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21.
Induced ferroelectric S*C phases are formed by non-chiral SC host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization Ps and the tilt angle Θ has been investigated as a function of the mole fraction xG of the chiral dopant. In most cases the reduced polarization P0 = PS/ sin Θ has been found to depend linearly on xG. The polarization power which is defined by δP=(∂P0/∂xG)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P0(xG) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P0. By assuming a local field of Lorentz type a theoretical relation for P0(xG) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large. 相似文献
22.
Günter Georg Hoffmann Hans-Jürgen Hochkamp 《Fresenius' Journal of Analytical Chemistry》1992,344(4-5):227-228
Summary The application of a digital signal processor (DSP) to Fourier-transform infrared spectroscopy is described, which allows for near real-time display of the transformed interferograms as spectra. 相似文献
23.
Pilz N. Hoffmann P. Lieser K. H. Dams W. Nelges K. H. 《Analytical and bioanalytical chemistry》1989,334(7):695-717
Analytical and Bioanalytical Chemistry - A non-destructive method and an experimental set-up are described by which the Pu content in UO2/PuO2 mixed oxide (MOX) pellets and in fuel rods,... 相似文献
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The basic obstacles for the general use of laser-ablation(LA)-ICP-MS in analytical laboratories are connected with its reproducibility and calibration. A mathematical relation deduced from the correlation function of the analytical signals allows the estimation of the number of craters needed for representative analyses. The procedure was applied to different samples such as manganese crusts and soils. The ion intensities of the major elements in the manganese crusts and nodules were used as internal standards, improving relative standard deviations by factors between 2 and 3. Selected samples of wood and manganese crusts were analyzed by LA-ICP-MS and the results compared with those obtained by solution ICP-MS. The agreement of the values is within the 95% confidence limits. Powdered reference materials and, in the case of wood analysis, cellulose doped with standard solutions were used for the calibration. 相似文献
27.
The bonding environment of boron is usually thought about in terms of localized 2c-2e/3c-2e bonding (as in diborane) or completely delocalized polyhedral bonding (as in B(12)H(12)(2)(-)). Recently, a number of boron compounds having a rhomboidal B(4) framework have been synthesized; these show an amazing variation in their skeletal electron count, one that cannot be interpreted in familiar ways. In this report, we systematically explore the origin of the range of electron counts in these compounds. We find that four skeletal MOs are primarily responsible for keeping the B(4) skeleton together. As a subunit in a macropolyhedral environment, termed rhombo-B(4), such an arrangement of B atoms deviates from Wade's rule by three electron pairs (if treated as a distorted arachno system derived from B(6)H(6)(2)(-)). Aided by this analysis, we examine the nature of bonding in Na(3)B(20), where the rhombo-B(4) unit forms linear chains fusing closo-B(7) units. Theory suggests that this structure requires one more electron per formula unit for optimal bonding. Finally, we study the nature of bonding in beta-SiB(3), where silicon atoms also adopt the rhomboid framework. 相似文献
28.
From a molecular orbital study of model systems we derive the electronic requirements for the Grob fragmentation. The necessary condition for an allowed fragmentation in an X-C1-C2-C3-N system, or the amino cation +C1-C2-C3-N is the level ordering A below S . This in turn is set by maximal through-bond coupling of the empty cation orbital and the nitrogen lone pair. The conformational dependence of through-bond coupling is exactly that derived by Grob, namely parallel orientation of the cation orbital (or the C-X bond), the C2-C3-σ-bond, and the N-lone-pair. When the C1-C2-C3 and C2-C3-N angles are small, the through-space interaction dominates, reversing the level ordering to S below A , and consequently makes the fragmentation forbidden even though the conformational requirements for it are met. Ring closure becomes allowed. Some examples exploiting this result are presented, as well as procedures for enhancing through-bond coupling and thus fragmentation. The through-bond-effect has also kinetic consequences, allowing the definition of a new type of remote neighbouring group participation operative through bonds and not by direct overlap. The position of equilibria in nitrogen inversion processes should also be influenced by remote substituents which are π-acceptors or donors. 相似文献
29.
A high-performance liquid chromatographic procedure was developed for the isolation and quantitation of coumarin from vanilla-based liquid flavorings of Mexican origin. Forty products representing fourteen different Mexican brands were assayed for coumarin, vanillin, and ethyl vanillin by the proposed method. The procedure has been adapted to the analysis of other products including domestic vanilla extracts and imitation vanilla flavorings for vanillin, ethyl vanillin, 4-hydroxybenzaldehyde and piperonal. Chromatographic retention data for thirty-seven compounds associated with vanillin and vanilla products employing two mobile phase systems are presented. 相似文献
30.
N-(2-Nitro)benzoylamino acids were prepared by 2-nitrobenzoylation of amino acids via the mixed ethylcarbonic anhydride procedure. They were reduced catalytically to N-(2-amino)benzoylamino acids which underwent cyclization to the corresponding lactams under a variety of conditions. The use of this reaction sequence for stepwise degradation of peptides seems possible. 相似文献