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181.
Chaves DC Assunção JC Braz-Filho R Lemos TL Monte FJ 《Magnetic resonance in chemistry : MRC》2007,45(5):389-392
Two new cucurbitane-type triterpenoids, 2beta,3beta,16alpha,20(R),25-pentahydroxy-9-methyl-19-norlanost-5-en-7,22-dione and 2beta,3beta,16alpha,20(R),25-pentahydroxy-9-methyl-19-norlanost-5-en-7,11,22-trione, were isolated from fruits of Cayaponia racemosa. The total (1)H and (13)C chemical shift assignment of these two closely related compounds is described, making use of one- and two-dimensional NMR techniques. 相似文献
182.
Amaro Chaves Ramos 《液相色谱法及相关技术杂志》2016,39(1):8-12
Solanum cernuum is a native shrub from Brazil that has been used as medicine since the nineteenth century. Although it has been utilized for a long time, there are few studies regarding its chemical composition and it motivated us to investigate this species. The dried leaves of S. cernuum were extracted with methanol and the obtained extract was further partitioned with different organic solvents. As part of our investigation, the n-hexane fraction was submitted to CCC with a nonaqueous solvent system composed of hexane, acetonitrile, and ethyl acetate (5:5:1, v/v/v). Three terpenoids were obtained by one-step separation and identified as 24-oxo-31-norcycloartanone (22.8 mg), lupeol (9.4 mg), and cycloeucalenone (13.4 mg). Complementarily, the solvents system used on the separation have had their compositions determined by gas chromotography–flame ionization detector (GC-FID). 相似文献
183.
L.G. Vieira O. Hernandez A. Almeida M. Quilichini J.L. Ribeiro M.R. Chaves A. Klöpperpieper 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):447-456
We report a combined experimental study by means of elastic neutron scattering and dielectric measurements of a partially
deuterated and brominated BCCD (Betaine Calcium Chloride Dihydrate) crystal. The lowest-temperature phase is one-dimensional modulated and
characterized by the coexistence of different commensurate domains (with = 1/4, 4/17, 2/9 and 1/5 on cooling), but with a clear predominance of the five-fold phase. A huge global thermal hysteresis
of the wave-vector of the modulation, attaining values of about 9 K in the incommensurate phase and up to 15 K in the “harmless”
low temperature part of the phase diagram, is observed up to . The role of lattice defects on this phenomenon is discussed. Similarly to the behaviour of the pure compound, the structural
modulation evolves on cooling towards a soliton regime (growth of third and fifth-order satellite peaks), probably with respect
to a non-stabilized non-modulated ferroelectric phase. The critical temperatures deduced from dielectric constant and pyroelectric
current measurements are in very good agreement with those obtained from neutron scattering. The dielectric anomaly observed
in at K, and known as the “-anomaly”, could not be related with any special feature detected in the neutron data, and in particular no correlation between
this anomaly and the appearance of the soliton regime can be established.
Received 26 October 1998 相似文献
184.
In this work we obtain a Simons' type inequality for a suitable tensor and apply it in order to obtain some results characterizing umbilical submanifolds and a product of submanifolds in a semi-Riemannian space form. 相似文献
185.
Maria de Ftima V. Marques rica Gervasoni Chaves 《Journal of polymer science. Part A, Polymer chemistry》2003,41(10):1478-1485
Because of the great economic interest in propylene‐based polymers and the possibility of designing materials with desired properties with metallocene catalyst mixtures, we investigated the characteristics of polypropylenes produced by mixtures of SiMe2Ind2ZrCl2: dimethylsilane‐bis(indenyl) zirconocene ( 1 ) and Et(Flu)(Cp)ZrCl2: ethylidene (fluorenyl cyclopentadienyl) zirconocene ( 2 ) in different proportions. The polymers were fractionated with solvents, and the fractions were characterized. We observed that the polymers produced by the different mixed systems showed lower weight‐average molecular weights and only slightly broader molecular weight distributions than polypropylenes synthesized by the individual catalysts. We concluded that catalyst 1 acted independently of catalyst 2 , producing polymers with the same isotacticity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1478–1485, 2003 相似文献
186.
We report a clear cut evidence of a temperature dependent diffusive central mode associated with the ferroelectric phase transition of LiTaO3; a detailed linshape analysis shows that this central mode is a Debye-like relaxation. We also show its influence in the dielectric constant through polariton measurements for several temperatures. 相似文献
187.
In this work we are mainly concerned with the study of special 2-parameter envelopes M
n
(n 4) of n-spheres in a simply connected space form Q
c
n+1
(c = 0, 1 or –1) which satisfy the Weingarten relation A + BH + CK = 0 between the mean curvature H and the Gauss–Kronecker curvature K of the immersion x : M
n
Q
c
n+1
, where A, B and C are real constants; such a relation includes the case H constant. 相似文献
188.
This article reports the numerical comparison of the quantities characterizing the extent of electron fluctuation and pair localization in the domains determined by the direct minimization of electron fluctuation with the domains resulting from the partitioning of the molecules based on the topological analysis of the so-called electron localization function (ELF). Such a comparison demonstrates that the ELF partitioning can be regarded as a feasible alternative to computationally much more demanding direct optimization of minimum fluctuation domains. This opened the possibility of the systematic scrutiny of the electron pair model of the chemical bond, and as it was demonstrated, the previous pessimistic claims about the applicability of this model are not completely justified. 相似文献
189.
Humberto César Chaves Fernandes 《International Journal of Infrared and Millimeter Waves》1996,17(8):1419-1430
The theory and numerical results of the asymmetric single and coupled bilateral and unilateral finlines and arbitrary antipodal finline considering the attenuation of the substrate are presented. Results are given to the complex propagation constant. The full wave analysis of the Transverse Transmission Line — TTL method is used in the FTD. Applying the moment method the complex propagation constant, including the attenuation constant and the phase constant, are obtained. The effective dielectric constant, eff,, and the characteristic impedance are also calculated. The results are compared with the references, and news results are presented for these parameters. 相似文献
190.