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排序方式: 共有229条查询结果,搜索用时 15 毫秒
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2.
John T. W. Lai Frankie W. Lau Damon Robb Peter Westh Gerda Nielsen Christa Trandum Aase Hvidt Yoshikata Koga 《Journal of solution chemistry》1995,24(1):89-102
The excess partial molar enthalpies, the vapor pressures, and the densities of dimethylsulfoxide (DMSO)–H2O mixtures were measured and the excess partial molar Gibbs energies and the partial molar volumes were calculated for DMSO and for H2O. The values of the excess partial molar Gibbs energies for both DMSO and H2O are negative over the entire composition range. The results for the water-rich region indicated that the presence of DMSO enhances the hydrogen bond network of H2O. Unlike monohydric alcohols, however, the solute-solute interaction is repulsive in terms of the Gibbs energy. This was a result of the fact that the repulsion among solutes in terms of enthalpy surpassed the attraction in terms of entropy. The data in the DMSO-rich region suggest that DMSO molecules form clusters which protect H2O molecules from exposure to the nonpolar alkyl groups of DMSO. 相似文献
3.
Antje Huefner Christa Hoeller Gottfried Reznicek 《Monatshefte für Chemie / Chemical Monthly》2004,9(2):447-459
The stereoselective syntheses of four unsaturated hydroxy fatty acids 13S,2E,4E,9E)- 13-hydroxy-2,4,9-tetradecatrienoic acid, (13S,9Z,11E)-13-hydroxy-9,11-tetradecadienoic acid, (13S,9E, 11E)-13-hydroxy-9,11-tetradecadienoic acid, and (13S,2E,4E,9E)-13-hydroxy-2,4,9,11-tetradecatrienoic acid, are described. Wittig reactions, regioselective oxidation of dialcohol 3, and diastereomerization were used. 相似文献
4.
The photochemistry of various Roussin's red ester compounds of the general formula Fe(2)(SR)(2)(NO)(4), where R = CH(3), CH(2)CH(3), CH(2)C(6)H(5), CH(2)CH(2)OH, and CH(2)CH(2)SO(3)(-), were investigated. Continuous photolyses of these ester compounds in aerated solutions led to the release of NO with moderate quantum yields for the photodecomposition of the ester (Phi(RSE) = 0.02-0.13). Electrochemical studies using an NO electrode demonstrated that 4 mol of NO are generated for each mole of ester undergoing photodecomposition. Nanosecond flash photolysis studies of Fe(2)(SR)(2)(NO)(4) (where R = CH(2)CH(2)OH and CH(2)CH(2)SO(3)(-)) indicate that the initial photoreaction is the reversible dissociation of NO. In the absence of oxygen, the presumed intermediate, Fe(2)(SR)(2)(NO)(3), undergoes second-order reaction with NO to regenerate the parent cluster with a rate constant of k(NO) = 1.1 x 10(9) M(-1) s(-1) for R = CH(2)CH(2)OH. Under aerated conditions the intermediate reacts with oxygen to give permanent photochemistry. 相似文献
5.
Mroginski MA Murgida DH von Stetten D Kneip C Mark F Hildebrandt P 《Journal of the American Chemical Society》2004,126(51):16734-16735
The chromophore structures in the parent states Pr and Pfr as well as in the photocycle intermediate Lumi-R of oat phytochrome phyA are determined by comparison of the experimental resonance Raman spectra with calculated Raman spectra that have been obtained by density functional theory calculations (B3LYP) using scaled force fields. The spectra were calculated for various tetrapyrrole geometries including more than twenty different methine bridge isomers. For the parent states Pr and Pfr the best agreement in terms of vibrational frequencies, isotopic shifts, and Raman intensities was achieved with the ZZZasa and ZZEssa geometry, respectively. For the first intermediate Lumi-R, the chromophore geometry is concluded to be the ZZEasa configuration. These finding imply that the primary step of the photoactivation of phytochrome is the Z/E isomerization of the C-D methine bridge double bond, whereas the single bond remains in the anti conformation. The subsequent transition to the physiologically active state Pfr includes a (partial) single bond rotation of the A-B methine bridge. 相似文献
6.
J. W. Einax O. Kampe D. Truckenbrodt 《Fresenius' Journal of Analytical Chemistry》1998,361(2):149-154
Partial least squares regression (PLS) as a method for multivariate data analysis has been applied to environmental data
of the German rivers Saale, Ilm and Unstrut. Main aspects of the study are to describe the relationships of the distribution
of metals between river water and river sediment using PLS. A simulation of the distribution of metals between the liquid
and solid phase by variation of some parameters (e.g. conductivity, DOC, dissolved oxygen, pH, phosphate and suspended matter)
is presented and compared with experimental results.
Received: 15 July 1997 / Revised: 6 November 1997 / Accepted: 7 November 1997 相似文献
7.
Bismuth Monoiodide, a Compound with Bi(O) and Bi(II) Bismuth monoiodide was synthesized in closed tubes from the elements as well as from Bi and HgI2 as a black coloured crystalline compound. With increasing temperature BiI passes two transitions. α-BiI is stable below 370 K and changes to β-BiI by a martensitic transition. γ-BiI is the stable modification above 564 K and decomposes at 585 K peritectically to BiI3 and a lower iodide. All three modification crystallize in the monoclinic space group C2/m. The structures (single crystal studies) of α-BiI and β-BiI are characterized by onedimensional infinite chains [Bi4I4] with covalent bonds but only weak interactions in between. The [Bi4I4]-chains are built up by two completely different Bi atoms. Bi(A) is only bonded to three Bi whereas Bi(B) has bonds to one Bi and four I. The average bond lengths are Bi? Bi = 304.5 pm and Bi? I = 313.7 pm respectively. The configuration of the Bi(A) atoms is typical for BiO and that one of the Bi(B) atoms is characteristic for Bi2+ with the electron configuration s2p1. Therefore, α-BiI and β-BiI are mixed valence compounds [BiOBi2+I4]. The structures are variants of the simple cubic polonium type of structure and differ in the stacking of connected units. The structures and their transitions, the possible configurations for monohalides BiX on principle as well as the energy balances of the disproportionation of Bi+ are discussed together in detail. 相似文献
8.
A simple and precise method was developed for the analytical and preparative reversed-phase HPLC separation of a mixture of epimeric pavine N-oxides containing 49.1% of (−)-thalimonine N-oxide A and 50.1% of (−)-thalimonine N-oxide B isolated from Thalictrum simplex L. (Ranunculaceae). A reversed-phase system with Nucleosil C18 analytical and preparative columns and ethanol-1.5% aqueous orthophosphoric acid (15:85) as the mobile phase was used. The epimeric pavine N-oxides were completely separated within 50 min. 相似文献
9.