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排序方式: 共有133条查询结果,搜索用时 31 毫秒
1.
Grotjahn DB Van S Combs D Lev DA Schneider C Incarvito CD Lam KC Rossi G Rheingold AL Rideout M Meyer C Hernandez G Mejorado L 《Inorganic chemistry》2003,42(10):3347-3355
Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions. 相似文献
2.
Charles Bock Mendel Trachtman Ann Schmiedekamp Philip George Tin S. Chin 《Journal of computational chemistry》1983,4(3):379-389
An ab initio study of O?N? N?S with full geometry optimization has been carried out to corroborate the presence of an interaction between the terminal atoms in this type of structure, which, in O?N? N?O, apparently stabilizes the cis conformer. Using the unscaled 4–31G basis set with a full set of d functions on the sulfur, there is a potential minimun at the trans but not the cis geometry. A gauche conformer with a torsional angle of 77.2° is the most stable. With N2O2 this basis set gives potential minima at both the cis and trans geometries, but the trans conformer is slightly more stable, contrary to experiment and the results of (7,3) basis-set calculations reported in the literature in which Gaussian lobe functions were employed. Using a (9,5) basis set there is no longer a potential minimum at the cis geometry, and a gauche structure is more stable than the cis conformer as in the case of N2OS with the less-extended basis set. Force constants (harmonic and anharmonic), compliance constants, relaxed force constants, and interaction-displacement coordinates for both molecules are compared for key structural elements. 相似文献
3.
4.
Weisshoff H Preiss A Nehls I Win T Mügge C 《Analytical and bioanalytical chemistry》2002,373(8):810-819
A hyphenated HPLC-(1)H NMR method for the identification and quantification of PAHs in soil samples has been developed and applied to a PAH reference sample provided by the Federal Institute for Materials Research and Testing (BAM, Berlin, Germany). The results were compared with those obtained by HPLC-DAD, HPLC-F, and GC-MS analyses of the same sample. 相似文献
5.
Niu W Wong EH Weisman GR Hill DC Tranchemontagne DJ Lam KC Sommer RD Zakharov LN Rheingold AL 《Dalton transactions (Cambridge, England : 2003)》2004,(21):3536-3547
Ethylene cross-bridging of the popular tetraazamacrocyclic ligand cyclam has led to metal complexes with enhanced kinetic inertness. The synthesis and spectral characterization of zinc(II), cadmium(II), and mercury(II) complexes of cross-bridged cyclam (L1) as well as cross-bridged cyclen (L2) are reported along with the details of our synthetic route to L2. X-ray structural studies revealed that all Zn(II) and Cd(II) cations are fully kappa(4)N-coordinated inside the respective ligand's molecular cleft with L1 providing the better fit for Zn(II). While Hg(II) is similarly coordinated to L2, it has been found to complex L1 outside the ligand cleft in a novel exo-kappa(2)N-mode. Solution NMR data of the kappa(4)N complexes are consistent with the presence of only a single cis-folded isomer in each case. Ligand (1)H and (13)C coupling to both (111,113)Cd and (199)Hg in their complexes can be clearly discerned. The relative kinetic inertness of representative cross-bridged complexes in acidic aqueous solution has been assessed and found to be in the following order: Zn(II) > Cd(II)[dbl greater-than] Hg(II). The data also reaffirm that cross-bridged cyclam ligand L1 forms a substantially more inert complex with zinc(II) than either the smaller cyclen analogue L2 or the unbridged 1,4,8,11-tetramethyl-cyclam L3. 相似文献
6.
A. Desii C. Duce L. Ghezzi S. Monti R. Solaro M. R. Tiné 《Journal of Thermal Analysis and Calorimetry》2009,97(3):791-796
The self-aggregation of two short ionic tetrapeptides (RWDW and RVDV, where R = arginine, V = valine, W = tryptophan, D = aspartic acid) was studied by different experimental techniques such as, atomic force microscopy (AFM), light scattering (LS), isothermal titration calorimetry (ITC) and viscosimetry. AFM was used to investigate the morphology of the aggregates; the AFM images showed the presence of fibrillar aggregates whose number and length increased with increasing oligopeptides concentration. On the other hand, the collected experimental calorimetric and physical chemical results consistently indicated a critical aggregation concentration (cac) around 0.5 and 2.0 mM for RWDW and RVDV respectively. 相似文献
7.
Tin Thanh Le 《Tetrahedron》2010,66(46):8893-8898
Starting from isosorbide and isomannide, two by-products from the starch industry, a family of chiral functionalized β-aminoalcohols presenting a THF ring has been synthesized as potential ligands for hydrogen transfer reduction of prochiral ketones. Under optimal conditions, more than 70% ee with an excellent conversion were obtained for the HTR of the acetophenone. 相似文献
8.
Rosemarie Philipp Olaf Hanebeck Sebastian Hein Wolfram Bremser Tin Win Irene Nehls 《Accreditation and quality assurance》2010,15(3):141-146
The Federal Institute for Materials Research and Testing (BAM), Germany, has issued a series of large volume ethanol in water
certified reference materials (CRMs), primarily developed for the calibration of evidential breath alcohol analyzers in Germany.
The certified parameter is the ethanol mass concentration at 20 °C. When used in a wet bath simulator, the solutions deliver
gas samples that meet the requirements set by the Organization of Legal Metrology for calibration of breathalyzers. The materials
were prepared gravimetrically by spiking of ethanol into water in single 5 L units. A complete uncertainty budget for the
preparation process has been established. The purity of the commercial ethanol stock solution was identified to be the main
source of uncertainty. For stability and homogeneity measurements and for the verification of the gravimetric mass concentration
of the CRMs, a robust high-precision gas chromatography, with flame-ionization detection method for ethanol determination
in aqueous samples was developed and validated. The good performance of this method has been demonstrated in several international
comparisons organized by the Consultative Committee for Amount of Substance—Metrology in Chemistry at the International Bureau
of Weights and Measures. 相似文献
9.
Yew Hoong Leong Jaguthsing Dindyal Tin Lam Toh Khiok Seng Quek Eng Guan Tay Sieu Tee Lou 《ZDM》2011,43(6-7):819-831
Problem solving is at the heart of the Singapore Mathematics curriculum. However, it remains a challenge for teachers to realise this curricular goal in practice. Here, we review the efforts of Singapore mathematics teacher educators in incorporating problem-solving (teaching) competency in teacher education and PD programmes. We discuss conceptual and practical issues, actions taken and changes made in building teachers?? capacity to enact a problem-solving curriculum in a school-based design experiment project. In the project, teachers learnt problem solving, observed and then carried out lessons, using the ??Mathematics Practical????akin to the science practical??as key to instruction and assessment. 相似文献
10.
V. M. Denisov S. M. Tin’kova L. T. Denisova L. A. Irtyugo 《Physics of the Solid State》2011,53(10):2025-2027
This paper reports on a study of the thermal conductivity of the PbGeO3 and PbGe3O7 glasses. 相似文献