用差谱法研究环氧树脂中基团的相对稳定性

环氧树脂是一类广泛应用的万能胶,其分子中各种官能团的相对热稳定性是实际应用的重要参数。本文用付里叶变换红外光谱为测试手段研究了室温与降解条件(300℃、1小时)下的己固化环氧618树脂,用差谱法得到了固化的环氧618中各官能团的热稳定性次序为:甲基>次甲基>对位取代苯环>醚健>同碳二甲基。     

土壤中常量及微量元素的ICP光谱分析

本文叙述了土壤中Ca、Mg、Na、Sr、Ba、Mn、Fe、V、Cr及Ti等元素简单快速的ICP-AES测定方法。用HClO_4、HNO_3及HF在聚四氟乙烯高压罐中溶解祥品,用H_3BO_3络合过量的HF。采用基体匹配法校正干扰。方法的准确度、精密度及检出限均较好。     

原子吸收法测定硅藻土中的铁,钙和镁

通过Al、SiO_2、HClO_4对Fe、Ca、Mg原子吸收法测定干扰的研究,对仪器工作条件、溶样方法的研究,建立了原子吸收法测定硅藻土中Fe、Ca、Mg的分析方法。此法测定Fe、Ca、Mg的相对标准偏差分别是Fe<2.0%、Ca<9.0%、Mg<10.0%,三元素回收率均在94-106%之间。     

Some chain conditions in pullbacks

The main purpose of this paper is to study when a (T, I, D) construction ring is a C-ring in the sense of Echi (Port Math 50:277–295, 1993) where I is not necessarily a maximal ideal of T. This study allows us to generalize some results proved in Echi (Port Math 50:13–23, 1994) and to give new examples of C-rings.     

Determination of the stoichiometry and stability constants of theβ-cyclodextrin-dextromethorphan inclusion complexes by liquid chromatography and UV spectroscopy

The inclosion of dextromethorphan (DMN) by -cyclodextrin (-CD) was studied by using chromatography, UV spectroscopy and circular dichroism methods at 25 °C, pH 7.4 and 4.2. It was found that the CD : DMN complex has 1 : 1 stoichiometry. It is more stable at pH 7.4 than at pH 4.2. with constants respectively equal to 8000 ± 800 M^–1 and 5750 ± 500 M^–i, as determined by chromatography. The stability of the complex at pH 7.4 decreases as the temperature increases. From the van 't Hoff dependence the standard entropy and enthalpy changes were determined at this pH.     

1,3-Alternate and C-1,2-Alternate Tetrahomodioxacalix[4]crowns. Crystal Structures and Metal Ion Complexation

Two novel tetrahomodioxacalix[4]crowns with crown-5 and crown-6 rings weresynthesized. From X-ray crystal structures, homooxacalix[4]crown-5 (3) andhomooxacalix[4]crown-6 (2) were found to be in the 1,3-alternate and theC-1,2-alternate conformations, respectively. Homooxacalix[4]crown-5 (1)shows a marked selectivity for cesium ion over other metal ions tested.     

Compartmental modeling of the fluorescence anisotropy decay of a cylindrically symmetric brownian rotor: Identifiability analysis.

We present the results of the deterministic identifiability analysis based on similarity transformation for models of one-state excited-state events of cylindrically symmetric rotors in isotropic environments undergoing rotational diffusion described by Brownian reorientation. Such an analysis on error-free time-resolved fluorescence (anisotropy) data can reveal whether the parameters of the considered model can be determined. The fluorescence delta-response functions I(parallel)(t) and I(perpendicular)(t), for fluorescence polarized respectively parallel and perpendicular to the electric vector of linearly polarized excitation, are used to construct, in convenient matrix form, expressions of the sum S(t) = I(parallel)(t) + 2I(perpendicular)(t), the difference D(t) = I(parallel)(t) - I(perpendicular)(t), and the time-resolved fluorescence anisotropy r(t) = D(t)/S(t). The identifiability analysis of r(t) demonstrates that the rotational diffusion coefficients D(parallel) and D(perpendicular) for rotation respectively about and perpendicular to the symmetry axis can be uniquely resolved. However, the polar and azimuthal angles defining the absorption and emission transition moments in the molecular reference frame are not individually identifiable. Nevertheless, the difference between the polar angles of these transition moments is uniquely determined.     

Electrocatalytic dimerisation of non-heteroatom-substituted manganese alkynylcarbene complexes

Upon controlled potential electrolysis (-1.30 V vs. SCE), the alkynylcarbene complexes MeCp(CO)2Mn=C(R)C [triple bond]CR' are seen to undergo an electrocatalytic dimerisation by coupling of the remote alkynyl carbon atoms to afford the ene-diyne dimanganese complexes [MeCp(CO)2Mn]2[eta 4-(E)-RC [triple bond] C(R')C=C(R')C [triple bond] CR].     

Experimental design for the estimation of photophysical parameters of the two-state excited-state proton-exchange reaction in the presence of pH buffer

The excited-state proton-exchange reaction of commonly used fluorescent pH probes at physiological pH becomes reversible upon addition of pH buffer. Using computer-generated fluorescence decay surfaces, we investigated under which experimental conditions (pH, buffer concentration, and excitation and emission wavelengths) the rate constants describing the excited-state processes (k(ij)) and the spectral parameters related to excitation ((~)b(1)) and emission ((~)c(1)) can be accurately and precisely estimated by global compartmental curve fitting. It was found that a minimum of three fluorescence decay traces should be collected for the pH probe in the presence of buffer. These three decays should be characterized by at least two different pH values and at least two different buffer concentrations. In addition to these three traces, a minimum of one trace corresponding to the pH probe without buffer has to be recorded. Furthermore, for the accurate estimation of k(ij), (~)b(1), and (~)c(1), at least two of these traces should be collected at the same pH and excitation and emission wavelengths. The experimental conditions should be chosen in such a way that decays with unambiguous biexponential character are obtained. For fluorescent pH probes with pK(a) approximately equal to 7 that are responsive in the near-neutral pH range, it is advisable to use buffers with pK(B)(a) values comparable to or higher than the pK(a) of the probe. Because the changes in the decay times are already apparent with a small quantity of buffer, there is no need to use excessively high buffer concentrations. From a practical point of view, the best experimental design is attained when one combines in a single fluorescence decay surface traces originating from samples characterized by different pH values at the same buffer concentration with traces characterized by different buffer concentrations at the same pH and decays of samples without buffer measured at several pH values.     

Effect of a new beta-sitosterol analogue on plasma lipid concentrations in rats

N-Substituted succinamic acid beta-sitosteryl ester derivatives were prepared and evaluated. Compounds 1 and 2 were prepared in 76-92% yields by the reaction of beta-sitosterol and succinic anhydride, followed by the activation of the resulting acid compound 1 by thionyl chloride or methyl chloroformate, and finally by amination with appropriate amines. Compound 2a (DANA87) was also easily obtained in one step by the direct addition of beta-sitosterol to dicyclohexylcarbodiimide (DCC) in 80% yield. Administration of the dietary compound DANA87 resulted in significant decreases in total plasma cholesterol (TC) and low-density lipoprotein (LDL) cholesterol concentrations compared with controls in a rat model. High-density lipoprotein cholesterol and plasma triglyceride levels were not affected. These findings indicate that DANA87 functions as TC and LDL cholesterol-reducing agent.