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991.
Mass spectra of phloroglucinol derivatives recently isolated from African Dryopteris species are presented. Phloroglucides with n-valeryl side chain were found in these ferns for the first time. In the mass spectra they show strong peaks corresponding to loss of propene (C3H6) produced by McLafferty rearrangement, as shown in model compounds. Aside of the rottleron change (discussed formerly) this must be considered in order to avoid wrong interpretation, particularly when mass spectra are used to analyse mixtures of homologues, as usually present in the plants. If valeryl side chains are present other methods must be used therefore to measure the amount or show the absence of lower homologues with acetyl side chains. 相似文献
992.
Maria Merces Marques C. Costa F. Lemos F. Ram?a Ribeiro A. R. Dias 《Reaction Kinetics and Catalysis Letters》1997,62(1):9-15
In this paper we present a kinetic approach to the analysis of steady-state homogeneous Ziegler-Natta polymerization activity
data. The influence of the number of monomeric species that are coordinated to the active site on the apparent rate law is
discussed and the equations are fitted to the experimental results. 相似文献
993.
Pyrolysis-gas chromatography was applied to the characterization of ground beef and soy protein isolate and its mixtures at the 50, 30 and 10% levels. A cryogenic trap pyrolysis mode was employed and the volatile pyrolysates were analysed by a high-resolution capillary gas chromatograph. Two phenolic derivatives, o-methoxy- and dimethoxyphenol, as well as 2,3-dithiabutane are uniquely observed at a large concentration in the soy pyrolysate. Quantification of these characteristic compounds allow the estimation of the level of soy inclusion in the mixture. This approach effectively denotes soy addition up to the 10% level in comminuted beef. 相似文献
994.
The syntheses and structures of two new compounds are reported. The first compound, [Cr(CO)4]2[C4F2(PPh2)4], obtained from Cr(CO)4(PPh2H)2 and CF3CCF3 in the presence of one equivalent of BuLi has a structure with the ligand 1,2,3,4-tetrakis(diphenylphosphino)-1,4-difluorobutadiene chelating to two Cr(CO)4 groups via the 1,4 and 2,3 phosphine groups. A mechanism for the formation of this compound is suggested which involves sequential deprotonation of a phosphine, nucleophilic attack on the fluorocarbon, and fluoride ion elimination. The second compound, CrC36H28P2O7, arises from a similar base promoted reaction of Cr(CO)4-(PPhH2)2 and PhCCCOOEt. Here the expected initial product from cyclization of these reactants acts as a nucleophile to attack a second equivalent of the acetylene. The intermediate carbanion from this reaction can undergo a ring closure by displacement of OEt-, giving the observed product. 相似文献
995.
Esahak Ali Ajit K. Chakravarty Satyesh C. Pakrashi Klaus Biemann Charles E. Hignite 《Tetrahedron》1977,33(11):1371-1377
The structure of solanoforthine, m.p. 208–10°, [α]D- 26.6°, a new steroidal alkaloid isolated from Solanum seaforthianum Andr., has been established as 3β-amino-22,26-epimino-16α,23-epoxy-22αH,25βH-cholest- 5- en -23β - ol 3, based on chemical and spectroscopic evidence and by its reduction to solanocapsine 2, another steroidal alkaloid encountered in the same species. The mass spectra of2 and 3 are discussed. 相似文献
996.
EPR studies of Gd3+ doped in single crystals of Nd2(SO4)3·(NH4)2SO4·8H2O (hereafter referred to as NASO) at room (RT) and liquid nitrogen (LNT) temperatures exhibit that (1) the metal aquo complex has a tetragonal symmetry with abnormally low magnitudes of crystalline field parameters at RT and (2) NASO undergoes a possible phase transition between RT and LNT. 相似文献
997.
Hennequin Y Evens M Quezada Angulo CM van Duijneveldt JS 《The Journal of chemical physics》2005,123(5):054906
Nearly athermal colloid-polymer mixtures were studied in the "protein limit." A fluid-fluid transition was observed in mixtures of stearyl-alcohol-coated silica particles and large polystyrene coils in toluene. The ratios of the polymer radius of gyration to the particle radii were q=4.1 and q=5.2. The binodal curves and the critical points were determined. Turbidity measurements and analysis for one set of particles allowed the systems to be mapped onto hard sphere-polymer mixtures. A comparison with recent predictions for the miscibility of model mixtures shows that the experimental binodals lie between the two extreme results for ideal and interacting polymers. The critical colloid volume fraction is also found to decrease with increasing size ratios. 相似文献
998.
A liquid chromatographic analysis with electrochemical detection of 5-methoxytryptamine, 5-methoxytryptophol, 5-methoxyindoleacetic acid and melatonin is described. Optimal elution conditions were determined by studying several variables: pH, buffer salt, counter ion and organic modifier. Measurement of 5-methoxyindoles in the pineal gland and plasma of hamsters has been performed after extraction. This method is specific and sensitive, and enables detection of 5-methoxyindoles in a pool of two hamster pineal glands. This is also the first time that these three 5-methoxyindoles have been measured simultaneously in plasma. 相似文献
999.
The selectivity of an analytical method may be defined as expressing the degree to which a component can be determined in the presence of other but similarly behaving components without interference. The method itself will usually serve for determination of the other components, under slightly different conditions. The selectivity is closely related to the resolution of the method and also to the resolution of the instrument used for obtaining the signals. A simple formula is suggested for expressing the percentual degree of selectivity of an analytical procedure, on the basis of signal overlap caused by the interfering components, and can be used quite generally. A distinction is made between the terms analytical selectivity and selectivity of an analytical procedure, and between selectivity and specificity. 相似文献
1000.