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31.
The first highly efficient, stereo- and regioselective palladium-catalyzed hydro-, sila-, and stannastannation of cyclopropenes to give multisubstituted cyclopropylstannanes have been developed. It was shown that the addition across the double bond of cyclopropene is generally controlled by steric factors and proceeds from the least hindered face. The directing effect of alkoxymethyl substituents in the hydrostannation reaction of 3,3-disubstituted cyclopropenes was demonstrated. This methodology represents a powerful and atom-economic approach toward a wide variety of highly substituted cyclopropylstannanes, important building blocks unavailable by other methods. 相似文献
32.
The measurement in thermal equilibrium of the vacancy contribution to the residual resistivity of metals has posed certain
difficulties. The recent experiment of Celasco and co-workers represents a new, powerful approach to this problem, via the
measurement of the power spectrum of the voltage noise generated by resistivity fluctuations. The latter originate in vacancy
number fluctuations. We develop a theory for the power spectrum, incorporating three basic features. Vacancies can be annihilated
in the material and they diffuse. Grain boundaries act as sources and sinks for vacancies. Both annihilation (a form of reaction)
and diffusion are noisy processes. We therefore set up and solve a reactive-diffusive stochastic equation for the instantaneous
density, with appropriatefinite boundary conditions. Assuming for simplicity that the grains are spherical, the power spectrum is evaluated exactly, in closed
form. A detailed comparison with experiment is made. The physical origins of different time scales in the problem and the
consequent frequency regimes in the power spectrum are analysed. Recognising the very general applicability of our theory,
we also mention possible applications to other problems. 相似文献
33.
We report the results of a Raman scattering study of (NH4)2CuCl42H2O at 300, 205 and 100°K, in order to elucidate the dynamics and phase transition in this double salt. A group theoretical calculation of the symmetry vectors, in the high temperature phase (D4h14), is made and the various modes are identified. The deuterated compound (ND4)2CuCl4·2D2O has also been investigated to help in identifying the modes involving motion of the ammonium ions and water molecules. Through a careful analysis of the spectra at 100°K, the space group in the low temperature phase has been established as D2d3. The important consequence of this result is that this leads to parallel spatial ordering of ammonium tetrahedra in this compound in the low temperature phase. 相似文献
34.
The heat transfer in a laminar incompressible plane wall jet due to a variably heated wall has been studied. It is assumed that the difference of temperatures between the wall and the issuing jet is inversely proportional to an arbitrary exponent of the distance from the slit. A similar solution of the energy equation is possible. The solutions, for arbitrary values of the Prandtl number and of the exponent are obtained. It is found that in some cases the heat transfer at the wall may become zero or negative. 相似文献
35.
36.
The variation of hyperfine field, isomer shift and Curie temperature with chromium concentration has been studied for the alloys (NiFe)1?xCrx for x = 0, 5, 10 and 15 at .%. The decrease in hyperfine field and isomer shift suggests an emptying of the majority spin d-band with increasing Cr concentration (reducing number of average 3d + 4s electrons per atom). The Curie temperature is also found to reduce rapidly with Cr concentration in agreement with magnetisation data. But the Curie temperatures as determined from Mössbauer spectra are considerably lower than those given by magnetisation studies and for the 10 and 15 at .% Cr concentrations the magnetic transition is spread out over a range of temperatures suggesting a mictomagnetic behaviour. 相似文献
37.
Detailed Raman scattering investigation of LiKSO4 in phases II and III across the transition temperatureT c ? 700 K is reported. Abrupt change in frequency and line width of the external and internal modes have been observed. Analysis of the results suggests lithium positional disorder and sulphate orientational disorder in the high temperature phase (II). The results also throw some light on the existence of twin domains in the crystal. 相似文献
38.
This paper presents a general theory of the forced response under convected loading of mono-coupled periodic systems with a single disorder. The general expressions derived have been used to study the response of an infinite periodic beam on simple supports with one of the support spacings different from all the others. Convected harmonic pressure fields and frozen random pressure fields have been considered. Computer studies are presented showing the moment response at supports and the space-time-averaged responses in the disorder and in the nearby periodic beam elements. High response levels can occur due to (i) resonances of the beam length disorder against the stiffness of the attached periodic systems and (ii) hydrodynamic coincidence vibration occurring in the periodic beam. The frequency zones in which these high responses may occur are identified. The high response due to the resonance (ii) is restricted to the vicinity of the disorder, whereas that due to coincidence occurs throughout the system. Computed results show that the highest response levels do not necessarily occur in the beam length disorder, but may occur in one of the nearby periodic beam elements. The dependence of the maximum response levels on the magnitude of the disorder has been investigated. The conditions under which small disorders may be neglected have been pointed out. 相似文献
39.
Oseen’ approximations are used to study the slow motion of a viscous, incompressible, electrically conducting fluid past a
circular cylinder in the presence of a uniform aligned magnetic field. Using series truncation method, the analytical solutions
for the first three terms in the Fourier sine series expansion of the stream function are obtained. Numerical values of the
tangential drag for different values of magnetic interaction parameter and viscous Reynolds number are calculated. 相似文献
40.
A subtle first order phase transition in LiKSO4 has been discovered with the help of a temperature dependent study of the Raman intensity measurements of certain polar modes in different polarization configurations. The room temperature hexagonal C66 (P63) phase transforms to trigonal C43v (P31c) phase at 201°K while cooling; the reverse transformation (on heating) takes place at 242°K. The phase transition appears to be primarily associated with a cooperative reorientation of SO4 tetrahedra in the crystal. 相似文献