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81.
William J. Evans 《Journal of organometallic chemistry》1978,154(3):C50-C51
The (Me3Si)3C group causes very large steric hindrance to nucleophilic displacement at a silicon atom to which it is attached, and (Me3Si)3CSiMe2Cl is even less reactive than t-Bu3SiCl towards base. The compounds (Me3Si)3CSiMe2X (X = Cl, Br, or I) are cleaved by MeOH/MeONa to give (Me3Si)2CHSiMe2OMe, possibly via the silaolefin (Me3Si)2 CSiMe2, and the correspondLug (Me3Si)3 CSiPh2X compounds undergo the analogous reaction even more readily. The halides (Me3Si)3CSiR2X (X = Cl or Br) and (Me3Si)3CSiCl3 do not react with boiling alcoholic silver nitrate, but the iodides (Me3Si)3CSiR2I are rapidly attacked. 相似文献
82.
Using single-crystal, automated diffractometer techniques, the linear coefficient of thermal expansion has been determined for La0.62Pb0.38MnO3 from 298 to 627 K. The linear coefficient of thermal expansion is observed to undergo a change from 7.2 × 10?5 Å/K for T < Tc to 10.8 × 10?5 Å/K for T > Tc. It is concluded that while the rhombohedral distortion in the (La, Pb)MnO3 system can be understood qualitatively on the basis of ionic size and polarizability considerations alone, the quantitative systematics of the distortion parameters and the change in the linear thermal expansion coefficient at Tc indicate a significant coupling between the elastic and magnetic exchange forces. 相似文献
83.
Rabindra N. Roy James J. Gibbons Guilio LaCross Jr. Charles W. Krueger 《Journal of solution chemistry》1976,5(5):333-343
The two thermodynamic dissociation constants of glycine at 11 temperatures from 5 to 55°C in 50 mass % methanol-water mixed solvent have been determined from precise emf measurements with hydrogen-silver bromide electrodes in cells without liquid junction. The first acidic dissociation constant (K
1)for the process HG+H++G± is expressed as a function ofT(oK) by the equation pK
1 = 2043.5/T – 9.6504 + 0.019308T. At 25°C, pK
1is 2.961 in the mixed solvent, as compared with 2.350 in water, with H°=1497 cal-mole–1, G°=4038 cal-mole–1, S°=–8.52 cal-°K–1-mole–1, and C
p
o
=–53 cal-°K–1-mole–1. The second acidic dissociation constant (K
2)for the process G±H++G– over the temperature range studied is given by the equation pK
2 = 3627.1/T – 7.2371 + 0.015587T. At 25°C, pK
2is 9.578 in MeOH–H2O as compared with 9.780 in water, whereas H° is 10,257 cal-mole–1, G° is 13,063 cal-mole–1, S° is –9.41 cal-°K–1-mole–1, and C
p
o
is –43 cal-°K–1-mole–1. The protonated glycine becomes weaker in 50 mass % methanol-water, whereas the second dissociation process becomes stronger despite the lower dielectric constant of the mixed solvent (=56.3 at 25°C). 相似文献
84.
Henry E. Montgomery Jr. 《The Chemical Educator》2002,7(6):334-338
Information entropy is introduced as a measure of quantum mechanical uncertainty. An uncertainty relation based on information entropy is obtained as an alternative to the Heisenberg inequality. In two typical examples, the entropic uncertainty relation is shown to be bounded in situations where the Heisenberg inequality diverges or grows too large to be useful. 相似文献
85.
M. B. Evans 《Chromatographia》1978,11(4):183-187
Summary The limitations of methods for the characterization of stationary phases, which require the use of a base or reference stationary
phase, are considered. In order to overcome these problems, a new method of column characterization, based upon measurements
of molecular retention index and ΔMe values for selected functional groups, is proposed. ΔMe values are shown to be dependent
upon liquid phase composition, nature of the support and liquid phase loading. 相似文献
86.
Treatment of meso-substituted metalloporphyrins [meso -substituent = OCOCF3, OCOCH3, OMe, CHO, CN, Cl; metal = Zn(II) or Cd(II)] with thallium(III) trifluoroacetate, followed by an acidic work-up, gives the corresponding β-substituted α-oxophlorins which were either characterised as such or else further derivatised. In all cases the major (or only) disubstitution product has the αβ orientation at the meso positions, indicating that the existing meso substituent directs the incoming one (trifluoroacetate) into the flanking, rather than opposite, meso position of the intermediate π-cation radical or π-dication. In contradistinction, meso substituted zinc(II) porphyrins which are able to lose protons (e.g. α-oxophlorins or α-aminoporphyrins) react with thallium(III) trifluoroacetate, and after a work-up with HCl the corresponding γ-chloro-α-oxophlorin or γ-chloro-α-aminoporphyrin is obtained. 相似文献
87.
Summary Inverse gas chromatography with sulphurated olefinic stationary phases and air as carrier gas has been found to yield characteristic
shifts of retention data. The resulting plots of retention index versus reaction time obtained for a range of squalene vulcanizates
have been found to be similar to those given by oxygen absorption measurements. Furthermore the chromatographic oxidation
curves have been found to be consistent with the known oxidation behaviour of the various sulphur networks formed on vulcanization.
This paper is dedicated to the memory of Dr.Graham Moore (1924–1968). 相似文献
88.
J. D. Jones J. M. Rottschafer H. B. Mark Jr. K. E. Paulsen G. J. Patriarche 《Mikrochimica acta》1971,59(3):399-404
Résumé Une méthode simple et rapide de détermination de traces de mercure est présentée utilisant l'activation de neutrons. Le mercure 197 sous forme de complexe (HgCl4
2–) est retenu sur une résine échangeuse d'anion qui est comptée directement. La méthode est appliquée à des milieux biologiques très divers. Les échantillons contiennent des quantités de mercure allant de 10 p. p. b. à 200 p. p. m. La déviation standard dans ces cas est de ± 10%.
Determination of traces of mercury in biological material by neutron activation
Summary A simple and rapid technique for the trace analysis of mercury by neutron activation is presented. The activated mercury, as an HgCl4 2– complex, is retained on an anion exchange resin which is counted directly. Results obtained for biological samples containing 10 p. p. b. to 200 p. p. m. are given. In this range, results had a ± 10% standard deviation.
Zusammenfassung Ein einfaches und rasches Verfahren zur Bestimmung von Spuren Quecksilber durch Neutronenaktivierung wurde beschrieben. Das aktivierte Quecksilber wird als HgCl4 2– an einem Anionenaustauscherharz adsorbiert und seine Aktivität direkt gemessen. Biologische Proben mit 10 ppb bis 200 ppm Hg wurden analysiert. In diesem Bereich betrug die Standardabweichung ± 10%.相似文献
89.
P. J. Ziemann A. W. Castleman Jr. 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,20(1):97-99
Gas phase (MgO) n + and (MgO) n Mg+ clusters were produced in a gas aggregation source and studied by using laser-ionization time-of-flight mass spectrometry. A MgO molecule apparently serves as the nucleus for cluster growth, to which Mg and O atoms add. The heat generated by the formation of metal-oxygen bonds, and that added to the cluster by ionization leads to the production of clusters with the stoichiometry of the stable high-temperature oxide. The abundance maxima observed in the mass spectra indicate that the clusters form compact cubic structures similar to pieces of the MgO crystal lattice. The primary fragmentation channel responsible for the observed patterns is probably the loss of MgO monomers. 相似文献
90.
Changes of the rheological properties of hyaluronic acid (sodium-magnesium salt) solutions after exposure to UV radiation
indicate a vigorous decrease in their viscosity, but its still strong shear rate dependence. Whereas the presence of the singlet
oxygen sensitizer (anthracene-1-sulphonic acid) brings about a loss of shear dependence; the studied solutions show newtonian
behavior. 相似文献