Ab Initio versus CNDO barrier calculations

Energy barrier curves to internal rotation in P₂H₄ and P₂F₄ have been studied by the ab initio SCF-LCAO-MO method in the gaussian approximation and by the CNDO method. For P₂H₄, two stable rotamers at 75 and 180 of equal energy and separated by a barrier of only 500 cal/mole are predicted, and the cis barrier computes to be 4.1 kcal/mole. A trans-only form of P₂F₄ is the theoretical structure with only an arrest in the barrier curve at the gauche position. The CNDO method does not produce reliable information concerning the number of stable rotamers, their precise configurations, or the heights of the barriers.

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Zusammenfassung Die Kurven der Energiebarriere bei der inneren Rotation von P₂H₄ und P₂F₄ wurden nach der ab initio SCF-LCAO-MO-Methode mit einer Basis von Gaussfunktionen sowie nach der CNDO-Methode untersucht. Beim P₂H₄ ergeben sich zwei stabile Rotamere von gleicher Energie bei 75 und 180 und getrennt durch eine Barriere von nur 500 cal/mol; die cis-Barriere berechnet sich zu 4.1 kcal/mol. Eine reine trans-Form von P₂F₄ ist die theoretische Struktur mit nur einem Sattelpunkt in der Energiekurve bei der gauche-Position. Die CNDO-Methode liefert keine genügend genauen Anhaltspunkte zur Zahl der stabilen Rotameren, ihren genauen Konfigurationen oder zu den Höhen der Barrieren.

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Résumé Les courbes d'énergie donnant les barrières de rotation interne ont été étudiées pour P₂H₄ et P₂F₄ par la méthode ab-initio SCF LCAO MO en base gaussienne et par la méthode CNDO. Pour P₂H₄ on prévoit deux rotamères stables de mÊme énergie à 75 et 180 séparés par une barrière de 500 cal/mole seulement, la barrière cis calculée étant de 4,1 kcal/mole. La structure théorique de P₂F₄ est la forme «tout-trans» avec seulement un arrÊt sur la courbe dans la position gauche. La méthode CNDO ne produit pas d'informations sûres en ce qui concerne les rotamères stables tant du point de vue de leurs configurations que de celui des barrières.

     

Radiohochspannungsionophoretische Untersuchungen der Isotopenaustauschreaktionen zwischen Polythionaten,Thiosulfat und Sulfit

The isotopic exchange of³⁵S between thiosulfate/sulfite and tetrathionate/thiosulfate is examined. The kinetic data are determined. In both cases the reactions are of second order. As the isotopic exchange between tetrathionate/thiosulfate is very fast, these reaction components are used in high dilution. Furthermore, the extent of exchange during the separation is determined.      

Synthesis of sodium tantalate nanorods by alkalide reduction

NaTaO3 nanorods were synthesized with high (>90%) yield by reduction of TaCl5 with THF solutions of the alkalide K+(15C5)2Na-, followed by product annealing under dynamic vacuum at 250 and 600 degrees C. In addition to the nanorods, the product is comprised of 5-10% approximately 10-nm diameter spheroidal NaTaO3 nanocrystals. The nanorods are generally longer than 500 nm, with some exceeding 1 mum, and 10-100 nm wide, with aspect ratios that vary between 10 and 20:1. Select area electron diffraction patterns of individual nanorods indicate that each nanorod is a single crystal with its axis oriented in the [010] direction.     

The structure of new lignans from the seeds of phytolacca americana

From the seeds of Phytolacca americana two new lignans, Americanin B and D, have been isolated and their structures elucidated mainly by spectroscopic methods     

Reactions with Addition Compounds Containing Activated Formic Acid

Addition compounds of formic acid with tertiary organic bases in ratios higher than that required for salt formation (3:1 and 2:1 adducts) contain formic acid in a highly activated form. In particular, the 3:1 adducts of formic acid with trimethylamine and triethylamine are liquid reducing agents that are convenient to handle and are suitable for use in many selective reductions. These addition compounds have a surprisingly strong reducing action on sulfur dioxide, which is rapidly converted, even at low concentrations, into crystalline rhombic sulfur. Sulfones and polysulfones can be prepared in good yields by three-component reactions from activated formic acid, sulfur dioxide, and polarized vinyl compounds.     

Studien über die naphthoresorcin-reaktion von tollens : Zur bestimmung von freier uronsaure neben uronosiden,polyuronosiden und neben anderen kohlenhydraten

A micro determination for free uronic acids based on Tollens' reaction is proposed. Sources of error in earlier methods are avoided by the use of phosphoric acid, a reagent solution in glacial acetic acid, and extraction of the dye with toluene. Free glucuronic acid can be determined in presence of glucuronosides of the phenol-ether type up to molar ratios of ca. 1 : 5. Reliable results can also be obtained in the presence of other carbohydrates.     

Synthetic Cu0.507(5)Pb8.73(9)Sb8.15(8)I1.6S20.0(2) nanowires

Nanowires of an iodine containing Pb-Sb-sulfosalt have been synthesized by chemical vapor transport. Their structure was studied using high-resolution transmission electron microscopy and X-ray powder diffraction. The lattice parameters show values equal to a=4.9801(4) nm, b=0.41132(8) nm (with two-fold superstructure), c=2.1989(1) nm and β=99.918(6)°. These parameters and the results of a multislice simulation are in good agreement with the mineral pillaite, Cu_0.10Pb_9.16Sb_9.84S_22.94Cl_1.06O_0.5 (space group C2/m, a=4.949(1) nm, b=0.41259(8) nm, c=2.1828(4) nm, and β=99.62(3)°). Microprobe and EDX analyses yielded a chemical composition of Cu_0.507(5)Pb_8.73(9)Sb_8.15(8)I_1.6S_20.0(2) which is close to natural pillaite but contains no oxygen and iodine instead of chlorine. The structure of the investigated material is based on chains of M-S polyhedra (M=Pb or Sb) typical for the architecture of sulfosalts implying iodine atoms in trigonal prismatic coordination with Pb atoms from the M-S polyhedra of neighboring chains. The [010] superstructure of the specimen was found to be unstable under electron beam irradiation with a rapid decrease of the b lattice parameter from 0.8 to 0.4 nm within 5 min.     

Thermal characterization of UHMWPE stabilized with natural antioxidants

This work presents a study of the thermal degradation of ultra-high molecular mass polyethylene (UHMWPE) stabilized with natural (α-tocopherol and carvacrol) and synthetic antioxidants. Thermogravimetric analysis in dynamic mode was used to determine the apparent activation energies of different samples. The stabilization of UHMWPE with low concentrations (around 0.3%) of α-tocopherol is enough to obtain an efficient thermal performance of the polymer. Carvacrol is also a good stabilizer for UHMWPE, but at higher concentration than in the case of α-tocopherol. The comparison of apparent activation energy between samples with natural and synthetic antioxidants in similar concentration shows that α-tocopherol is a better stabilizer in terms of their thermal performance.     

Preparation and Chemical and Biological Studies of the Novel Complex μ-1,4,5-triphenyl-1,3,4-triazole-2-thiolate-bis[dichlorotriethylphosphineplatinum(II)]a

The mesoionic compound, 1,4,5-triazole-2-thiolate, is a versatile ligand, capable of binding metal centres in different ways. The present study shows how it reacts with the bridging platinum(II) complex μ-dichlorobis[(chlorotriethylphosphineplatinum(II)], [Pt₂(PEt₃)₂Cl₄], originating a bimetallic complex. Both the sulphur atom and nitrogen 3 behave as basic sites, coordinating different platinum centres. An interesting isomerization process, shown by ³¹P{¹H} n.m.r. spectroscopy, occurs at the S-bonded platinum moiety, resulting in a fluxional behaviour of the complex. It is also shown to target CT-DNA with moderate binding affinity.To the memory of Dr. Wagner M. Teles (1971–2004).     

High-performance liquid chromatographic separation and quantitative determination of valepotriates in valeriana drugs and preparations (author's transl)

It is possible directly to separate and analyse, quantitatively and qualitatively, the valepotriates from Valeriana crude extracts or from commercial Valeriana preparations by high-performance liquid chromatography. The separations are achieved on 4 or 8 mm I.D. columns packed with silica gel (particle size 10 micron) with n-hexane-ethyl acetate mixtures as eluent. A refractive index detection system is necessary for determining all of the valepotriates. If the concentration differences between didrovaltratum and valtratum are very great, an ultraviolet (UV) detector must be used and the determination must be conducted in two steps. For valtratum drugs UV detection alone will suffice. As internal standards p-dimethylaminobenzaldehyde should be used for extracts and preparations from valtratum races, and benzaldehyde in the presence of didrovaltratum races. This determination is superior to the combined thin-layer chromatographic-hydroxamic acid method used hitherto with respect to time consumption, precision, and sensitivity.