Thermochemical behaviour of crown ethers in the mixtures of water with organic solvents: Part VII. Enthalpy of solution of 15-crown-5 and benzo-15-crown-5 in the mixtures of water with acetone at 298.15 K

Enthalpy of solution of crown ethers (15-crown-5 and benzo-15-crown-5) in water-acetone mixtures have been measured within the whole range of mole fraction at 298.15 K. The obtained data have been compared with those of the solution enthalpy of both crown ethers in the mixtures of water with dimethyl sulfoxide. The replacement of SO group with CO in the molecule of the organic solvent brings about an increase in the exothermic effect of the solution of 15-crown-5 and benzo-15-crown-5 ethers, especially in the mixtures with a medium water content. The observed effect is connected with the preferential solvation of the molecules of both crown ethers by acetone molecules in the water-acetone mixtures. The process of preferential solvation of 15-crown-5 and benzo-15-crown-5 ethers does not take place in the water-dimethyl sulfoxide mixture.     

四氮杂大环与镧系金属配合物的热力学研究

The stability constants of the 1:1 complexes of H4L2 (5,12-dipheny1-7, 14-dimethyl-1,4,8,11-tetraazamasrocyclotetradecane-N', N", N"',N""-tetraacetic acid) with Ln3+ (Ln=La, Nd, Pr, Sm, Eu, Gd, Dy, Yb) were determined by potemtiometric titrations in 0.5mol•L-1 KCl at 40±0.1℃, 50±0.1℃ and 60±0.1℃ respectively. The △H, △S and △G of the coordination reactions of H4L2 with Ln3+ were given. Influences of the steric effect and temperature changes on the stability lanthanide complexes were discussed thermodynamically.     

Measurement of the enthalpies of vaporization and sublimation of solids aromatic hydrocarbons by differential scanning calorimetry

An experimental procedure is proposed for direct measurement of the heat involved in the vaporization of a solid organic compound above its normal melting temperature. This technique consists on the fusion of a solid aromatic hydrocarbon, which is then vaporized by a sudden decrease of the pressure. The direct register of heat flow as function of time by differential scanning calorimetry allows the quantifying of the enthalpy of vaporization of compounds such as phenanthrene, β-naphthol, pyrene, and anthracene. Enthalpies of vaporization were measured in an isothermal mode over a range of temperatures from 10 to 20 K above the melting temperatures of each compound, while enthalpies of fusion were determined from separate experiments performed in a scanning mode. Enthalpies of sublimation are computed from results of fusion and vaporization, and then compared with results from the literature, which currently are obtained by calorimetric or indirect techniques.     

A Theoretical Study of the Interaction Between Cytosine and BX<Subscript>3</Subscript> (X = F,Cl) Systems

The B3LYP method of DFT and HF theories of ab initio with 6-311+G^** basis sets were used to predict the geometries of the cytosine-BX₃ (X,=F, Cl) complex systems. Four conformers were obtained with no imaginary frequencies, respectively. The binding energies, enthalpies and Gibbs energies of cytosine-BX₃ have been obtained. The analyses of the combinations between cytosine and BX₃ using the natural bond orbital (NBO) method and thermodynamics indicate that the complexes (a) and (e), which depend on the proton affinities of the oxygen on the cytosine and boron in BX₃, are the most stable ones with their combination energies of −234.21 and −228.23 kJ.mol⁻¹ (B3LYP method, BSSE corrected). Based on the calculation results, a reasonable method was employed to calculate the change in the enthalpies and Gibbs energies to form eight complexes in the gaseous state at 298.15K and 101.325 kPa. It can be shown that the conformers (a) and (e) are the most stable and form readily.     

Interactions of ether groups with saccharides. Enthalpy of dilution of aqueous systems containing S-trioxane and inositol,mannitol, or cyclohexanol

Enthalpies of dilution of aqueous systems containing trioxane, and trioxane with each of mannitol, inositol and cyclohexanol have been determined at 25°C. The data have been treated in terms of the Savage-Wood additivity principle and, in conjunction with literature data on ether compounds a revised estimate of the pairwise group-interaction enthalpy for CH₂/–O– and a first estimate for CHOH/–O– are presented. Systems of saccharides and saccharides with alcohols do not conform to the Savage-Wood principle and an explanation in terms of the specific hydration properties of saccharides is offered.     

Enthalpy Change of Allosteric Transition in Human Haemoglobin A

Oxygen equilibria of haemoglobin were analysed according to a binding isotherm proposed by Amire ( Bull. Chem. Soc. Jpn. 1994, 67, 7 )¹ to obtain the intrinsic oxygen association constants to the molecule. Two sets of binding sites in haemoglobin were identified, which were ascribed to R₂ and T forms of the molecule. The average intrinsic association constants determined as a function of temperature gave a heat of oxygenation of‐76 ± 4 kJ mol^?;1 (tetramer). A microcalorimetrically determined heat of deoxygenation of oxyhaemoglobin by dithionite gave ?267 ± 10 kJ mol^?1 (tetramer). From these results, the heat of allostery of ?234 ± 24 kJ mol^?1 for haemoglobin tetramer was obtained, yielding allosteric energy per salt bridge of‐29 ± 3 kJ. This result suggests that salt‐bridge may, in fact, be thermochemically equivalent to hydrogen bonds.     

Enthalpies of absorption and solubility of CO2 in aqueous solutions of methyldiethanolamine

A flow mixing unit using a SETARAM C-80 calorimeter, developed for measuring the enthalpy of solution of two fluids, has been used to measure enthalpies of absorption of carbon dioxide in a 30 wt.% aqueous solution of methyldiethanolamine (MDEA) at three temperatures 313.15, 353.15, 393.15 K and three pressures 2.0, 5.0, 10.0 MPa. We have established that the effect measured by calorimetry corresponds not only to the absorption of CO₂ in the aqueous solution but also to the vaporisation of water into the carbon dioxide depending on the temperature and the pressure of the experiment. The enthalpies measured by calorimetry were compared with those calculated from solubility measurements and a reasonable agreement within the accuracy of measurement and calculation was found.     

The enthalpy of fusion and degree of crystallinity of polymers as measured by DSC

Crystalline polymers are not in thermal equilibrium and thermodynamic parameters such as enthalpy of fusion as determined by differential scanning calorimetry from the area under the melting endotherm over a wide temperature range have not been measured under equilibrium conditions. Accordingly measurements of the degree of crystallinity based on the enthalpy of fusion reflect experimental conditions, are incorrect in that they do not usually agree with those determined by other analytical procedures, such as density and WAX scattering, particularly when measured at ambient temperatures. While this has been repeatedly pointed out procedures used to determine the fractional crystallinity of polymers based on the enthalpy of fusion continue to be widely used.Using the First Law method, the enthalpy of fusion and fractional crystallinity have been measured as a function of temperature for metallocene polyethylene and polyethylene terephthalate during heating. The crystallinity measured on the sample prior to heating is in good agreement with that determined by density and wide angle X-ray diffraction.     

光声量热法测定辅酶B12光解的焓变和体积变化

用时间分辨的光声量热法研究了浦防B12的光解反应，首次测定了脉冲激光诱导输酶B12光解反应的治变和反应体积变化，其值分别为131±12kJ·mol-1和6±1mL·mol-1，推测该反应体积变化可能与Co－C健断裂引起的柔性咕啉环构象变化有关．     

Enthalpies of dilution of urea solutions in six polar solvents at several temperatures

Using the isothermal displacement calorimeter, enthalpies of dilution are obtained for solutions of urea in water, methanol, ethanol, formamide, N,N-dimethyl-formamide, and dimethyl sulfoxide. The results are discussed in terms of solute-solvent and solute-solute interactions.Now Diane Beaumont.