3-氯-1，2-环氧丙烷的同步辐射光电离

杂环类化合物在自然界中分布十分广泛,且许多具有生物活性,它们大多数在生物的生长 发育、遗传和衰亡过程中起着关键的作用［１,２］．含氧三元杂环体系是最基本、最简单的杂环化合 物,分子中的含氧三员环,具有很大的张力,且存在很强的极性共价键,使其易于发生定向开环 反应,具有高度反应性．利用光电高质谱法对这类分子进行光电离与光离解研究,可以获得这 类物质一些重要的物理化学数据． 我们已对环氧乙烷和环氧丙烷进行了光电离和光离解研究,并结合量化理论计算分析了 它们的光电离解离通道［３,４］,３－氯－１,２－环氧丙…     

Temperature variation effects on the determination of acidity constants through the internal standard–capillary electrophoresis method

The internal standard (IS) CE (IS‐CE) method is an interesting alternative to other methods for the determination of acidity constants of compounds. Although some of the advantages of this method have been already reported, the method has not been tested yet as regards to temperature effects. This has been the aim of this work, where it is demonstrated that the method can be applied successfully for the determination of pK_as at different temperatures, if the acidity constant of the IS at the desired temperature is known. The fact of obtaining the acidity constants at different temperatures allows the calculation of some thermodynamic quantities, such as the molar standard enthalpy and the molar standard entropy in a fast way. It is also demonstrated that if the IS and the test compound have similar standard enthalpy increment, the IS compensates uncontrolled possible temperature fluctuations (e.g., due to Joule heat) inside the capillary obtaining reliable acidity constant values at the desired temperature.     

Flip–flop Motion of Circular Hydrogen Bond Array in Thiacalix[4]arene

Thiacalix[4]arene contains a circular array of four equivalent hydrogen bonds on its lower rim. The array undergoes a flip–flop motion between two possible directions. The rate of this motion in the temperature range 223–313 K is assessed by means of measurements of the nuclear spin relaxation. The values of the activation enthalpy (38.7 kJ mol^? 1) and of the activation entropy ( ? 15 J mol^? 1 K^? 1) were determined. In addition, correlation times of molecular tumbling have been determined in the same temperature range. The measured properties of thiacalix[4]arene are compared to those of the “classical” calix[4]arene in order to utilize them for fine tuning of the building blocks in supramolecular chemistry.     

Thermodynamics of Complexation of Tetraalkylammonium and Rare-earth Cations with Two Sulfonatocalix[n]arenes (n=4 and 6) in Aqueous Solution

We report results of a microcalorimetry study of the association of inorganic and organic cations with two p-sulfonatocalix[n]arenes (host 1: n = 4; host 2: n = 6) in aqueous solution at 298.15 K. First, we have determined the thermodynamic parameters for the complexation between the host 2 and a series of quaternary ammonium cations. We have evaluated the influence of the pH on the structure and energetics of these organic complexes. We have also reported the association constant and enthalpy of reaction for the complexation of some rare-earth cations (Sm³⁺, Dy³⁺, Y³⁺ and Sc³⁺ cations) with the two hosts. In all cases we have observed the formation of 1:1 complexes.

Whereas the association is driven by a favourable entropy change for the inorganic cations (ΔH > 0 and TΔS>>0), it is controlled by a favourable enthalpy change for the organic cations (ΔH < < 0 and TΔS < 0 or >0). In acidic solution, the complexes formed between host 2 and tetraalkylammonium cations are weaker than those formed with the cyclic tetramer. In neutral solution this effect is not observed. All the results are in line with a conformational change of host 2 with the pH.     

Comparison of four cholesterol‐based stationary phases for the separation of steroid hormones

The chromatographic behavior of steroid hormones on four cholesterol‐bonded stationary phases with different structures in binary methanol/water mobile phases was studied. Of the stationary phases tested, the commercially available stationary phases Cogent UDC cholesterol? and COSMOSIL cholester? provided better separations of steroid hormones in comparison to homemade aminocholesterol and diaminocholesterol stationary phases. The results show that the temperature has a significant influence on the retention and selectivity for steroid hormones separation. The temperature increase may cause changes in the elution order. From the dependences of the retention (ln k) on temperature (1/T), the standard partial molar enthalpy and standard partial molar entropy were calculated and their enthalpic and entropic contributions to the retention were compared. The enthalpic effects principally control the retention mechanism.     

Thermodynamics of solvation and solvophobic effect in formamide

Using semi-adiabatic calorimetry, we measured the enthalpies of solution for various low-polar compounds including alkanes, aromatic hydrocarbons and their halogenated derivatives in formamide at temperature of 298 K. For the same compounds, the values of limiting activity coefficients in formamide were determined using GC headspace analysis at 298 K, and Gibbs free energies of solution and solvation were calculated. Based on these data and the available literature values of the Gibbs free energy of solvation in formamide for a number of other low-polar solutes, a study of the solvophobic effect in this solvent is performed, and its resemblance to the hydrophobic effect in aqueous solutions is demonstrated. It is shown that the contribution of the solvophobic effect into the solvation Gibbs free energy in formamide is much higher than that in aliphatic alcohols, but lower than that in water. Like in water, the magnitude of this contribution for different solutes linearly increases with the solute molecular volume. Solvophobic effect also significantly affects the enthalpies of dissolution in formamide, causing them to be more negative in the case of alkanes and more positive in the case of arenes.     

Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr₂Nb₂O₇ and calcium niobate Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form C_pm = 248.0 + 0.04350T − 3.948 × 10⁶/T² J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and C_pm = 257.2 + 0.03621T − 4.434 × 10⁶/T² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, S_m°(298.15 K) = 238.5 ± 1.3 J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and S_m°(298.15 K) = 212.4 ± 1.2 J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇, were evaluated from the low-temperature heat capacity measurements.     

Paradigms and paradoxes: energetics of the oxidative cleavage of azo compounds (diazenes)

The enthalpy of oxidative cleavage of azo compounds (diazenes) is discussed and compared with that of symmetric olefins. The roles of substituents and of electronegativity are explicitly discussed.     

Phase transitions of K<Subscript>2</Subscript>TaF<Subscript>7</Subscript> within 680–800°C

Three thermal effects on heating/cooling of K₂TaF₇ in the temperature interval of 680–800°C were investigated by the DSC method. The values determined for the enthalpy change of the individual processes are: Δ_transIIH_m(K₂TaF₇; 703°C) = 1.7(2) kJ mol⁻¹, Δ_transIH_m(K₂TaF₇; 746°C) = 19(1) kJ mol⁻¹ and Δ_transIIIH_m(K₂TaF₇; 771°C) = 13(1) kJ mol⁻¹. The first thermal effect was attributed to a solid-solid phase transition; the second to the incongruent melting of K₂TaF₇ and the third to mixing of two liquids. These findings are supported by in situ neutron powder diffraction experiments performed in the temperature interval of 654–794°C.