Size effect on crystal structure and chemical bonding nature in BaTiO<Subscript>3</Subscript> nanopowder

Summary The size effect on the crystal structure including the chemical bonding nature has been investigated for several kinds of BaTiO₃ nanopowder with the particle sizes down to 50 nm in diameter, by means of powder diffraction using high-energy synchrotron radiation. The Rietveld refinement reveals that the BaTiO₃ nanopowder consists of tetragonal and cubic structure components at 300 K. The feature of coexistence can be illustrated by the core/shell model for the particle, in which the shell with a cubic structure covers the core with a tetragonal structure. The thickness of the cubic shell is almost constant irrespective of the particle sizes, and is estimated as approximately 8 nm. Hence, the critical particle-size, where the entire particle is covered with the cubic shell, is suggested as 16 nm. The charge density distributions of the BaTiO₃ nanopowder in the cubic phase at 410 K are revealed by the maximum entropy method. Changes in the bonding electron density and the ionic valence expected are not observed clearly even in the 50 nm crystal compared with the bulk crystal.     

Conductance Study of the Thermodynamics of Complexation of K+, Rb+, Cs+ and Tl+ Ions with Dibenzo-24-crown-8 in Binary Acetonitrile–Nitromethane Mixtures

The complexation reactions between dibenzo-24-crown-8 (DB24C8) and K⁺, Rb⁺, Cs⁺ and Tl⁺ ions were studied conductometrically in different acetonitrile–nitromethane mixtures at various temperatures. The formation constants of the resulting 1:1 complexes were calculated from the computer fitting of the molar conductance–mole ratio data at different temperatures. At 25 °C and in all solvent mixtures used, the stability of the resulting complexes varied in the order Tl⁺ > K⁺ > Rb⁺ > Cs⁺. The enthalpy and entropy changes of the complexation reactions were evaluated from the temperature dependence of formation constants. It was found that the stability of the resulting complexes increased with increasing nitromethane in the solvent mixture. The TΔS° vs. ΔH° plot of all thermodynamic data obtained shows a fairly good linear correlation indicating the existence of enthalpy–entropy compensation in the complexation reactions.     

Thermodynamics of ionic solid solutions: III. An addendum

In two earlier papers (C.A. 117:259320(1;121:19411y) the activity coefficients of the salts in binary solid solutions at 25‡C for 38 salt pairs, in which the members of each pair differ with respect to only one kind of ion, were determined. While the activity data are correct, the conclusions regarding deviations from ideality for eight of these pairs, namely those in which there are two moles of replaceable ion per mole of salt, require modification in order to be consistent with ideal entropies of mixing. By changing the formulation of the component salts to one-half of what is usual, the inconsistencies disappear. This half-mole approach, applied to the salt pairs CU_1/2(NH₄/K)SO₄-3H₂O, Mg_1/2NH₄(SO₄/ CrO₄)-3H₂O, Mg_1/2NH₄(SeO₄/SO₄)-3H₄O, Mg_1/2NH₄(SeO₄/CrO₄)-3H₄O, Mg_1/2 (K/NH₄)SeO₄-3H₂O, (NH₄/K)(SO₄)_1/2, and Ba_1/2(ClO₃/BrO₃)-1/2 H₂O shows that these solid solutions exhibit positive, not negative, deviations from ideality at 25‡C. Only the system Pb_1/2(C1/Br) still deviates negatively.     

Synthesis and acid catalyzed hydrolysis of B2,5 type conformationally constrained glucopyranosides: incorporation into a cellobiose analogue

Isopropyl and p-nitrophenyl α- and β-d-glucopyranosides, restrained in a conformation close to B_2,5 via an oxymethylene bridge have been synthesized. These four glucopyranosides were found to be hydrolyzed at similar rates, close to those observed for the parent unconstrained glucosides. In such derivatives, either α or β, the exocyclic cleaved bond is synperiplanar to an endocyclic oxygen lone pair. This conformationally locked glucopyranosyl moiety was also incorporated into a disaccharide, affording a conformationally restrained cellobiose analogue which was assayed against various glycosidases.     

In 2- und 3-Stellung verzweigte,enantiomerenreine 4,4,4-Trifluor-3-hydroxy-buttersäure-Derivate aus 6-Trifluormethyl-1,3-dioxan-und -dioxin-4-onen

Preparation of Enantiomerically Pure 4,4,4-Trifluoro-3-hydroxy-butanoic Acid Derivatives, Branched in the 2- or 3-Position, from 6-Trifluoromethyl-1,3-dioxan- and -dioxin-4-ones Enantiomerically pure 3-hydroxy-3-trifluoromethyl-propionic acid and esters, substituted in the 2- or 3-position, are prepared (13 examples) from (R)- or (S)-4,4,4-trifluoro-3-hydroxy-butanoic acid. Key intermediates are the 2-t-butyl-6-trifluoromethyl-1,3-dioxan- and -dioxin-4-ones. The Li enolate of the cis-dioxanone is generated with t-BuLi and reacts with electrophiles (alkyl halides, aldehydes, imines, nitroolefins, Br₂, I₂) with predominant formation of trans,trans-2,5,6-trisubstituted dioxanones (9 examples). Elimination of HBr from the 5-Br-substituted dioxanone gives the (R)- or (S)-dioxinone, a chiral derivative of 4,4,4-trifluoro-3-oxo-butanoic acid (trifluoro-acetoacetate). Michael additions of cuprates or of CuCl-doped Grignard reagents to the dioxinone produce 6,6-disubstituted dioxanones (10 examples) bearing a CF₃ group in the 6-position. In most cases this addition is highly diastereoselective, with the new substituent winding up in the trans position. There are, however, surprising exceptions, such as the product formed with benzylmagnesium chloride which is an abnormal adduct with a p-quinoid structure ( 26 ) and with the newly introduced group in the cis position with respect to the t-Bu group. The structures of four trisubstituted dioxanones bearing CF₃ groups are determined by X-ray crystal structure analysis (Figure 1, Table 1), one of them including the absolute configuration (by anomalous diffraction). Besides the well-known sofa, a twist-boat conformation of dioxanones appears to be favorable. The solution conformations of the different types of CF₃-substituted dioxanones are derived from Nuclear Overhauser NMR measurements and compared with the crystal structures (Figure 3).     

EFFECT OF THE COMPOSITION OF m-CRESOL/H2O SOLVENT ON HYDRODYNAMIC PROPERTIES OF NYLON 6

The intrinsic viscosity [ η], Huggins constant (KH), [ η]0, α3 and flow activation energy values of nylon 6 have been measured in water/m-cresol (0/100-20/80) systems at different temperatures (20-60℃). It has been found that the intrinsic viscosity, [η]0 and α3 increase with the increase in water contents in m-cresol up to 15% and then decrease. They increase with the increase in temperature irrespective of solvent composition. It has been noted that the percent increase of α3 is the highest at 60℃ and the lowest at 20℃ for a particular solvent system. The intrinsic viscosity data obey Arrhenius equation over the considered conditions. The activation energy and the KH values decrease very sharply with the addition of water,giving a minimum value at 15% of water and then increase slowly. The variation of all the parameters has been explained in terms of variation in thermodynamic quality of solvent with the addition of water to m-cresol and change in temperature,resulting in the change of conformational and orientational properties of polymer molecules. This change of solvent quality also results in variation of selective sorption of solvent over the polymer, such as hydrogen bonding, etc.     

Thermodynamic characteristics of thermal dissociation of platinum dichloride

The dissociation pressure for the process PtCl₂(s) → Pt(s) + Cl₂(g) was measured by the static method with diaphragm zero-pressure gauges. The approximating equation for the temperature dependence on the dissociation pressure for the above reaction was found. The enthalpy (137.7±0.3 kJ mol⁻¹) and entropy (163.6±0.4 J mol⁻¹ K⁻¹) of PtCl₂(s) dissociation and enthalpies of formation and absolute entropies of platinum di- and trichlorides at 298.15 K were calculated. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1345–1348, June, 2005.     

Structure elucidation of dicarboxylate complex of Sn porphyrin with a ring current effect model

A new simple model of porphyrin ring current effect was proposed based on a line current approximation. It can reproduce the porphyrin-induced shifts for several Sn(IV)(tpp) and Sn(IV)(oep) dicarboxylate complexes quite satisfactorily. Perpendicular arrangement of the aromatic rings in the diaromatic-carboxylate complexes of Sn(IV)(tpp) and Sn(IV)(oep) was clarified with this porphyrin ring current effect model. There are two structures, exo and endo, in solution in dinaphthalene-1- and 2- carboxylate complexes of Sn(IV)(tpp) and Sn(IV)(oep). The exo conformer is in dynamic equilibrium with the endo form in solution. Thermodynamic data of these conformational equilibria are given.     

甲壳素类液晶高分子的研究Ⅶ.N,O-苄基壳聚糖的胆甾螺旋行为

N ,O-苄基壳聚糖在浓溶液中形成胆甾液晶相 .用圆偏光二向色性谱 (CD)研究了这一聚合物的螺旋行为 ,主要包括螺距和螺旋方向 .浓度越高 ,螺距P越大 ,意味着胆甾相的扭转力随浓度增加而减弱 .CD谱图上观测到两类吸收 ,即在 5 70nm附近较宽但较强的吸收和 330nm附近较尖但较弱的吸收 .前者归属于胆甾相层片的超分子螺旋构象 ,而后者可以归属于分子链的螺旋构象 .改变浓度或溶剂性质时这两个层次的构象都会发生符号的变化 .提高浓度 (固定二氧六环为溶剂 )时两种螺旋结构先后发生反转 .以氯仿为溶剂 (固定浓度为 6 5 % )时两种螺旋结构均为左旋 (正Cotton效应 ) ,但二氧六环和四氢呋喃为溶剂时均变为右旋 (负Cotton效应 ) .溶剂的影响可能与溶剂和高分子间形成氢键的能力有关     

多标度分形热力学及其相变

用与热力学和统计物理相类比的方法,对多重分形引入了熵、自由能函数。这些函数对多重分形的几何特性给出了充分的描述。同时按照热力学相变理论刻画了多重分形谱函数的非解析性及相变。