1H NMR direct observation of enantiomeric exchange in palladium(II) and platinum(II) complexes containing N,N' bidentate aryl-pyridin-2-ylmethyl-amine ligands

The complexes [MCl(2)(kappa2-N approximately N')] (N approximately N' = 2-C(5)H(4)N-CH2-NHAr; Ar = 4-MeC(6)H(4), a; 2,6-Me(2)C(6)H(3), b; 4-MeOC(6)H(4), c; 4-CF(3)C(6)H(4), d; M = Pd, 1a-d; Pt, 2a-d) have been prepared and fully stereochemically characterized both in the solid state and in solution. Their behavior in DMSO-d6 solution is dependent on the substituents of the aryl group and on the metal. Complexes of palladium with substituents at the para position (1a, 1c, 1d) display a dynamic 1H NMR pattern when the solutions are heated. An enantiomeric exchange Slambda/Rdelta is suggested to explain such behavior. On the basis of the calculated negative DeltaS values, an associative mechanism involving the solvent is proposed. Under the same conditions, analogous complexes of platinum (2a, 2c, 2d) proved to be unstable, and release of the N approximately N' ligand was observed. Complexes 1b and 2b show temperature-variable 1H NMR spectra without any evidence accounting for enantiomeric exchange or decoordination. DFT calculations on models of 1a and 1b show that diastereomeric exchange Sdelta/Slambda is a process where the complex with the higher sterical hindrance, 1b, has a lower energy barrier.     

Diastereoselectivity in the self-assembly of As2L2Cl2 macrocycles is directed by the As-pi interaction

The As-pi interaction, in conjunction with reversible As-thiolate bond formation, is used to direct the self-assembly of dinuclear As2L2Cl2 (L = a dithiolate) macrocycles that exist as equilibrium mixtures of both syn and anti diastereomers. The diastereomeric excess of these self-assembly reactions is controlled in a predictable manner by prudent choice of different achiral, isomeric ligands. A general method for the preparation of As2L2Cl2 macrocycles is established, and strategies to control the diastereoselective self-assembly of regioisomeric macrocycles in solution and the crystalline state are described. A mechanism for the interconversion between diastereomers (a slow process on the NMR time scale) is suggested, and variable-temperature NMR spectroscopic data show that the diastereomeric excess (de) decreases with increasing temperature. anti-As2(L2,6)2Cl2 crystallizes in monoclinic space group P2(1)/n with a = 6.3949(13), b = 19.675(4), c = 10.967(2) A, beta = 106.817(3) degrees , and Z = 2. anti-As2(L1,5)2Cl2 crystallizes in monoclinic space group P2(1)/c with a = 6.813(4), b = 19.085(12), c = 10.277(6) A, beta = 107.788(10) degrees , and Z = 4. syn-As2(L1,4)2Cl2.CHCl3 crystallizes in triclinic space group P(-) with a = 19.313(4), b = 19.923(4), c = 24.508(5) A, alpha = 78.110(4) degrees , beta = 78.860(5) degrees , gamma = 89.183(5) degrees , and Z = 12. As2(L1,4)2Cl2.C6H6 crystallizes in monoclinic space group P2(1)/n with a = 10.3332(7), b = 34.375(2), c = 17.8593(12) A, beta = 98.9650(10) degrees , and Z = 8.     

Recent studies of the electrospray ionisation behaviour of selected drugs and their application in capillary electrophoresis-mass spectrometry and liquid chromatography-mass spectrometry

This review is concerned with recent studies of electrospray ionisation-mass spectrometry (ESI-MS) of selected small molecular mass drugs and their application in qualitative and quantitative analytical methods using the techniques liquid chromatography mass spectrometry (LC-ESI-MS) and capillary electrophoresis mass spectrometry (CE-ESI-MS). The publications reviewed are taken from the Web of Knowledge database for the year 2006. The drugs have molecular mass less than 1000 Da and are chosen according to selected drug classifications in which they give ESI signals primarily as [M+H]+ ions. The drug classifications are antibiotics/antibacterials, steroids, anti-tumour drugs, erectile dysfunction agents, anti-epileptic drugs, antiasthmatic drugs, psychoactive drugs and miscellaneous drugs. Details are given on the fragmentations, where available, that these ionic species exhibit in-source and in ion trap, triple quadrupole and time-of-flight mass spectrometers. Analytical methods for the detection and determination of these small molecular mass drug molecules are also discussed, where appropriate, under the particular drug classifications. Analytical information on, for example, sample concentration techniques, separation conditions, recoveries from biological media and limits of detection/quantitation (LODs and LOQs) are provided.     

Binaural interference and auditory grouping

The phenomenon of binaural interference, where binaural judgments of a high-frequency target stimulus are disrupted by the presence of a simultaneous low-frequency interferer, can largely be explained using principles of auditory grouping and segregation. Evidence for this relationship comes from a number of previous studies showing that the manipulation of simultaneous grouping cues such as harmonicity and onset synchrony can influence the strength of the phenomenon. In this study, it is shown that sequential grouping cues can also influence whether binaural interference occurs. Subjects indicated the lateral position of a high-frequency sinusoidally amplitude-modulated (SAM) tone containing an interaural time difference. Perceived lateral positions were reduced by the presence of a simultaneous diotic low-frequency SAM tone, but were largely restored when the interferer was "captured" in a stream of identical tones. A control condition confirmed that the effect was not due to peripheral adaptation. The data lend further support to the idea that binaural interference is affected by processes related to the perceptual organization of auditory information. Modifications to existing grouping-based models are proposed that may help account for binaural interference effects more successfully.     

Improved precision in measurements of acoustic impedance spectra using resonance-free calibration loads and controlled error distribution

Resonances and/or singularities during measurement and calibration often limit the precision of acoustic impedance spectra. This paper reviews and compares several established techniques, and describes a technique that incorporates three features that considerably improve precision. The first feature is to minimize problems due to resonances by calibrating the instrument using up to three different acoustic reference impedances that do not themselves exhibit resonances. The second involves using multiple pressure transducers to reduce the effects of measurement singularities. The third involves iteratively tailoring the spectrum of the stimulus signal to control the distribution of errors across the particular measured impedance spectrum. Examples are given of the performance of the technique on simple cylindrical waveguides.     

Locally Farkas-Minkowski linear inequality systems

This paper introduces thelocally Farkas-Minkowski (LFM) linear inequality systems in a finite dimensional Euclidean space. These systems are those ones that satisfy that any consequence of the system that is active at some solution point is also a consequence of some finite subsystem. This class includes the Farkas-Minkowski systems and verifies most of the properties that these systems possess. Moreover, it contains the locally polyhedral systems, which are the natural external representation of quasi-polyhedral sets. TheLFM systems appear to be the natural external representation of closed convex sets. A characterization based on their properties under the union of systems is provided. In linear semi-infinite programming, theLFM property is the more general constraint qualification such that the Karush-Kuhn-Tucker condition characterizes the optimal points. Furthermore, the pair of Haar dual problems has no duality gap.     

2,5‐Dichloro‐2,5‐dihydro­thio­phene 1‐oxide: determinaton of the complete stereochemistry

The title five‐membered heterocycle, C₄H₄Cl₂OS, adopts an envelope conformation with the S atom at the tip of the flap. All three ring substituents, viz. the sulfoxide O atom and the two Cl atoms, are cis to each other. The two C atoms α to the sulfoxide group are also bonded to chlorine. The electron‐withdrawing chlorine substituents give rise to weak C—H·O hydrogen bonds with the sulfoxide O atom of a symmetry‐related mol­ecule [H·O = 2.44 (2) and 2.61 (2) Å, C·O = 3.143 (3) and 3.302 (2) Å and C—H·O = 129.9 (19) and 135.1 (19)°]. There is also a possible weak C—H·Cl inter­action. Chains of mol­ecules held together by these weak inter­actions run parallel to the a axis.     

Secondary interactions in the iso­morphous compounds 2,6‐bis­(chloro­methyl)­pyridinium chloride and 2,6‐bis­(bromo­methyl)­pyridinium bromide

The title compounds, C₇H₈Cl₂N⁺·Cl⁻ and C₇H₈Br₂N⁺·Br⁻, are isomorphous. In the crystal packing, layers parallel to the ac plane are formed by a classical N⁺—H⋯X⁻ hydrogen bond (X = halogen) and two X⋯X contacts. A third X⋯X contact links the layers, and a fourth, which is however very long, completes a ladder‐like motif of halogen atoms. Hydro­gen bonds of the form C—H⋯X play at best a subordinate role in the packing.