including repulsion, giving rise to breaking of the coulombic ordering arising from coordination bond formation in the IL phase. 相似文献
Abstract Egelstaff, March and McGill proposed the idea that electron-electron correlation functions for conduction electrons in liquid metals could be extracted from experiment by combining X-ray, neutron and electron diffraction studies. Though electron diffraction studies are still only in early stages of development, evidence has nevertheless gathered that conduction electrons have a significant degree of ordering near the melting point of liquid metals. In the latter part of the review, this conclusion is brought into contact with current ideas on strongly correlated electronic ground states in crystalline metals. In both areas, resonating bonds proposed by Pauling seem an essential ingredient in understanding the electronic behavior. 相似文献
Abstract A method for sample work-up and enrichment using Supported Liquid Membrane (SLM) and liquid chromatographic determination of triazine herbicides in natural waters was investigated. A porous PTFE membrane was impregnated with a water immiscible organic solvent, forming a barrier between two aqueous phases. With a flowing donor and a stagnant acceptor solution, an enrichment of the triazines was obtained. The various factors that influence the extraction efficiency and selectivity of the extraction procedure were experimentally studied. The obtained results were in good agreement with the developed theoretical model. The pH of the acceptor solution was found to be the critical limiting factor in obtaining higher extraction efficiencies and led to an extraction efficiency decrease with an increase in enrichment time. By increasing both the trapping capacity of the acceptor solution and the donor flow rate, the method detection limit of the triazines ranged from 0.03 to 0.16 μgL?1 in natural waters with 20 minutes extraction time. 相似文献
The formation of brass-type shear bands (SBs) in twinned microstructures of
oriented copper single crystals, has been investigated after channel-die deformation at 77K. Setting up a system for making high-resolution orientation maps using transmission electron microscopy (TEM) has opened new advantageous circumstances for the analysis of orientation changes within SBs. This method with spatial resolution higher than 10nm allows an examination of microstructure images composed of nanoscale sub-cells forming SBs structure. The early stages of SBs formation are the result of equally effective operation of two slip systems operating on the {111} slip planes which could be represented by a resultant super-system of
type. This process leads to the lattice rotation about
axis and to the rise of Goss orientation within the band. A minor group of components is observed near
, arising from the near primary matrix orientation. For well-developed SBs, a second rotation about
direction is observed. This tendency, resulting from the
type slip systems operation, is usually accompanied by activation of new slip systems. Hence the process of SBs formation is regarded as a strictly crystallographic one. 相似文献
The suitability of reversed-phase high-performance liquid chromatography for determining the distribution coefficients of organic compounds in the hexane-acetonitrile system was considered. A version of the necessary additional step in the sample preparation procedure was proposed, namely, changing from nonpolar hexane to a solvent unlimitedly miscible with the eluent. The distribution coefficients were determined for 20 pesticides by the proposed method and compared with those determined by gas chromatography. 相似文献
A new magnetic ionic liquid (MIL) with 3D antiferromagnetic ordering has been synthetized and characterized. The information obtained from magnetic characterization was supplemented by analysis of DFT calculations and the magneto‐structural correlations. The result gives no evidence for direct iron‐iron interactions, corroborating that the 3D magnetic ordering in MILs takes place via super‐exchange coupling containing two diamagnetic atoms intermediaries. 相似文献
Made to order : Aqueous dispersions of polymer‐encapsulated liquid crystal (LC) droplets were synthesized with precise interfacial chemistry and sizes in the micrometer‐to‐sub‐micrometer range. Size‐dependent changes in LC ordering could be observed. Study of the competition between size and interfacial chemistry on LC ordering enables size‐dependent properties of LC droplets to be exploited in applications such as photonics and sensing.
The artificial neural network (ANN) model with back-propagation of error is used to study the quantitative structure-activity relationship of para-substituted phenol derivatives between the biological activity and the physicochemical property parameters. Network parameters are optimized, and an empirical rule for dynamically adjusting the network's learning rate is proposed to improve the network's performance. The results showthat the three-layer ANN model gives satisfactory performance, with f(x)=1/(1+exp(-x)) as the network node's input-output transformation function and the number of hidden nodes 10. The network gives the mean square error (rose) of 0.036 when predicting the biological activity of 26 para-substituted phenol derivatives. This result compares favourably with that obtained by the conventional methods. 相似文献