Recent Developments in Spectrophotometric Methods for Determination of Vanadium

Owing to the toxic and essential nature of vanadium in biological systems, there has been considerable interest in the determination of its content in different kinds of samples. Among several analytical techniques for the determination of vanadium, spectrophotometric methods are very popular due to their simplicity and low-cost instrumentation. Various attempts have been made to modify these methods in order to improve their sensitivity and selectivity. This review shows recent developments in the spectrophotometric determination of vanadium and highlights those which can be used for speciation analysis of this element.     

Mechanical aspects of fibre spinning process in molten polymers

Summary The process of stream broadening as observed in molten polymers spun into fibres has been investigated. It has been established that the maximal broadening existing in the first millimeters below the spinneret increases with a mean velocity within the channel, and mean velocity gradientD=d V/dr, decreases with melt temperature and the length of channel. The experiments have been carried out on molten polycapronamide and polystyrene. The explanation of the mechanism of the observed phenomenon has beeen based on the relaxation effects: elasticity of the melt and disorientation of macromolecules. The significance of this phenomenon for the theory of fibre formation has been discussed and the general scheme of fibre spinning given as based on the changes of parallel velocity gradientG=d V/dl along the spinning way.

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Zusammenfassung Der Proze? der Zwiebelbildung, wie er bei geschmolzenen Polymeren beim Spinnen zu Fasern beobachtet wird, wurde untersucht. Es lie? sich feststellen, da? die maximale Verbreiterung, die in den ersten Millimetern hinter der Spinndüse auftritt, mit der mittleren Geschwindigkeit im Spinndüsenkanal und mit dem mittleren GeschwindigkeitsgradientenD=d V/dr ansteigt und da? sie mit der Schmelztemperatur und der L?nge des Düsenkanals abnimmt. Die Versuche wurden an geschmolzenem Polycapronamid und Polystyrol durchgeführt. Die Erkl?rung des Mechanismus des beobachteten Ph?nomens wurde auf Relaxationseffekte begründet: Elastizit?t der Schmelze und Desorientierung der Makromoleküle. Die Bedeutung dieses Ph?nomens für die Theorie der Faserbildung wird diskutiert und ein allgemein gültiges Schema für das Spinnen von Fasern gegeben, das auf die ?nderungen des parallelen GeschwindigkeitsgradientenG=d V/dl l?ngs des Spinnweges gegründet ist.

     

Molecular electrostatic potential of the B-DNA helix. I. Region of the guanine–cytosine base pair

The electrostatic molecular potential minima around the guanine–cytosine base pair within a B-DNA minihelix are computed, taking into account the contributions of the sugar-phosphate backbone and of the adjacent base-pairs. The calculations are based on ab initio SCF wave functions of the different constituents of the nucleic acid. The results point to significant differences in the potential between the isolated nucleic acid bases or base-pairs and those within the DNA. Altogether the minima in the G–C regions are strongly enhanced in the minihelix. They benefit from the field created by the neighboring phosphates. From the purely electrostatic viewpoint an ambiguity remains as concerns the relative affinity of N₇ and N₃ of guanine for electrophiles. On the other hand, guanine should altogether be more susceptible than cytosine to such reagents, this ordering concerning also its NH₂ group compared to that of cytosine.     

Fourier transform NMR investigations of 13C labeled silyl and stannyl acetylenes

The ¹³C NMR spectra of ¹³C labeled mono- and di-substituted silyl- and stannyl-acetylenes have been studied. It was found that the values of ¹J(CC) coupling constants between acetylenic carbons decrease very sharply in the series Alk₃SiCCH, Alk₃SnCCH, Alk₃SiCCSiAlk₃ and Alk₃SnCCSnAlk₃. These results and the observed changes in the geminal hetero-atom β-acetylenic carbon couplings suggest a very strong p_πd_π interaction between the π-electrons of the triple bond and the vacant d orbitals of silicon and tin.     

Diastereoselective Diels-Alder Reaction of 2-Thienyl and 2-Furyl Substituted 3-Propenethioamides with Electron Deficient Dienophiles

Summary. The hetero-Diels-Alder reaction of N-aryl-3-(2-thienyl)-2-propenethioamides with N-phenylmaleimide and maleimide yielded a mixture of endo- and exo-2-arylimino-4-(2-thienyl)tetrahydrothiopyran[2,3-c]pyrroles. Cycloaddition to diethyl fumarate required acylation and furnished a mixture of diastereoisomeric 5-(N-acetylphenylamino)-2,3-bis-(ethoxycarbonyl)-4-(2-thienyl)-3,4-dihydro-2H-thiopyrans. Reactions of 3-(2-furyl)-2-propenethioamides with N-arylmaleimides furnished the correspondent 2-arylimino-4-(2-furyl)tetrahydrothiopyran[2,3-c]pyrroles. In the cycloadditions of the heterodienes with N-arylmaleimides the endo-cycloadducts were formed as the major products.     

Measurement of Small One-Bond Proton–Carbon Residual Dipolar Coupling Constants in Partially Oriented C Natural Abundance Oligosaccharide Samples: Analysis of Heteronuclear JCH-Modulated Spectra with the BIRD Inversion Pulse

Two 2D J-modulated HSQC-based experiments were designed for precise determination of small residual dipolar one-bond carbon–proton coupling constants in ¹³C natural abundance carbohydrates. Crucial to the precision of a few hundredths of Hz achieved by these methods was the use of long modulation intervals and BIRD pulses, which acted as semiselective inversion pulses. The BIRD pulses eliminated effective evolution of all but ¹J_CH couplings, resulting in signal modulation that can be described by simple modulation functions. A thorough analysis of such modulation functions for a typical four-spin carbohydrate spin system was performed for both experiments. The results showed that the evolution of the ¹H–¹H and long-range ¹H–¹³C couplings during the BIRD pulses did not necessitate the introduction of more complicated modulation functions. The effects of pulse imperfections were also inspected. While weakly coupled spin systems can be analyzed by simple fitting of cross peak intensities, in strongly coupled spin systems the evolution of the density matrix needs to be considered in order to analyse data accurately. However, if strong coupling effects are modest the errors in coupling constants determined by the “weak coupling” analysis are of similar magnitudes in oriented and isotropic samples and are partially cancelled during dipolar coupling calculation. Simple criteria have been established as to when the strong coupling treatment needs to be invoked.     

Soil stress and deformation determination under a landing airplane on an unsurfaced airfield

This paper presents a multidisciplinary approach to a problem of soil–wheel interaction during the landing of an airplane on a grass airfield, with terramechanical analysis of the forces acting between a wheel and the surface. The experiment on stress and deformation state in soil under a wheel on touchdown was performed in the field. The soil stress state (major stress S1, minor stresses S2 and S3, stress invariant in an octahedral system, MNS and OCTSS) was determined with the use of an SST (stress state transducer), which was installed at 15 cm depth. Soil deformation was determined with an inertial measuring system, integrated with the SST. These soil data were captured exactly at the moment touchdown. In the experiment, a four-passenger, STOL multirole airplane was used and pilots performed two landing patterns: normal landing and emergency landing (without propeller thrust).