Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes

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.     

Intramolecular interaction effects and the structure of N2OS and N2O2: An Ab initio study

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.     

Development of an HPLC-NMR method for the determination of PAHs in soil samples - a comparison with conventional methods

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.     

Inside or outside a ligand cleft? Synthetic, structural, and kinetic inertness studies of zinc, cadmium, and mercury complexes of cross-bridged cyclam and cyclen

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.     

Investigation of the self-assembly of hydrophobic self-complementary ionic tetrapeptides

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.     

New class of β-aminoalcohol ligands derived from isosorbide and isomannide: application in hydrogen transfer reduction of prochiral ketones

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.     

Ethanol/water solutions as certified reference materials for breath alcohol analyzer calibration

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.     

Teacher preparation for a problem-solving curriculum in Singapore

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.     

Thermal conductivity of the PbGeO<Subscript>3</Subscript> and PbGe<Subscript>3</Subscript>O<Subscript>7</Subscript> glasses

This paper reports on a study of the thermal conductivity of the PbGeO₃ and PbGe₃O₇ glasses.