Site-selective EXAFS in mixed-valence compounds using high-resolution fluorescence detection: a study of iron in Prussian Blue

A quantitative analysis is presented for the site-selective Fe K-edge absorption spectra of Prussian Blue: Fe(4)[Fe(CN)(6)](3) x xH(2)O (x = 14-16). The site-selective spectra were recorded using high-resolution fluorescence detection of the K beta emission from a polycrystalline sample. The K beta fluorescence lines arising from the high-spin and low-spin sites are shifted in energy. Since the emission features partially overlap, fluorescence-detected absorption spectra using different emission energies represent different linear combinations of the pure high-spin and low-spin EXAFS. A numerical method was used to extract the individual site EXAFS spectra from the experimental data. The analysis yields a range of solutions. A unique solution can be obtained if homovalent model compounds are used to simulate the K beta fluorescence emission from the two Fe sites in Prussian Blue. EXAFS analysis of the range of spectra obtained in the numerical method yields almost identical interatomic distances for the different spectra while the Debye-Waller factors vary considerably. The distances obtained in the EXAFS fit correspond to the crystallographic distances.     

Spectroscopic characterization of microscopic hydrogen-bonding disparities in supercritical water

The local hydrogen-bonding environment in supercritical water (380 degrees C, 300 bars, density 0.54 gcm3) was studied by x-ray Raman scattering at the oxygen K edge. The spectra are compared to those of the gas phase, liquid surface, bulk liquid, and bulk ice, as well as to calculated spectra. The experimental model systems are used to assign spectral features and to quantify specific local hydrogen-bonding situations in supercritical water. The first coordination shell of the molecules is characterized in more detail with the aid of the calculations. Our analysis suggests that approximately 65% of the molecules in supercritical water are hydrogen bonded in configurations that are distinctly different from those in liquid water and ice. In contrast to liquid water the bonded molecules in supercritical water have four intact hydrogen bonds and in contrast to ice large variations of bond angles and distances are observed. The remaining approximately 35% of the molecules exhibit two free O-H bonds and are thus either not involved in hydrogen bonding at all or have one or two hydrogen bonds on the oxygen side. We determine an average O-O distance of 3.1+/-0.1 A in supercritical water for the H bonded molecules at the conditions studied here. This and the corresponding hydrogen bond lengths are shown to agree with neutron- and x-ray-diffraction data at similar conditions. Our results on the local hydrogen-bonding environment with mainly two disparate hydrogen-bonding configurations are consistent with an extended structural model of supercritical water as a heterogeneous system with small patches of bonded molecules in various tetrahedral configurations and surrounding nonbonded gas-phase-like molecules.     

Catalytic hydroamination of fullerene C<Subscript>60</Subscript> with primary and secondary amines

Catalytic 1,2-hydroamination of fullerene C₆₀ with primary and secondary amines in the presence of Ti, Zr, and Hf complexes gave the corresponding alkyl-, aryl-, and hetarylaminodihydrofullerenes.     

Auswahl,Vergleich und Bewertung optimaler Arbeitsbedingungen für die quantitative Mehrkomponenten-Analyse

Precision criteria for multi-component analysis procedures are examined on a statistical basis for calibrating and analytical procedures. The standard deviation σ or its only by measuring approachable estimate s shows to be unsuitable when looked at more closely. By the aid of statistical methods which base on combinatorial rules, and by using redundant calibrating and analytical procedures, the expected value Δ and the second moment of the distribution of the differences of nominal values can be calculated. The expected value Δ _E of calibration increases monotonously with the number of over-determinations in calibration samples, while that one for analytical procedures (Δ _F ) decreases monotonously, so that there exists a mutual value Δ ⁺ for both distributions of differences of nominal values. This is a characteristic magnitude, which only depends on the procedure and the equipment, and it can be applied for the statistical evaluation of calibrating and analytical procedures. The characters $$G_E = \frac{1}{{\Delta ^ + }}{\text{ }}(\Delta ^ + - \Delta _{\rm E} ){\text{ and }}G_F = \frac{1}{{\Delta ^ + }}{\text{ (}}\Delta _F - \Delta ^ + )$$ present themselves as characters of quality for the precision of calibrating and analytical systems. Both characters always are non-negative and decrease monotonously with increasing number of calibration samples. The postulate G _E ≤0.2 defines the minimum number of calibration samples, which secures the required compensation of errors as well as an average difference of nominal values for analytical samples Δ _F , which is comparable with those for calibration samples. With a great number of calibration samples, however, the second moment is still significantly greater than that one for analytical samples.     

Enantioselective photochemical reactions of 2-pyridones in solution

[reaction: see text] Two key photochemical reactions of prochiral 2-pyridones were studied in the presence of a chiral host. The [4 + 4]-photocycloaddition with cyclopentadiene (CpH) proceeded smoothly and with high enantioselectivity (84-87% ee). The absolute configuration of the endo-diastereoisomer was established by X-ray crystallography. The electrocyclic [4pi]-ring closure to 3-oxo-2-azabicyclo[2.2.0]-5-hexenes occurred with lower enantioselectivity (20-23% ee at -20 degrees C). The velocity of the latter reaction slowed significantly with decreasing temperature.     

Nachweis und Bestimmung organischer S?uren

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