Linear crystallization and induction-period studies of the growth of calcium sulphate dihydrate crystals

The linear crystallization of well-formed crystallites of calcium sulphate dihydrate from supersaturated solutions has been studied by a microscopic method at temperatures ranging from 25 degrees to 55 degrees . The activation energy is 16 +/- 1.5 kcal/mole for the fast-growing faces. Induction periods in the crystallization of calcium sulphate dihydrate have been determined as a function of solution concentration at temperatures of 25 degrees and 90 degrees .     

Atomic-absorption spectrometric determination of cobalt,nickel, and copper in geological materials with matrix masking and chelation-extraction

An atomic-absorption spectrometric method is reported for the determination of cobalt, nickel, and copper in a variety of geological materials including iron- and manganese-rich, and calcareous samples. The sample is decomposed with HP-HNO₃ and the residue is dissolved in hydrochloric acid. Ammonium fluoride is added to mask iron and 'aluminum. After adjustment to pH 6, cobalt, nickel, and copper are chelated with sodium diethyl-dithiocarbamate and extracted into methyl isobutyl ketone. The sample is set aside for 24 h before analysis to remove interferences from manganese. For a 0.200-g sample, the limits of determination are 5–1000 ppm for Co, Ni, and Cu. As much as 50% Fe, 25% Mn or Ca, 20% Al and 10% Na, K, or Mg in the sample either individually or in various combinations do not interfere. Results obtained on five U.S. Geological Survey rock standards are in general agreement with values reported in the literature.     

On the mechanism of reorientational and structural relaxation in supercooled liquids: the role of border dynamics and cooperativity

Molecular dynamics simulation and analysis based upon the many-body potential energy landscape (PEL) are employed to characterize single molecule reorientation and structural relaxation, and their interrelation, in deeply supercooled liquid CS(2). The rotational mechanism changes from small-step Debye diffusion to sudden large angle reorientation (SLAR) as the temperature falls below the mode-coupling temperature T(c). The onset of SLAR is explained in terms of the PEL; it is an essential feature of low-T rotational dynamics, along with the related phenomena of dynamic heterogeneity and the bifurcation of slow and fast relaxation processes. A long trajectory in which the system is initially trapped in a low energy local minimum, and eventually escapes, is followed in detail, both on the PEL and in real space. During the trapped period, "return" dynamics occurs, always leading back to the trap. Structural relaxation is identified with irreversible escape to a new trap. These processes lead to weak and strong SLAR, respectively; strong SLAR is a clear signal of structural relaxation. Return dynamics involves small groups of two to four molecules, while a string-like structure composed of all the active groups participates in the escape. It is proposed that, rather than simple, nearly instantaneous, one-dimensional barrier crossings, relaxation involves activation of the system to the complex, multidimensional region on the borders of the basins of attraction of the minima for an extended period.     

Effect of macromolecules and Triton X-100 on the triplet of aggregated chlorophyll in aqueous solution

Chlorophyll a (Chla) in aqueous solution (2-6% acetone) is present as mono- and dihydrated aggregated forms which are characterized by specific ground state absorption spectra. The amount of dihydrated form is larger in the presence of macromolecules, such as bovine serum albumin (BSA), lysozime and polyvinyl alcohol (PVA), increasing from BSA to lysozime and PVA. Chla in aqueous acetone with and without macromolecules is characterized by low fluorescence and the absence of triplet-triplet (T-T) absorption. The ratio of dihydrated to monohydrated forms is significantly influenced by triton X-100. For lower triton X-100 concentrations, i.e. smaller than the critical micelle concentration of 0.26 mM (cmc), dihydrated forms are converted into monohydrated in both aqueous acetone and the presence of BSA or lysozime. In the presence of PVA dihydrated forms appeared to be resistant to triton X-100 action. Moreover, for triton X-100 concentrations of 2-3 times higher than cmc the amount of these forms is increased with time. T-T absorption of both mono- and dihydrated Chla aggregates was not detected in the presence of [triton X-100] < cmc. The lack of T-T absorption in aqueous acetone solution as well as in the presence of macromolecules implies that the triplet lifetime of the chlorophyll aggregates is short (tauT<10 ns) and/or the quantum yield of intersystem crossing is small (<5 x 10(-3)). The Chla monomers start to be formed as solubilized in the micelle for [triton X-100] larger than cmc, showing substantial fluorescence and T-T absorption.     

Neues verfahren zur bestimmung der acetylgruppe

A simple and accurate method is described for the determination of acctyl groups in various organic compounds. The compound is dissolved or suspended in absolute methanol and saponified with a known quantity of potassium methylate solution. The excess potassium methylate is then hydrolysed and determined as potassium hydroxide by titration, an appropriate indicator being used.     

Indolopyridines with a bridging heteroatom

Reaction of 2-acetylpyridine with 4-[2.2]paracyclophanyllithium or 4-acetyl[2.2]paracyclophane with 2-pyridllithium gives 1-([2.2]paracyclophan-4-yl)-1-(2-pyridyl)ethanol. Its molecular and crystalline structures have been studied by x-ray analysis. It was found that heating this alcohol in acid medium causes dehydration and heterocyclization to give 1-paracyclophanyl-1-pyridylethylene and 10-methyl[2.2]paracyclophano[4,5-bindolizine.For communication 9 see [1]. This report is also communication [5] in the series Synthesis, Structure, and Biological Activity of [2.2]Paracyclophanes (for communication 4 see [2]).Russian University of National Friendship, Moscow 117198. A. N. Nesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, Moscow 117813. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 793–800, June, 1997.