New chiral phosphine-phosphite ligands in the enantioselective palladium-catalyzed allylic alkylation

A series of chiral phosphine-phosphite ligands 1-6 have been synthesized and used in the enantioselective palladium-catalyzed reaction of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands 1a, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butylphenylphosphinoborane. The use of dynamically resolved Li phosphide (-)-sparteine provided the optically pure ligands. Crystals of the allylpalladium (6a) complex were obtained, suitable for X-ray crystal structure determination. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indicating that the attack of the nucleophile takes place at the carbon trans to the phosphine moiety. This was confirmed by the fact that the phosphine moiety did not affect the enantioselectivity directly. Under mild reaction conditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand 1e. Systematic variation of the ligand bridge and the phosphite moiety showed that the configuration of the product is controlled by the atropisomerism of the biphenyl substituent at the phosphite moiety. The conformation of the biphenyl group, in turn, is controlled by the substituent at the chiral carbon in the bridge. Ligands with large bite angles yielded higher enantioselectivities.     

Intercalation of Benzamide into Kaolinite

Well-crystallized kaolinite (K) was initially reacted at 60 degrees C with a water/dimethylsulfoxide (DMSO) mixture and the resulting intercalation derivative (K-DMSO) was characterized by powder X-ray diffractometry (PXRD), thermal analysis (simultaneous TG and DSC), and Fourier-transformed infrared spectroscopy (FTIR). Benzamide crystals were then melted with the K-DMSO derivative at 140 degrees C for 4 days, when a gradual displacement of DMSO by benzamide was observed within the interlayer spacing of the modified kaolinite. The resulting material, after extensive washing with acetone, was characterized and compared to the results obtained previously for the K-DMSO composite. Benzamide intercalation proceeded by gradual displacement of DMSO molecules until completion. The structural stabilization of the K-BZ derivative was explained through the establishment of hydrogen bonds between the carbonyl oxygen atoms of the intercalated benzamide and aluminol groups present at the surface of the kaolinite layer. The interlamellar spacing of K-BZ was shown to be possibly occupied by benzamide molecules that were located at a 68 degrees orientation in relation to the layer surface. Unlike most intercalation molecules such as DMSO, variations in the interplanar spacing of kaolinite were consistent with the nonkeying of any other part of the molecule between the aluminosilicate interlayers. Copyright 2000 Academic Press.     

Direct determination of intermolecular potentials from gaseous transport coefficients alone

Intermolecular pair-potential energy functions are given for all the unlike interactions of the monatomic gases. The potentials are obtained by direct inversion of experimental measurements of low density binary mixture viscosity and diffusion coefficients. In those cases where these data extend to sufficiently low temperatures to enable the potential well depth ε/k to be determined, the values obtained for this parameter are Kr-Xe 220 ± 5 K, Ar-Xe 170 ± 5 K, Ar-Kr 165 ± 10 K, Ne-Xe 70 ± 5 K, Ne-Kr 60 ⁺¹⁰ _-5 K, Ne-Ar 60 ± 5 K. For the systems He-Xe, He-Kr, He-Ar and He-Ne only the repulsive branch of the potential energy function is obtained. For all systems the potentials are shown to reproduce well other macroscopic data not used in their construction. The agreement with thermal diffusion data is particularly encouraging. The independent determination of this wide range of potentials provides a direct test of both the hypothesis that they are conformal and the validity of parameter mixing rules. Significant deviations from conformality are found at the extremes of small and large intermolecular separations. No simple combining rules reproduce the potential parameters quantitatively, but the direction in which empirical modifications should be made is clearly suggested.     

Dual-echelon single-shot femtosecond spectroscopy

A dual-echelon technique for performing real-time pumpprobe spectroscopy with sub-100-fs temporal resolution is demonstrated. In a single laser shot, measurements at 400 different temporal delays are recorded to cover a total temporal range of 10 ps in 25-fs steps. Data from a glass sample and an explosive single crystal, each irradiated with intensity above the permanent damage threshold, are presented.     

A novel fluid densimeter for high pressures

Abstract

A new type of densimeter for the measurement of fluid densities at pressures up to 500 MPa is described. Thie densimeter makes use of the effect of buoyancy on a mass suspended from a wire to determine the resonant frequency of the wire for transverse vibrations. The difference in resonant frequency between oscillations in vacuo and in the fluid thereby permits measurement of the fluid density. Preliminary results demonstrate that the precision and reproducibility of the instrument is one of ± 0.1% at pressures up to 100 MPa.     

Post‐Synthetic Ligand Exchange in Zirconium‐Based Metal–Organic Frameworks: Beware of The Defects!

Post‐synthetic ligand exchange in the prototypical zirconium‐based metal–organic framework (MOF) UiO‐66 was investigated by in situ solution ¹H NMR spectroscopy. Samples of UiO‐66 having different degrees of defectivity were exchanged using solutions of several terephthalic acid analogues in a range of conditions. Linker exchange only occurred in defect‐free UiO‐66, whereas monocarboxylates grafted at defect sites were found to be preferentially exchanged with respect to terephthalic acid over the whole range of conditions investigated. A 1:1 exchange ratio between the terephthalic acid analogue and modulator was observed, providing evidence that the defects had missing‐cluster nature. Ex situ characterisation of the MOF powders after exchange corroborated these findings and showed that the physical‐chemical properties of the MOF depend on whether the functionalisation occurs at defective sites or on the framework.     

Thermal conductivity of two binary mixtures of gases of equal molecular weight

The paper reports new measurements of the thermal conductivity of two binary mixtures at a nominal temperature of 27.5°C but over a pressure range. The two systems, N₂-CO (three compositions up to 12MPa) and N₂O-CO₂ (four mixtures up to 4.2 MPa) have been chosen because they consist of molecules of similar structure and equal molecular weight (28.01 and 44.01, respectively). Moreover, the zero-density viscosity is identical within each mixture.The results have been extrapolated and fitted to equations in the usual way. The zero-density thermal conductivity in each system varies systematically with composition and differs by about 4% for the pure components in each case. This is a measure of the effect of the internal degrees of freedom on thermal conductivity.In the absence of a reliable theory, it is shown that the equations of kinetic theory can be used to represent the composition-dependence of zero-density thermal conductivity with a judicious choice of two quantities which cannot be calculated independently.In both systems, both the excess thermal conductivity λ(T,?) - λ(T, 0) and the ratio λ(T, ?)?λ(T, 0) are sole functions of density with a remarkable degree of precision.The variation of the thermal conductivity with density could be predicted quit accurately with the aid of the theory due to Mason.     

A further contribution to the theory of the transient hot-wire technique for thermal conductivity measurements

We study the density of states of a one-dimensional tightbinding electron model with random hopping elements. The Hamiltonian is $\text{H = -∑}{}_{\mathrm{i}}\text{J}{}_{\mathrm{<mtext>i+</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{(a}{}^{\mathrm{+}}{}_{\mathrm{i}}\text{a}{}_{\mathrm{i+1}}\text{+a}{}^{\mathrm{+}}{}_{\mathrm{i+1}}\text{a}{}_{\mathrm{i}}\text{)}$, where the $\text{J}{}_{\mathrm{<mtext>i+</mtext><mtext>1</mtext><mtext>2</mtext>}}$'s are independent identically distributed random variables. It is proved that the single particle density of states D(E) diverges near E = 0 as $\text{1}\text{|(E}\text{log}{}^3\text{|E|)|}$.