Fermi surface and thermoelectric power of (Bi1−x Sbx)2Te3<Ag, Sn> mixed crystals

The Fermi surface anisotropy of (Bi_1?x Sb_x)₂Te₃ single crystals (0.25 ≤ x ≤ 1) was studied by analyzing the angular dependence of the frequency of Shubnikov-de Haas oscillations and the effect of tin and silver doping on the thermoelectric power in these crystals in the temperature range 77 ≤ T ≤ 300 K. It was shown that silver doping of (Bi_1?x Sb_x)₂Te₃ mixed crystals produces acceptors, while silver in Bi₂Te₃ acts as a donor. Tin also exhibits acceptor properties. Both tin and silver doping of p-(Bi_1?x Sb_x)₂Te₃ mixed crystals decrease the thermoelectric power due to an increase in the hole concentration.     

A simple low-vacuum environmental cell.

The environmental cell device discussed in this paper provides a modest low-vacuum scanning electron microscopy (SEM) capability to a standard SEM without requiring additional pumping. This environmental cell confines a volume of low vacuum in contact with the sample surface using a container that has an aperture for admitting the primary electron beam. The aperture is large enough to permit a limited field of view of the sample, and small enough to limit the outflow of gas into the SEM chamber to that which can be accommodated by the standard SEM pumping system. This environmental cell also functions as a gaseous detector device.     

Effects of neutron skin on the excitation of isovector modes of neutron-rich nuclei

The existence of a neutron skin in neutron-rich nuclei is discussed in connection with the excitation of isovector dipole and quadrupole giant modes via isoscalar nuclear probes. In the case of large neutron excess, important contributions are obtained from the nuclear excitation, which may even become predominant according to proper kinematical conditions. At variance with the usual situation encountered in inelastic processes, constructive interference can be found between nuclear and Coulomb contributions.     

Synthesis and anticonvulsant activity of new fluorinated N-phenyl- and N-benzyl-2-azaspiro[4.4]nonane- and [4.5]decane-1,3-dione derivatives: Part III

A series of N-phenyl- and N-benzyl-2-azaspiro[4.4]nonane- and [4.5]decane-1,3-diones containing a fluoro or trifluoromethyl substituents at the aryl ring was synthesized and tested for their anticonvulsant activity in the maximal electroshock (MES) and subcutaneous metrazole (sc.Met) tests. Among them, the most active were N-benzyl derivatives with fluoro and trifluoromethyl substituents especially at position-2 of the aryl moiety. The introduction of the phenyl ring at the imide nitrogen atom resulted in less active compounds. The results obtained showed that incorporation of fluoro or trifluoromethyl substituents increased the anticonvulsant activity in comparison to respective chloro, methoxy or methyl analogues. Crystallographically obtained conformation for one active and two inactive derivatives with trifluoromethyl substituents at position-2 or -3 of phenyl ring were initially used for molecular electrostatic potentials (MEP) calculation. The MEP distribution at carbonyl oxygen atoms was different for active and inactive molecules.     

Accurate determination of pyridine-poly(amidoamine) dendrimer absolute binding constants with the OPLS-AA force field and direct integration of radial distribution functions

OPLS-AA force field and direct integration of intermolecular radial distribution functions (RDF) were employed to calculate absolute binding constants of pyridine molecules to amino group (NH2) and amide group hydrogen atoms in and first generation poly(amidoamine) dendrimers in chloroform. The average errors in the absolute and relative association constants, as predicted with the calculations, are 14.1% and 10.8%, respectively, which translate into ca. 0.08 and 0.06 kcal/mol errors in the absolute and relative binding free energies. We believe that this level of accuracy proves the applicability of the OPLS-AA, force field, in combination with the direct RDF integration, to reproducing and predicting absolute intermolecular association constants of low magnitudes (ca. 0.2-2.0 range).     

Oxidation-Reduction Conversions of Derivatives of 1,4-Dihydropyridine and 4H-pyran on Interaction of Alicyclic 1,5-Diketones wth 4-Aminoazobenzene

The interaction of 2,2'-methylenedicyclohexanone and 2,2'-methylenedicyclopentanone with 4-aminoazobenzene in acetic acid leads to reduction of the azo group of 1,4-dihydropyridine and 4H-pyran derivatives formed as intermediates. The reduction products were isolated as the corresponding pyridinium salts.     

Oxidative Coupling in a Series of Derivatives of 4a,9-Diaza-1,2,4a,9a-tetrahydrofluorene. 6. Reactions with Substituted 2-Aminoethanols

Oxidative coupling of derivatives of 4a,9-diaza-1,2,4a,9a-tetrahydro-9H-fluorene with 2-ethyl-, 2,2-dimethyl-, and 2-hydroxymethyl-2-methylaminoethanol in the presence of MnO₂ led to the selective formation of the corresponding mono- and di(hydroxymethyl)quinonediimines, subsequent cyclization of which gave the products of 6,7-annelation. Coupling with 2,2-di(hydroxymethyl)aminoethanol gave the annelation products directly.     

Spectrum of positive entropy multidimensional dynamical systems with a mixed time

It is shown that if an abelian countable group is such that is a finite group and every aperiodic positive entropy action of on a Lebesgue probability space has a countable Haar spectrum in the subspace , where denotes the Pinsker -
algebra of , then every aperiodic positive entropy action of on has the same property. A positive answer to the question of J.P. Thouvenot is obtained as a corollary.

     

Theoretical study of some heterocyclic amines with applications to the chemistry of 9-amino-1,2,3,4-tetrahydroacridine

9-Amino-1,2,3,4-tetrahydroacridine (THA ), a potent cholinesterase inhibitor, was recently used in the treatment of Alzheimer's disease. On attempting to prepare a dihydropyridine ? pyridinium salt-based redox chemical delivery system (CDS ) to enhance brain delivery of THA , several of the practical synthetic challenges were examined by using a theoretical MO approach. The structures, reactivities and stability of THA , derivatives of THA and a model compound, 4-aminopyridine, a simple dibasic heterocyclic amine, were studied in the framework of the AM -1 approximation. The study included the possible protonated forms of THA and 4-aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4-aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4-aminopyridine, consistent with experimental results. Electron delocalization is responsible for the remarkable stability of these molecules and for the observed lack of reactivity of the amino group, both in the basic and protonated forms. The site of N-alkylation of the 9-nicotinamide derivative of THA (an intermediate in the synthesis of THA -CDS ) is controlled by electronic, thermodynamic, and steric factors.