Eine Methode zum Nachweis und zur qualitativen Trennung von Ferrocyaniden, Ferricyaniden und Rhodaniden

Ohne Zusammenfassung     

Initial state and transition state solvation effects in the cobaltitungstate oxidation of iodide in binary aqueous solvent mixtures

Summary Rate constants are reported for 12-tungstocobaltate(III) [CoW₁₂O₄₀]^5– oxidation of iodide in water and in binary aqueous solvent mixtures containing up to 40% methanol, 40% acetonitrile, or 60% dimethyl sulphoxide. From these kinetic results, solubility measurements on potassium 12-tungstocobaltate(III), and published data on Gibbs free energies of transfer of appropriate ions, it has been deduced that the dominant factor in determining the marked decrease in rate observed on going from water into the binary aqueous solvent mixtures is destabilisation of the transition state for the electron-transfer reaction.     

1-Ethyl-1,4-dihydro-4-oxo-7-(pyridinyl)-3-quinolinecarboxylic acids. I. Synthesis of 3- and 4-(3-aminophenyl)pyridine intermediates

A series of substituted 3- and 4-(3-aminophenyl)pyridines has been prepared as intermediates for the synthesis of some 1-alkyl-1,4-dihydro-4-oxo-7-pyridinyl-3-quinolinecarboxylic acids. The Hantzsch, Hauser and other pyridine syntheses were used. 4-(3-Aminophenyl)pyridine was prepared via 3-(4-pyridinyl)-2-cyclohexen-1-one using the Semmler-Wolff reaction.     

Radium-224 in natural waters measured by γ-ray spectrometry

A method based on Ge(Li) γ-ray spectrométry is applied to the determination of ²²⁴Ra (t_{$\text{1}\text{2}$}= 3.64 days) in natural waters. The ²²⁴Ra is first removed from several hundred liters of water by preconcentration onto manganese dioxide-impregnated acrylic fibers. The fibers are leached, radium is coprecipitated with barium sulfate, and the γ-ray activity is counted so that activity ratios among ²²⁴Ra, ²²⁵Ra and ²²⁶Ra can be calculated. Concentrations are determined by using the ²²⁶Ra concentration determined on a small separate sample. Results from samples collected from ground water, estuarine, and continental shelf environments are presented.     

An ab initio study of the geometry,energy, and selected force constants for the three planar conformers of carbonic acid,and the bicarbonate ion; and of the energy for the reaction H2O + CO2 → H2CO3

Ab initio calculations using the unscaled 4-31G basis set have been carried out on the cc, tc, and tt conformers of carbonic acid and the bicarbonate ion, with full geometry optimization assuming the structures to be planar. The complete harmonic force field is reported for the (most stable) tt conformer and for the bicarbonate ion, also selected quadratic force constants for the cc and tc conformers. The changes in certain bond lengths and stretching force constants in the cc → tc, tc → tt, and cc → tt conformer conversion reactions are indicative of intramolecular hydrogen bonding, C?O…H? O and H? O…H? O, which is examined in greater detail by partitioning the overall conformer conversion energy into distortion and bonding energy components. The fundamental vibration frequencies for the tt conformer and the bicarbonate ion are calculated from the force constant matrices, and hence, using a scaling factor based on a comparison of calculated and experimental values for the bicarbonate ion and trans-formic acid, a value is predicted for the zero-point energy of the tt conformer. A new estimate of ΔH? for the hydration reaction, H₂O + CO₂ → H₂CO₃, at 298 K in the gas phase; is made from thermochemical data, +20.2 ± 3.4 kJ mol^?1, which, together with estimates of (H₂₉₈? – H₀?) and the zero-point energy for H₂CO₃, gives +8.1 ± 7.0 kJ mol^?1 for ΔE_T(expt). ΔE_T calculated from the 4-31G basis set data is -29.1 kJ mol^?1. Comparison of the experimental value, the Hartree–Fock limit value, and values calculated with a variety of basis sets for the bond separation reaction, CO₂ + CH₄ → 2H₂CO, suggests that the differences, ΔE_T(expt) minus ΔE_T(SCF ), are due mainly to basis set limitations and not substantial correlation energy contributions.     

Approaches to charge calculations in molecular mechanics

Alternative methods of estimating atomic charges in haloalkanes are presented, derived from quantum mechanical and classical treatments. A scheme based on a breakdown of the transmission of charge by polar atoms into one-bond, two-bond, and three-bond additive contributions is given, in which the one-bond effect is proportional to the difference in the electronegativities of the bonded atoms, and the two- and three-bond effects functions of the atomic electronegativity and polarizability. Suitable developments of the basic scheme, including an iterative self-consistent process, give calculated dipole moments for a variety of haloalkanes in good agreement with the observed values. The atomic charges obtained by this scheme are compared with other estimates of these charges. They are similar to those derived from a simple LCAO –MO scheme but differ from those obtained by population analysis of more refined quantum mechanical calculations.     

Magneto-optical Cotton—Mouton effect of molecular oxygen. A comparison

Measurements of the magneto-optical Cotton—Mouton effect at 632.8 nm of O₂(³Σ) over a range of temperature (≈299–464 K) and pressure (up to 613 kPa) are reported and analysed. The study enables a useful comparison to be made of data obtained under different conditions by the two groups that have undertaken systematic measurements of this effect. Agreement between the experimental results and also the derived molecular properties is found to be satisfactory.     

Cubanes: Starting Materials for the Chemistry of the 1990s and the New Century

The study of non-natural products has led to a broad understanding of bonding and reactivity in organic chemistry. Many times, compounds thought impossible have been realized in the course of such studies. Cubane, a landmark in the world of “impossible” compounds, has been found to have a rich chemistry, full of the unexpected. The recent renaissance of cubane chemistry, triggered by potential applications of the system to the production of high-energy fuels and the like, has led to many discoveries including the first methods for systematic substitution on strained, saturated systems and a new process for the metalation of arenes, ortho magnesiation. Reactive intermediates with exceptional bonding parameters have been uncovered and characterized including 1(9)-homocubene, the most twisted olefin; cubene, the most pyramidalized olefin; cubyl cation, once the “least likely” cation; cubylmethyl radical, a saturated radical that rearranges on the picosecond timescale; and many other extraordinary species. There is certainly good reason to believe that future work in the cubane arena will be at least as productive (probably more so), and that it will help develop a deeper understanding of chemistry.