Hexaphosphapentaprismane: a new gateway to organophosphorus cage compound chemistry

Several independent synthetic routes are described leading to the formation of a novel unsaturated tetracyclic phosphorus carbon cage compound tBu4C4P6 (1), which undergoes a light-induced valence isomerization to produce the first hexaphosphapentaprismane cage tBu4C4P6 (2). A second unsaturated isomer tBu4C4P6 (9) of 1 and the bis-[W(CO)5] complex 13 of 1 are stable towards similar isomerization reactions. Another starting material for the synthesis of the hexaphosphapentaprismane cage tBu4C4P6 (2) is the trimeric mercury complex [(tBu4C4P6)Hg]3 (11), which undergoes elimination of mercury to afford the title compound 2. Single-crystal X-ray structural determinations have been carried out on compounds 1, 2, 9, 11, and 13.     

Über die Anwendung der Kohlepaste-Elektrode zur inversvoltammetrischen Bestimmung kleiner Quecksilbermengen

The applicability of the carbon paste electrode to the determination of trace quantities of mercury has been investigated in a base electrolyte of 0.1 N KSCN + 0.025 N HCl containing 25 ng/ml copper. The detection limit of the mercury determination has a value near 2.5 ng/ml (1.25×10^?8 M), if pre-electrolysis is carried out 10 min at ?1.0 V and the current voltage curve is registered with a scan rate of 16.7 mV/sec between ?0.5 and +0.5 V. The peak height is directly proportional to the concentration in the range from 1.25×10^?8 to 1.25×10^?6 M. With appropriate preconditioning the carbon paste electrode can be used for several measurements without renewing the surface.     

Assignment of Relative Configuration to Acyclic Compounds Based on (13)C NMR Shifts. A Density Functional and Molecular Mechanics Study

1,3-Dimethylated hydrocarbon segments occur frequently as structural elements in polyketide natural products. The (13)C NMR chemical shifts of a series of model compounds containing such segments can be well reproduced by a combination of molecular mechanics and SOS-DFPT/IGLO calculations. (13)C NMR chemical shifts are calculated on MM3 geometries and are Boltzmann weighted according to the MM3 energies. On the basis of the resulting thermally averaged chemical shifts, all diastereomers of the model compounds can be unequivocally distinguished. Significant differences in chemical shifts occur at methyl groups and methylene groups that are adjacent to a single stereogenic center. The method is applied to predict the relative configuration of two stereocenters in the side chains of two natural products, sambutoxin and the bradykinin inhibitor L-755,897.     

Mass-selected resonance-enhanced multiphoton ionisation spectra of laser-desorbed molecules for environmental analysis: 16 representative polycyclic aromatic compounds

Mass-selected gas-phase UV spectra of laser-desorbed molecules at room temperature have been measured via resonance-enhanced multiphoton ionisation (REMPI) and time-of-flight mass selection. The wavelength range of 260 to 320 nm is optimal for detection of polycyclic aromatic hydrocarbons (PAHs) using REMPI-mass spectrometry. A new laser desorption/laser ionisation source has been used which features a compact size and thermal equilibrium of the desorbed molecules. 16 PAHs have been investigated which have been selected by the US Environmental Protection Agency (EPA). These 16 EPA-PAHs are commonly used world-wide to characterise the PAH-load of environmental samples.     

On Bachet's diophantine equationx 3=y 2=k

Using Eisenstein's law of cubic reciprocity we investigate cases in whichx ³=y ²+k is unsolvable in the ring of rational integers In particular we show, that for all primesp ± 1 (mod 9),p3, the equationx ³=y ²+3p(p±9) has no solutions in .     

Femptovoltmeter (10−15 Volt) based on a double Josphson junction resistive interferometer

A voltmeter of sensitivity is based on direct conversion of the voltage to be measured into a frequency according to the Josephson voltage-frequency relation. Selfpumped parametric amplification in a d.c. biased loop containing two Josephson junctions and a measuring resistanceR _s=4.7×10^–9 amplifies the femptovolt level voltages acrossR _s to microvolt levels that can be observed with a room temperature amplifier. The sensitivity of the femptovoltmeter is determined by Johnson-Nyquist noise voltages at 4.2 K in the 4.7×10^–9 resistance.     

A theoretical study of the electromagnetic properties of ferrocene and the ferrocenium ion

An iterative extended Hückel molecular orbital calculation was used to obtain wave functions for the ground state of ferrocene and several low lying states of the ferrocenium ion. Photoemission spectra in terms of relative ionization potentials and the electric field gradient at the iron nucleus were calculated for ferrocene andg values and the electric field gradient were calculated for the ferrocenium ion. These values are in good agreement with experiment. The nature of the molecular orbitals was compared with results of previous semi-empirical andab initio calculations.     

Structures of Addition Products of Acetylenedicarboxylic Acid Esters with Various Dinucleophiles. An application of C,H-spin-coupling constants

Heterocyclic compounds obtained by addition of acetylenedicarboxylic acid esters to thioureas, cyclic amidines and o-difunctionalized aromatic systems have been studied by ¹³C-NMR. In particular, C, H-spin-coupling constants over two and three bonds were used to differentiate between the various constitutional isomers and to establish the configuration of trisubstituted exocyclic C, C-double bonds. The configurational significance and diagnostic value of vicinal cis and trans C,H-spin coupling is again demonstrated in the present series.     

Influence of moderate Joule heating on electroosmotic flow velocity, retention, and efficiency in capillary electrochromatography

The influence of Joule heating on electroosmotic flow velocity, the retention factor of neutral analytes, and separation efficiency in capillary electrochromatography was investigated theoretically and experimentally. A plot of electrical current against the applied electrical field strength was used to evaluate the Joule heating effect. When the mobile phase concentration of Tris buffer exceeded 5.0 mM in the studied capillary electrochromatography systems using particulate and monolithic columns (with an accompanying power level of heat dissipation higher than 0.35 W/m), the Joule heating effect became clearly noticeable. Theoretical models for describing the variation of electroosmotic flow velocity with increasing applied field strength and the change of retention factors for neutral analytes with electrical field strength at higher Tris buffer concentrations were analyzed to explain consequences of Joule heating in capillary electrochromatography. Qualitative agreement between experimental data and implications of the theoretical model analysis was observed. The decrease of separation efficiency in capillary electrochromatography with macroporous octadecylsilica particles at high buffer concentration can be also attributed to Joule heating mainly via the increased axial diffusion of the analyte molecules and dispersion of solute bands by a nonuniform electroosmotic flow profile over the column cross-section. However, within a moderate temperature range, the contribution of the macroscopic velocity profile in the column arising from radial temperature gradients is insignificant.