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81.
Paristerone, a novel phytoecdysone has been isolated from the tubers of P. polyphylla and its structure and stereochemistry has been established as 2α, 3β, 14α, 20(R), 22(r), 25-hexahydroxy-5β-chloest-7-en-6-one (2-epiecdysterone) on the basis of chemical and physical evidence. 相似文献
82.
A cathode-ray polarographic method has been developed for the determination of microgram quantities of gadolinium with the use of an azo-dye, Solochrome Violet RS. Disadvantages in the use of 0sd1M potassium chloride, commonly recommended as electrolyte for the polarographic determination of rare earths, have been demonstrated. The influence of pH, temperature, ligand concentration and purity, and the stability of the complex, have been investigated for the optimum performance of the method. The linear range is 0.5-12 mug/ml of gadolinium. Interferences by some lanthanides and other cations have been examined. 相似文献
83.
Asymptotic behavior of the pion form factors has been discussed treating pion as a bound state of the elementary nucleon and antinucleon system. The matrix-element of the electromagnetic current is written in terms of the Bethe-Salpeter amplitude for the nucleon-antinucleon bound system. Using suitable approximations the Bethe-Salpeter equation for nucleon-antinucleon bound state with nucleons as Dirac spinors has been solved and the corresponding off-shell pion form factors are determined. The form factors are strongly interaction dependent. For coupling the form factors vanish asymptotically like t?1 and for the form factors still vanish but less rapidly than t?1. 相似文献
84.
Agostic interactions and weak hydrogen bonds to metal acceptors, M, are chemically and geometrically distinct, and this distinction must be maintained in the classification of new C-H...M interactions. 相似文献
85.
A series of polymer nanocomposite films based on intercalation of (PAN)8LiCF3SO3 into the nanometric clay channels of an organomodified clay has been prepared using the standard solution-casting technique.
The role of organoclay concentration on polymer–ion interaction, ion–ion interaction, and ion–clay interaction in clay-based
nanocomposite films has been analyzed using Fourier transform infrared (FTIR) analysis. Substantial ion dissociation is observed
even at a very low clay loading (1–2 wt.%) in the nanocomposites. FTIR results suggest the presence of both uncoordinated
CF3SO3
− (free-anions) and ion pairs in the nanocomposite evidenced by changes in CF3SO3
− symmetry from C3ν to Cs and marked asymmetry in the profile of degenerate δd(CF3
−) mode. The experimental results suggest a direct correlation of clay-assisted ion dissociation process with variation in
conductivity (σ
dc) and glass transition temperature (T
g) as a function of clay concentration. A model has been proposed to explain the observed correlation on the basis of polymer–ion–clay
interaction. The proposed scheme of ion transport mechanism appears to be consistent with the experimental observation. 相似文献
86.
The effect of lanthanum substitution on the structural, dielectric and piezoelectric properties of lead calcium titanate (PCT) ceramics is studied. Samples with compositional formula Pb0.76? x La x Ca0.24Ti0.98Mn0.02O3 (PLCT) (x?=?0–0.08, in steps of 0.02) were prepared by the conventional dry ceramic technique. Phase and structural analysis was carried out using XRD and SEM respectively. X-ray analysis confirmed the formation of single-phase compound with tetragonal crystal structure. Dielectric properties were studied in detail as a function of frequency and temperature. The Curie temperature (T C) and tetragonality (c/a) was found to decrease with increase in lanthanum content. The Curie temperature was also confirmed from the thermal-expansion behaviour of the sintered samples. Piezoelectric coefficients (d 33, d 31, g 33, g 31, g h, d h, k t and k p) were measured at room temperature. From this study it is found that La3+ substitution helps in the improvement of electro-mechanical anisotropy. 相似文献
87.
An analysis of phase-modulated heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance 总被引:1,自引:1,他引:0
Thakur RS Kurur ND Madhu PK 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2008,193(1):77-88
The design of variants of the swept-frequency two-pulse phase modulation sequence for heteronuclear dipolar decoupling in solid-state NMR is reported, their performance evaluated, and compared with other established sequences like TPPM and SPINAL. Simulations performed to probe the role of the homonuclear (1)H-(1)H bath show that the robustness of the decoupling schemes improves with the size of the bath. In addition, these simulations reveal that the homonuclear (1)H-(1)H bath also leads to broad baselines at high MAS rates. Results from a study of the SPINAL decoupling scheme indicate that optimisation of the starting phase and phase increment improves its performance and efficiency at high MAS rates. Additionally, experiments performed on a liquid crystal display the role of the initial phase in SPINAL-64 and sequences in the SW(f)-TPPM family. 相似文献
88.
89.
90.
A new series of 1,2,4‐triazole was designed, synthesized, and characterized as remarkable antimicrobial and antioxidant agents. These heterocycles have been prepared from the cyclization reactions of Schiff bases 3 ( a‐k ) with phenylhydrazine by refluxing under the alkaline medium. The Schiff bases in turn were realized in good yields from the condensation reactions of N‐phenylurea with different aromatic aldehydes. The structures of the intermediates 3 ( a‐k ) and final heterocycles 4 ( a‐k ) have been fully characterized through their spectral parameters. 相似文献