Single crystals of Sm4OCl6 and KSm2Cl5 have been obtained by metallothermic reductions of SmCl3 with lithium (in the presence of Sm2O3 or SmOCl) and potassium, respectively, at elevated temperatures in sealed tantalum containers. Sm4OCl6 (hexagonal, P63mc, Z = 2, a = 946.59(4), c = 717.88(4) pm) and KSm2Cl5 (monoclinic, P21/c, Z = 4, a = 888.06(6), b = 784.81(5), c = 1262.77(8) pm, ß = 90.085(6)°) are true divalent samarium compounds, Sm4OCl6 with remarkably short Sm2+–O2? distances (236.0, 237.6 (3x) pm) within the [Sm4O] tetrahedron. 相似文献
The long standing problem of solvent dependent absorption spectra of certain bile pigments such as bilirubindimethylester has been solved by measurement of molecular masses by means of vapour pressure osmometry. Using this method and the investigation of the concentration and temperature dependence of the absorption spectra on several model compounds an association equilibrium for pigments of the rubinoid structural type could be confirmed. Moreover the optical properties of the coexisting species and the corresponding equilibrium constants could be deduced by spectroscopic techniques. Verdinoid pigments lacking free carboxylic functions on the other hand tend to be monomeric in solution. A heteroassociated species involving equimolar amounts of a verdinoid and a rubinoid pigment could be verified existing on adsorbents like silicagel.
37. Mitt.:Falk, H., Thirring, K., Tetrahedron, im Druck. 相似文献
The adsorption of N-acetyl-L-cysteine from ethanol solution on gold has been studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and a quartz crystal microbalance. After an initial fast adsorption, in situ ATR-IR revealed two considerably slower processes, besides further adsorption. The appearance of carboxylate bands and the partial disappearance of the carboxylic acid bands demonstrated that part of the molecules on the surface underwent deprotonation. In addition, the C=O stretching vibration of the carboxylic acid group shifted to lower and the amide II band to higher wavenumbers, indicating hydrogen-bonding interactions within the adsorbate layer. Based on the initial ATR-IR spectrum, which did not reveal deprotonation, the orientation of the molecule within the adsorbate layer was determined. For this, density functional theory was used to calculate the transition dipole moment vectors of the vibrational modes of N-acetyl-l-cysteine. The projections of the latter onto the z-axis of the fixed surface coordinate system were used to determine relative band intensities for different orientations of the molecule. The analysis revealed that the amide group is tilted with respect to and points away from the surface, whereas the carboxylic acid is in proximity to the surface, which is also supported by a shift of the C-O-H bending mode. This position of the acid group favors its deprotonation assisted by the gold surface and easily enables intermolecular interactions. Periodic acid stimuli revealed reversible protonation/deprotonation of part of the adsorbed molecules. However, only non-hydrogen-bonded carboxylic acid groups showed a response toward the acid stimuli. 相似文献
Simple and readily accessible aryl bromides are useful building blocks for thiol end-capped molecular wires. Thus, 4-bromophenyl tert-butyl sulfide and 1-bromo-4-(methoxymethyl)benzene serve as precursors for a variety of oligo(phenylenevinylene) and oligo(phenyleneethynylene) wires via efficient synthetic transformations as presented in this paper. 相似文献
Zusammenfassung 2-Hydroxy-6,7-dihydro-4H-benzo[a]chinolizin-4-one (3a—p) werden durch Kondensation von 1-Alkyl-3,4-dihydro-isochinolinen (1a—g) mit Malonsäure-bis-2,4,6-trichlorphenylestern (2a—e) erhalten. Die Ausbeuten sowie die erforderlichen Reaktionszeiten und Temperaturen sind stark von der Art der Substituenten abhängig.
Syntheses of heterocycles, CXXXV: Quinolizines and indolizines, VI. A synthesis of 2-hydroxy-4H-benzo[a]quinolizin-4-ones
The condensation of 1-alkyl-3.4-dihydro-isoquinolines (1a tog) with 2.4.6-trichlorophenyl malonates (2a—e) yields 2-hydroxy-6,7-dihydro-4H-benzo[a]quinolizin-4-ones (3a—p). Yields, required reaction-periods and temperatures are depending on the nature of the substituents present in the malonyl and isoquinoline residue.
The Ramanujan Journal - Let V(T) denote the number of sign changes in $$\psi (x) - x$$ for $$x\in [1, T]$$ . We show that $$\liminf _{T\rightarrow \infty } V(T)/\log T\ge \gamma _{1}/\pi +... 相似文献
The Ramanujan Journal - We study fixed points in integer partitions viewed, respectively, as weakly increasing or weakly decreasing structures. A fixed point is a point with value i in position i.... 相似文献
This work introduces and compares approaches for estimating rare-event probabilities related to the number of edges in the random geometric graph on a Poisson point process. In the one-dimensional setting, we derive closed-form expressions for a variety of conditional probabilities related to the number of edges in the random geometric graph and develop conditional Monte Carlo algorithms for estimating rare-event probabilities on this basis. We prove rigorously a reduction in variance when compared to the crude Monte Carlo estimators and illustrate the magnitude of the improvements in a simulation study. In higher dimensions, we use conditional Monte Carlo to remove the fluctuations in the estimator coming from the randomness in the Poisson number of nodes. Finally, building on conceptual insights from large-deviations theory, we illustrate that importance sampling using a Gibbsian point process can further substantially reduce the estimation variance.