Stereoselective peracid epoxidation of allylic and δ-hydroxyallylic diphenylphosphine oxides

The title compounds were epoxidised by $\text{m}$-chloroperbenzoic acid with high stereoselectivity induced by the Ph₂PO or hydroxyl groups.     

Application of palladium-catalyzed allylic arylation to the synthesis of a (+/-)-7-deoxypancratistatin analogue

Palladium-catalyzed coupling of an aryl siloxane and an allylic carbonate proceeded in good yield to give an adduct that was converted to an analogue of (+/-)-7-deoxypancratistatin.     

The concentration and variability of selenium and mercury measured in vacuum-packed tuna fish

Methylmercury (meHg) is a known toxin commonly found in fish. Fish is also a rich source of the trace nutrient selenium which has been hypothesized to modify the toxicity of meHg. We analyzed 28 samples of commercially packaged albacore and light tuna fish for selenium and mercury using standard comparator instrumental neutron activation analysis. Significant differences in the concentration of mercury and selenium were associated with the type of fish, brand and batch. Fish consumers should vary the brand of tuna fish to avoid routine consumption of a brand high in mercury.     

Universal high energy anomaly in the angle-resolved photoemission spectra of high temperature superconductors: possible evidence of spinon and holon branches

A universal high energy anomaly in the single particle spectral function is reported in three different families of high temperature superconductors by using angle-resolved photoemission spectroscopy. As we follow the dispersing peak of the spectral function from the Fermi energy to the valence band complex, we find dispersion anomalies marked by two distinctive high energy scales, E1 approximately 0.38 eV and E2 approximately 0.8 eV. E1 marks the energy above which the dispersion splits into two branches. One is a continuation of the near parabolic dispersion, albeit with reduced spectral weight, and reaches the bottom of the band at the Gamma point at approximately 0.5 eV. The other is given by a peak in the momentum space, nearly independent of energy between E1 and E2. Above E2, a bandlike dispersion reemerges. We conjecture that these two energies mark the disintegration of the low-energy quasiparticles into a spinon and holon branch in the high Tc cuprates.     

Time-resolved vibrational spectroscopy of [FeFe]-hydrogenase model compounds

Model compounds have been found to structurally mimic the catalytic hydrogen-producing active site of Fe-Fe hydrogenases and are being explored as functional models. The time-dependent behavior of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) and Fe(2)(μ-S(2)C(2)H(4))(CO)(6) is reviewed and new ultrafast UV- and visible-excitation/IR-probe measurements of the carbonyl stretching region are presented. Ground-state and excited-state electronic and vibrational properties of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) were studied with density functional theory (DFT) calculations. For Fe(2)(μ-S(2)C(3)H(6))(CO)(6) excited with 266 nm, long-lived signals (τ = 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps) bands are observed in addition to CO-loss product. Short-lived transient absorption intensities are smaller for 355 nm and much larger for 532 nm excitation and are assigned to a short-lived photoproduct resulting from excited electronic state structural reorganization of the Fe-Fe bond. Because these molecules are tethered by bridging disulfur ligands, this extended di-iron bond relaxes during the excited state decay. Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized exited-state geometry of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) with a semibridging CO is reminiscent of the geometry of the Fe(2)S(2) subcluster of the active site observed in Fe-Fe hydrogenase X-ray crystal structures. We suggest these wavelength-dependent excitation dynamics could significantly alter potential mechanisms for light-driven catalysis.     

The conductance of electrolytes in acetone-2-propanol and acetone-1,1,1,3,3,3-hexafluoro-2-propanol mixtures at 25°C1

Precise conductance measurements are reported for tetrabutylammonium chloride, bromide, iodie, and perchlorate and lithium chloride in acetone-2-propanol (2-PrOH) and acetone-1,1,1,3,3,3-hexafluoro-2-propanol (HFP) mixtures at 25°C. Densities, viscosities and dielectric constants of the mixtures were determined. The dielectric constant vs mole % acetone curve for the acetone (ε=19.4)-2-propanol (ε=20.5) goes through a minimum at 40% acetone (ε=17.4), while that for acetone-HFP (ε=16.8) goes through a maximum at 50% acetone (ε=26.87). The variations ofK _A with ε in acetone-HFP are in accord with the predictions of electrostatic theory, while those for acetone-2-PrOH show more complex behavior. Ionic association in these mixtures is discussed in terms of an interplay between solvent structure and a multiple-step association process.