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81.
Ngouela S Zelefack F Lenta BN Ngouamegne ET Tchamo DN Tsamo E Connolly JD 《Natural product research》2005,19(7):685-688
A xanthone derivative, 3,6,7-trihydroxy-1-methoxyxanthone has been isolated from the stem bark of Allanblackia monticola together with other known compounds, 2,6-dihydroxy-1-methoxyxanthone, allanxanthone A, epicathechin and oleanolic acid acetate. The structure of the new compound was elucidated by spectroscopic methods. 相似文献
82.
Etched capillaries for use in open tubular electrochromatography are modified by silanization/hydrosilation and organosilanization. The migration behavior of both types of capillaries is evaluated with small basic molecules, peptides and proteins. Comparisons of peak symmetry and efficiency are used to measure the effectiveness of the two methods for modifying the etched surface. From this information, the suitability of each method for use with etched capillaries can be determined. 相似文献
83.
The approximation of hyperpolarizabilities through the use of operator inequalities is sketched. Previously applied methods to ordinary polarizabilities are extended. They lead to expressions involving moments of oscillator strength distributions related through sum rules to ground state properties. Systematic applications of new formulae are suggested. 相似文献
84.
The properties of n-type thin polycrystalline films of antimony trisulphide obtained by tarnishing reactions have been studied by photoelectrochemical and spectroellipsometric techniques. The photoelectrochemical behaviour is analysed using a recombination model. 相似文献
85.
The use of inductively coupled plasma mass spectrometry (ICP-MS) coupled with separation techniques for the purpose of elemental speciation has recently gained a lot of attention. Much of this is due to ever improving separation capabilities of Chromatographic techniques, the high sensitivity of ICP-MS, and the continuing development of better interface techniques. Additionally, there is a growing awareness of the need to monitor various species of an analyte, rather than just total analyte concentrations, due to their often varying natures. For the sake of learning from different elemental speciation approaches, this review brings together some selected types of elemental speciation which have been recently seen in literature. These include separations using various forms of liquid chromatography, such as reversed phase, reversed phase ion pairing, micelle, ion exchange, and size exclusion. Elemental speciation employing gas Chromatographie separations and supercritical fluid separations are discussed as well as elemental speciation using capillary electrophoresis. 相似文献
86.
In contrast to adducts I of bi-1-cycloalken-1-yls and p-benzoquinone, their reduction products II do not exhibit a double hydrogen migration from δ positions accompanying a ‘retro-Diels-Alder’ type fragmentation. An ordinary retro-Diels-Alder fragmentation was found to take place, with charge retention in the diene portion of the molecule. A double hydrogen migration has been detected in II leading to m/e 112 ion c2, which differed, however, from that in I in charge retention and in the origin of the migrating hydrogen atoms. Adducts III of di-1-cycloalken-1-yls and naphthoquinone behave similarly to II. They exhibit relatively low abundance ions a, however, due to a double hydrogen migration from δ positions, similarly to I. The origin of the migrating hydrogen atoms have been determined by deuterium labelling. Mechanistic suggestions are presented to explain the observed facts. 相似文献
87.
Joseph Rabinowitz 《Helvetica chimica acta》1969,52(1):255-261
Comparison of the NMR. spectra in CDCl3 of the heterocyclic bases obtained from the cyclisation of ω-(N-thiocarbamoylamino) ethyl (or propyl)-alcohols (or their orthophosphoric or sulfuric monoesters) to those of model compounds II (n = 1 or 2) and III (n = 1 or 2) has shown that: (1) In the case of five membred rings the C?N double bond is always endocyclic (Ib, n = 1) should R be aromatic, araliphatic or aliphatic; (2) In the case of six membered rings the C?S double bond is cnclocyclic when R is aliphatic or araliphatic (Ib, n = 2), and exocyclic when R is aromatic (I a, n = 2), with the exception of 2-(o-carboxyphcnylamino)-dihydro-δ2-m (Ib, n = 2, K = o-carboxyphcnyle). In CF, COOH, all five membered rings (I b, n = 1) show a triplet for the C-4 methylenic protons, whereas all the six membered rings (Ia or I b, n = 2) with the exception of I b, n = 2, R = o-carboxyphenyle, are represented b y a double triplet for the C-4 protons (samt. protonated spccics). Only one triplet is observed when the 3 position is substituted. Thiocarbamoylation of hydrazinoethanol or its orthophosphoric or sulfuric monoesters canoccur at either of the two nitrogen atoms, thus yielding upon cyclization five- (IT′) or six-membered rings (Va or Vb). The NMR spectra of compounds I V in (CIl,), SO show a singlet for 2 amino pro-tons (3-amino) and there is no further structural problem. The NMR spectra of compounds T′ in (CT), SO show a triplet for one amino proton coupling with the neighboring methylenic protons. I n this case, mode1 compounds are needed to assign the position of the C?N double bond ( e x cyclic V a or cndocyclic V b). When R = o-carboxyphenylc, the C?N double bond is probably endoc, yclic (Vb) because this ccimpound and 2-(o-carboxyphenvlarnino)-dihydro-δ2 have very similar UV spectra. 相似文献
88.
We have used energy minimization calculations to study a number of conformations of uncomplexed valinomycin. In certain cases, x-ray diffraction atomic coordinates were used directly as input coordinates, while in other cases, conformations were found by altering the x-ray coordinates prior to minimization. Five calculated conformations are reported along with their relative energies. The conformation found theoretically to be the most stable is in agreement with earlier, cruder calculations, but does not correspond to the predominant conformation observed in nonpolar solvents. A possible rationale is presented. 相似文献
89.
Summary We consider the two-dimensional Helmholtz equation u+u=0 inD with the boundary conditionsu=0 on D. D is the Swiss Cross — a region consisting of five unit squares. A method based on the concept of Coherence is utilized to determine an approximation for the first eigenvalue=
1 more accurate than calculated by classical difference methods. The numerical result is used to illustrate isoperimetric upper and lower bounds for
1, and to test some conjectures on its relations with torsional rigidity.Dedicated to the memory of Professor Lathar Collatz 相似文献
90.
Four new solutions in general relativity have recently been derived as representing static spherically symmetric stiff matter,=p. It is pointed out that the equation of state is, in fact,+p=0. It is further shown that two of the solutions are physically reasonable, turning out to represent the vacuum, one of them with a term. 相似文献