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991.
992.
Thallium (0.02-20 mug) was successfully preconcentrated on silica gel C18 from 0.1M hydrochloric acid in the presence of various cationic surfactants as ion pairs with tetrachlorothallate(III), and subsequently eluted with 96% ethanol. Of various atomic spectrometric procedures considered, emission spectrometry in a nitrous oxide-acetylene flame is suitable for the final determination of thallium in plants, this is more sensitive for thallium than flame atomic absorption spectrometry. 相似文献
993.
T. Sommer M. Vojta K.W. Becker 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(3):329-339
The two-layer square lattice quantum antiferromagnet with spins 12 shows a zero-field magnetic order-disorder transition at a critical ratio of the inter-plane to intra-plane couplings. Adding
a uniform magnetic field tunes the system to canted antiferromagnetism and eventually to a fully polarized state; similar
behavior occurs for ferromagnetic intra-plane coupling. Based on a bond operator spin representation, we propose an approximate
ground state wavefunction which consistently covers all phases by means of a unitary transformation. The excitations can be
efficiently described as independent bosons; in the antiferromagnetic phase these reduce to the well-known spin waves, whereas
they describe gapped spin-1 excitations in the singlet phase. We compute the spectra of these excitations as well as the magnetizations
throughout the whole phase diagram.
Received 23 April 2001 相似文献
994.
Smectic‐A liquid single crystal elastomers (SA‐LSCE) are materials where the rubber elasticity of the polymer network is combined with the one‐dimensional positional long‐range order of mesogenic groups which are covalently attached to the network. In the systems investigated so far, a mechanical deformation of the network causes significant reorientation processes of the layered structure. We present a new type of SA‐LSCE in which this structure remains unaffected on mechanical deformation both parallel and perpendicular to the director. The thermoelastic behavior, macroscopic dimensions, and stress–strain measurements parallel as well as perpendicular to the director are investigated. X‐ray studies confirm that a deformation parallel and perpendicular to the layer normal does not alter the macroscopically ordered lamellar structure with respect to the order parameter and the layer correlation length. We propose a simple picture where defects within the lamellar structure might account for these findings.
995.
Smectic‐A elastomers combine the positional long‐range order of mesogenic molecules in one dimension with the rubber elasticity of a polymer network. Upon mechanical deformation, completely different responses of the phase structure have been reported. We present a highly distorted system which shows a breakdown of smectic layering but no reorientation under deformation along the layer normal, while the phase structure stays unaffected under uniaxial stress in the perpendicular direction. The thermoelastic properties, macroscopic dimensions and stress–strain behaviour are investigated parallel and perpendicular to the layer normal. SAXS measurements supply evidence for a breakdown of the macroscopically ordered layer structure indicated by the small angle intensities and correlation length, whereas the orientational order is preserved. We propose defects in the smectic layer structure to be the origin of the different responses of different smectic elastomers.
996.
A spiro[5.5]ketal library embodying the core structure of numerous biologically active natural products was synthesized employing a double intramolecular hetero Michael reaction as a key transformation. 相似文献
997.
A variety of α,β-unsaturated amides (RHCCH2CONR′2, R=H, Me, Ar; R′=H or Et) readily condense with benzene at room temperature in the presence of an excess of aluminum chloride to give the corresponding 3-phenylpropionamides in excellent yields. This simple, one-pot procedure proved to be efficient and very clean. The mechanism of these and related reactions is discussed and the participation of superelectrophilic dicationic intermediates is suggested. 相似文献
998.
L. Sommer 《Fresenius' Journal of Analytical Chemistry》1965,210(1):64
Ohne Zusammenfassung 相似文献
999.
L. Sommer 《Fresenius' Journal of Analytical Chemistry》1965,215(2):112-114
Ohne Zusammenfassung 相似文献
1000.