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101.
J.S. Yadav B.V. Subba Reddy Ashutosh Pratap Singh A.K. Basak 《Tetrahedron letters》2007,48(42):7546-7548
Baylis-Hillman adducts undergo smooth one-pot oxidative Michael addition with allyltrimethylsilane in the presence of 2-iodoxybenzoic acid (IBX)/Sc(OTf)3 under mild conditions to afford homoallyl β-ketoesters in good yields with high 1,4-selectivity. 相似文献
102.
Rare earth cobalties, LnCoO3, can be conveniently prepared by the thermal decomposition of the precursor LnCo(C2O4)3·nH2O (La, Ce, n=9; Pr, Nd, n=8). CeCo(C2O4)3·8H2O, unlike the other oxalato compounds thermally decompose to a mixture of CeO2 and Co3O4. Although LnCoO3are formed from the precursors at a temperature lower than 800°C, thermal analysis of a mixture of La2(C2O4)3·10H2O and CoC2O4·2H2O at 900·C shows the residue containing mainly La2O3 and Co3O4 with a small amount of LaCoO3. 相似文献
103.
Activated aryl halides undergo smooth nucleophilic substitution reactions with secondary amines in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6) or 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) ionic liquids (ILs) at room temperature to afford the corresponding arylamines in excellent yields under mild and neutral conditions. 相似文献
104.
Assessing model fit by cross-validation 总被引:8,自引:0,他引:8
Hawkins DM Basak SC Mills D 《Journal of chemical information and computer sciences》2003,43(2):579-586
When QSAR models are fitted, it is important to validate any fitted model-to check that it is plausible that its predictions will carry over to fresh data not used in the model fitting exercise. There are two standard ways of doing this-using a separate hold-out test sample and the computationally much more burdensome leave-one-out cross-validation in which the entire pool of available compounds is used both to fit the model and to assess its validity. We show by theoretical argument and empiric study of a large QSAR data set that when the available sample size is small-in the dozens or scores rather than the hundreds, holding a portion of it back for testing is wasteful, and that it is much better to use cross-validation, but ensure that this is done properly. 相似文献
105.
We take a unified approach to central limit theorems for a class of irreducible multicolor urn models with constant replacement
matrix. Depending on the eigenvalue, we consider appropriate linear combinations of the number of balls of different colors.
Then under appropriate norming the multivariate distribution of the weak limits of these linear combinations is obtained and
independence and dependence issues are investigated. Our approach consists of looking at the problem from the viewpoint of
recursive equations. 相似文献
106.
Proton multiple quantum (MQ) nuclear magnetic resonance (NMR) is applied for the first time to probe chain dynamics in the polycarbonate of 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene (chloral) over a temperature range of 195–280 K. Since chloral polycarbonate contains only one essentially indistinguishable proton, any dynamical information is associated with the phenylene group motion, the dominant sub-Tg motion. First, the fractional multiple quantum (fMQ) signal which can be refocused at a fixed MQ preparation time decreases with increase in temperature from 195 to 200 K, associated with the onset of the phenylene π flips. Since π flips do not modulate dipolar couplings between phenylene protons at positions 2 and 3 on the same ring, this decrease in fMQ can be attributed to modulation of the remaining couplings on the time scale of the MQ experiment. Next, over the range 200–240 K, the fMQ maintains a reduced, nonfluctuating value. This suggests a broad distribution of correlation times exists for the π flips, since at any one temperature in this intermediate range, only a small portion of the phenylene groups undergo motion at the precise kHz frequency required to refocus the MQ signal. The increase in the signal strength as the temperature rises above 240 K is due to the majority of π flips occurring in the fast motional limit. These observations are in agreement with previous single quantum proton NMR studies on phenylene group rotation in choral polycarbonate. Furthermore these fMQ measurements show several similarities to those for PTFE (poly(tetrafluoroethylene)), measured by 19F MQ-NMR. One notable difference is that in as-polymerized PTFE at intermediate temperatures (200–220 K) rapid oscillations of fMQ were noted as the temperature was changed by a few degrees. Such behavior is not observed in the chloral polycarbonate. © 1994 John Wiley & Sons, Inc. 相似文献
107.
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109.
Ajoy K. Ghatak 《Zeitschrift für Physik A Hadrons and Nuclei》1969,226(5):454-462
The response of a “Lorentz gas” to a pulsed a.c. electric field and its relaxation after the cessation of this field has been studied by solving the Boltzmann's transfer equation. Explicit expressions for the electron distribution function and the current density are obtained under the assumption that the collision frequency is independent of the electron velocity. 相似文献
110.
This paper is a first comprehensive study on the correlated ion transport mechanisms contributing to the overall conductivity behavior in a new class of poly(ethylene oxide)-polyurethane/polyacrylonitrile (PEO-PU/PAN) semi-interpenetrating polymer networks (semi-IPNs)-salt complex polymer electrolytes. A simultaneous investigation of the electrical response on PEO-PU/PAN/LiClO(4) and PEO-PU/PAN/LiCF(3)SO(3) semi-IPNs with varying EO/Li mole ratios (100, 60, 30, 20, 15, 10) has been carried out by impedance spectroscopy. Analysis of the complex plane and spectroscopic plots indicated the presence of two microscopic phases corresponding to the PEO-PU and PAN domains, which leads to space charge polarization in these systems. A suitably modified approach based on the universal power law (UPL) considering the independent contribution from the individual microphases of semi-IPNs facilitates a complete interpretation of the spectroscopic profiles for the real component of conductivity (sigma'(omega)). The sigma'(omega) spectroscopic profiles were fitted with two power law equations, where the frequency region up to approximately 300 kHz is the conductivity profile associated with the PAN phase and beyond this is the superimposed contribution of the PEO-PU phase. Simulated fits using the UPL equation revealed two relaxation times (tau(PEO)(-)(PU), tau(PAN)) related to ionic hopping in the PEO-PU and PAN phases in addition to the conductivity relaxation time (tau(peak)) determined from the Debye peaks. The respective power law exponents (n(PEO)(-)(PU) approximately 0.5-0.8, n(PAN) approximately 1.0-1.6) indicate that though cationic hopping in the softer PEO-PU phase is favored, anionic hopping in the PAN phase contributes significantly to the charge transport processes in these semi-IPNs. Correlation of the experimental results with the simulated fits enable us to distinguish the effects of semi-IPN composition, temperature, morphology, ion-ion, and ion-polymer interactions, which influence the microscopic molecular events, involved in the charge transport in these complex semi-IPN polymer electrolytes. 相似文献