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971.
Using the results obtained from an MO SCF ab initio calculation on P2H4 in four different conformations, the 1J(PP) NMR coupling constants have been calculated. The 1J(PP) values are highly dependent upon the rotational angle φ (?238.0 Hz in the eclipsed conformation, 10.9 Hz in the staggered one). The shape of the theoretical 1J(PP) plot seems to be in good agreement with experimentally measured 1J(PP) values. 相似文献
972.
Langlet J. Claverie P. Pullman B. Piazzola D. Daudey J. P. 《Theoretical chemistry accounts》1977,46(2):105-116
The effect of water on the conformational preferences of acetylcholine has been studied within the discrete, the continuum and the combined discrete-continuum models described in parts I and II of this series. All the models lead to the conclusion that the trans-gauche form which is, following refined quantum-mechanical computations, the intrinsically preferred one and the one observed in the crystal of acetylcholine and of a number of analogues should remain also the preferred conformation in water. This result agrees with NMR studies. The results of the empirical discrete model used here compare favorably to those obtained by an ab initio super-molecule treatment. The continuum model utilized here represents a net improvement above such models utilized in other works. 相似文献
973.
Gregory D. Cuny 《Tetrahedron letters》2004,45(26):5167-5170
A synthesis of the alkaloid (±)-aporphine is reported. The initial key step of the synthesis involves a Pictet-Spengler cyclization of N-tosyl tyramine with 2-bromophenylacetaldehyde in trifluoroacetic acid. This step was followed by the second strategic transformation a palladium-mediated intramolecular phenol ortho-arylation reaction utilizing tricyclohexylphosphine as co-catalysts in the presence of cesium carbonate. Finally, de-oxygenation of the phenol, removal of the tosyl group and methylation gave the desired alkaloid. 相似文献
974.
975.
Zusammenfassung In der vorliegenden Arbeit wird das Sedimentationsgleichgewicht der Systeme Na6[H2W12O40]—NaCI—H2O und Na6[H2W12O40]—NaClO4— H2O untersucht. Die scheinbare molare Masse Mapp des Natriummetawolframats wird in beiden Systemen als Funktion der Natriummetawolfrarnat-Anfangspartialdichte
0
PX
z gemessen. Durch lineare Extrapolation der Funktion Mapp = Mapp (PP,, ) auf die Polymerelektrolyt-Anfangspartialdichte Null können die Grenzwerte der scheinbaren molaren Masse (Mapp)o ermittelt werden. Aus den (Mapp)o-Werten wird die Abhängigkeit der Größe (ln
PXz
/m
BX
)
mPXz von der NaCI- bzw. NaClO4-Molalität berechnet.Die Bestimmung der molaren Masse und der (stöchiometrischen) Ladungszahl eines Polymerelektrolyten aus den experimentell ermittelten (Mapp)o-Werten unter Anwendung der Lamm-Williams-Gleichung wird diskutiert. Zur Ermittlung der beiden Größen müssen mehrere Versuchsreihen mit zwei verschiedenen Fremdelektrolyten durchgeführt werden. Externe Messungen sind nicht erforderlich.Man erhält auf diese Weise verläßliche Werte so-wohl der molaren Masse als auch der (stächiometrischen) Ladungszahl.
Mit 5 Abbildungen und 3 Tabellen 相似文献
Summary The sedimentation equilibria of the systems sodium metatungstate-sodium chloride-water and sodium metatungstate-sodium perchlorate-water are investiated. In both systems the apparent molecular mass Mapp of sodium metatungstate (PX z) is measured in dependence on partial density 0 PX z before centrifugation. By linear extrapolation of the function Mapp = Mapp ( 0 PX z the limiting value (Mapp)o for 0 PX z = 0 is obtained. The variation with sodium chloride and sodium perchlorate molality of the quantity (ln PX z m BX )mPXz is calculated from (Mapp)o data.The determination of the molecular mass and the algebraic valency of a polymeric ion by means of the (Mapp)o values calculated by Lamm-Williams equation is discussed. In order to obtain these two quantities at least two series of experiments are required, each of which using a different supporting electrolyte. External measurements are not necessary. In this way reliable values for the molecular mass as well as for the stoichiometric valency are obtained.
Mit 5 Abbildungen und 3 Tabellen 相似文献
976.
Decomposition of isopropanol on V2O5 and the bonzes Li0.02V2O5, Na0.02V2O5, Na0.06V2O5, Li0.33V2O5, and Na0.33V2O5 has been studied in the temperature range 168–300°C. The main reaction was found to be dehydration to propene with negligible dehydrogenation to acetone on the first four catalysts. Dehydration on these catalysts increased with the alkali metal content, but the energy of activation remained unchanged. On the last two catalysts, dehydration and dehydrogenation proceeded at comparable rates. A tentative mechanism for the dehydration of isopropanol is proposed, based on the effect of the product on the initial rate, the electric conductivity of the catalysts and their ESR spectra.
V2O5 : Li0,02V2O5, Na0,02V2O5, Na0,06V2O5, Li0,33V2O5 Na0,33V2O5 186–300°C. . , . . , , .相似文献
977.
R. Van der Meer A. L. German D. Heikens 《Journal of polymer science. Part A, Polymer chemistry》1977,15(7):1765-1772
A short review of the effect of pressure on copolymerization kinetics shows the necessity of simple models for a better understanding of activation volumes. Therefore, a simple concept, possibly generally valid for free-radical polymerization, is proposed, based on the assumption that molar volumes of activation can be expressed as an addition of a characteristic radical and a monomer contribution, regardless of the combination involved. The scheme may facilitate the visualization of the transition state and contribute to the understanding of reaction mechanisms of radical polymerizations. Ethylene–vinyl acetate copolymerization at 62°C with tert-butyl alcohol as solvent agrees with the proposed scheme, appearing from the pressure independence of the product of reactivity ratios at the different levels (35,600, and 1200 kg/cm2). Implicitly it can be shown that an ethylene monomer contributes about 2 cm3/mole more to the activation volumes of the propagation reactions than does the vinyl acetate monomer, whereas for the radicals the difference of the respective contributions to the activation volumes is opposite in sign. 相似文献
978.
A rapid non-destructive neutron-activation method for the determination of uranium in rocks is described. The method is based on activation with epithermal neutrons and subsequent measurement of the 74 keV gamma-ray of (239)U. Results given for some standard rocks are in good agreement with literature data. The precision of the method is about 5 % and the limit of detection is of the order of 0.1 ppm in silicate rocks. 相似文献
979.
A convenient and compact automatic derivative spectrophotometric titrator is described which is suitable for automatically performing a wide variety of the well known color indicator titrations without any procedure modifications and usually with few, if any, pretitration considerations and instrument adjustments The results for many acid-base and oxidation-reduction titrations are presented to illustrate the general applicability and important characteristics of the instrument 相似文献
980.
N. V. Makarova Zh. V. Koronova A. V. Plotnietse D. Ya. Tirzite G. D. Tirzit G. Ya. Duburs 《Chemistry of Heterocyclic Compounds》1995,31(8):969-973
There has been synthesized a series of 1,4-dihydropyridines having an N-alkylpyridinium substituent at position 4 with a varying length of hydrocarbon chain. Their affinity to model (liposomal) membranes has been studied. It was found that this affinity increased with lengthening of the hydrocarbon chain on the Npyridinium substituent at the 4-position of the 1,4-dihydropyridine ring. However, lengthening of the hydrocarbon chain in the 3,5-ester groups of the 1, 4-dihydropyridine ring led to a decrease in the binding to the liposome when a 4-(N-hexadecylpyridinium) substituent was present.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1112–1117, August, 1995. Original article submitted June 6, 1995. 相似文献