The effect of fluorine substituents on the structure of cyclopropane. The structure of 4,4,8,8-tetraf1uorotricyclo-(5.1.0.0.3,5) octane

The first x-ray structure determination of a fluorine-substituted cyclopropane is reported. The title compound was synthesized via $\text{bis}$ difluorocarbene addition to 1,4-cyc1ohexadiene and its structure determined via x-ray crystallographic techniques. Its cyclopropane bondlengths (1.552 and 1.452) are very similar to those reported earlier in a microwave study of the parent 1,1-difluorocyclopropane, while its cyclohexane ring is virtually planar.     

Photochemical behaviour of 5-perfluoroalkenyl uracils

Phototransformations of derivatives of 5-fluoroalkenyl uracils depend strongly on the fluorinated substituents. 1,3-Dimethyl-5-trifluorovinyluracil when irradiated in water with UV light (λ>300 nm) gives 1,3-dimethyl-(5,6-dihydrourac-6-yl)-difluoroacetic acid as the only product, while the analogous 1,3-dimethyl-5-(E-pentafluoropropenyl)uracil isomerizes to its Z isomer. It is suggested that the first transformation is thermodynamically controlled while the second one is kinetically controlled, the difference being due to torquoselectivity.     

Narrow molecular weight distribution poly(styrenesulfonic acid). Part I. Preparation,solution properties,and phase separation

A method is described for the conversion of polystyrene to poly(styrenesulfonic acid) without change in the molecular weight distribution; the reaction is performed at room temperature in 100% H₂SO₄ and uses Ag⁺ catalyst. The resulting polyelectrolyte has solubility and other characteristics significantly different from those of previously investigated poly(vinylsulfonic acid). This permits a study of the influence of the aromatic group on the local and long range interactions in solution. The barium salt of poly(styrenesulfonic acid) is unusual in showing both upper and lower consolute temperatures in solution.     

Experimental and theoretical NMR study of selected oxocarboxylic acid oximes

1H and 13C NMR spectra of the oxocarboxylic acid oximes 2-hydroxyiminopropanoic acid (1), 2-(4-methylthiazol-2-yl)-2-(hydroxyimino)acetic acid (2) and 2-cyano-2-(hydroxyimino)acetic acid (3) were measured in DMSO-d6, D2O and acetone-d6 solutions. The data indicate the presence of hydrogen bonding in 1 and 2 and a strong electron-withdrawing effect due to the cyano group in 3. The effect of intra- and intermolecular hydrogen bonding on the hydrogen and carbon chemical shifts in these molecules was studied theoretically. Total energy calculations of the stability of various hydrogen-bonded species, in addition to equilibrium parameters and chemical shifts, were calculated using ab initio methods (RHF, MP2) and density functional theory (B3LYP), implemented in the Gaussian 98 software package. The gauge-including atomic orbital (GIAO) method was used to predict magnetic shielding constants. Chemical shift calculations for the most stable species agree fairly well with the observed data, especially for the hydroxyl protons. Substituents adjacent to the alpha-carbon show some influence of the oximic and carboxyl groups on the 13C chemical shifts, as expected for groups with different polar and anisotropic character.     

Studies on pyrazinyl-pyrazylidene tautomerism of pyrazine-acetonitrile derivatives

The equilibria between the pyrazinyl and 2(1H)-pyrazylidene forms of pyrazineacetonitrile derivatives were studied by IR,¹H-NMR and UV spectroscopy. The excess charges and dipole moments for the two tautomers were calculated by the CNDO/2-MO method.

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Untersuchungen über die Pyrazinyl-Pyraziliden-Tautomerie von Pyrazinacetonitril-Derivaten

Zusammenfassung Die Gleichgewichte zwischen Pyrazinyl- und 2(1H)-Pyrazyliden-Formen von Pyrazinacetonitril-Derivaten wurden mittels IR-, PMR- und UV-Spektroskopie untersucht. Die Überschußladungen und Dipolmomente der zwei Tautomeren wurden nach dem CNDO/2-MO-Verfahren berechnet.

     

Determination of major and trace elements in biological materials by microwave induced plasma optical emission spectrometry (MIP-OES) following tetramethylammonium hydroxide (TMAH) solubilization

A simple, alternative method to acid digestion is presented for the preparation of biological materials for major and trace elements by microwave induced plasma optical emission spectrometry (MIP-OES). Standard reference materials were solubilized using a methanolic solution of tetramethylammonium hydroxide (TMAH) by the application of ultrasonic agitation. Following dilution with water suspensions were pumped to a V-groove Babington-type nebulizer then into argon plasma. Limits of detection and precision are reported for major (Na, K, Ca, Mg) and trace (Cu, Fe, Sr, Zn) elements. Standard addition procedures were used to minimize possible matrix interferences. The method offers relatively good precision (R.S.D. ranged from 6 to 12%). Measured concentrations are in satisfactory agreement with certified values for the biological reference materials: TORT-1 (Lobster hepatopancreas), IAEA-153 (Milk powder) and NIST 1577 (Bovine liver). The simple procedure could be useful as a routine analysis of biological materials.     

TEM Orientation Mapping Applied to the Study of Shear Band Formation

The formation of brass-type shear bands (SBs) in twinned microstructures of oriented copper single crystals, has been investigated after channel-die deformation at 77K. Setting up a system for making high-resolution orientation maps using transmission electron microscopy (TEM) has opened new advantageous circumstances for the analysis of orientation changes within SBs. This method with spatial resolution higher than 10nm allows an examination of microstructure images composed of nanoscale sub-cells forming SBs structure. The early stages of SBs formation are the result of equally effective operation of two slip systems operating on the {111} slip planes which could be represented by a resultant super-system of type. This process leads to the lattice rotation about axis and to the rise of Goss orientation within the band. A minor group of components is observed near , arising from the near primary matrix orientation. For well-developed SBs, a second rotation about direction is observed. This tendency, resulting from the type slip systems operation, is usually accompanied by activation of new slip systems. Hence the process of SBs formation is regarded as a strictly crystallographic one.     

Determination of dependencies between the specific retention volumes and the parameters characterising adsorbent properties

Propane adsorption isotherms have been chromatographically determined on active carbon for different amounts of the injected adsorbate on column. The dependencies between the specific retention volume corrected to the standard temperature (273.15 K), Vg(273), and the molar differential work of adsorption, A, have been calculated on the basis of the propane isotherms and using the retention times of the peak maxima. The obtained equations: ln Vg(273) = f1(A) and (dW/dA)T.F(C) = f2(ln Vg(273)) have been used to explain the dependency between the chromatographic peak profile and the distribution function of pore volumes filled with propane with respect to the molar differential work of adsorption at different column temperatures (303-318 K).     

'Self-organization' processes in proton and deuteron mixtures in open-chain hydrogen bond systems: polarization IR spectra of 4-mercaptopyridine crystals.

This paper is devoted to IR spectroscopic studies in polarized light of 4-mercaptopyridine (4-MPD) hydrogen-bonded crystals. These studies were preceded by determination of the 4-mercaptopyridine X-ray structure. Polarization spectra of 4-mercaptopyridine crystals were measured in the frequency ranges of V(N-H) and V(N-D) bands at room temperature, and also at the temperature of liquid nitrogen, for the two different crystalline faces: 'bc' and 'ac'. When investigating 'residual' v(N-H) band shapes for crystals that were diluted by deuterium, strong dichroic effects in the spectra were still observed, providing evidence for the existence in the hydrogen bond chains of domains containing exclusively protons or deuterons. This phenomenon proves the existence of a new kind of H/D isotopic effects in open chain hydrogen bond systems, namely 'self-organization' effects. Such effects, however, were not observed for other open chain hydrogen bond systems, e.g. alcohol crystals. Solid-state spectra of 4-mercaptopyridine were quantitatively interpreted, along with the strong polarization and the isotopic effects, when based on the 'strong-coupling' theory for linearly arranged hydrogen bond dimers.