Structures of the gauche conformers of some substituted dimethyl ethers. Effect of adjacent atom lone pairs on methyl group asymmetry

The complete equilibrium structures of CH₃OCH₃ and of the gauche conformers of CH₃OCH₂F, HOCH₂F, CH₃OCH₂Cl and H₃OCH₂CN have been determined by ab initio gradient computation at the Hartree—Fock, double zeta-plus-polarization level. The very large asymmetries in C---H bond distances previously reported from microwave substitution structures are shown to be non-existent in the equilibrium structures and are presumably artifacts. Small differences, different in direction from those reported from the experiments and nearly an order of magnitude smaller in size, do exist. They reflect three factors: (1) a lengthening of a C---H bond which is trans to a lone pair on an adjacent atom, (2) a general shortening of C---H bonds originating at a carbon atom bearing a highly electronegative substituent, and (3) a specific interaction in which a C---X substituent shortens the nearly parallel C---H bond on the other methyl group. The last interaction, not previously reported, is mediated by withdrawal of electron density from the oxygen lone pair which is trans to both groups. Other structural features derived from the microwave studies are supported by the new results. p ]Inclusion of polarization functions in the basis set for oxygen is essential for correct determination of the COC angle and the dihedral angles. The dihedral angles of CH₃OCH₂F and HOCH₂F are not correctly determined by the computation even at this level, although the computed values are improved when d functions are used for oxygen and still more by use of two sets of oxygen d functions. Polarization functions on carbon or on fluorine have no effect on the computed torsional angles. There is no problem in computing the correct dihedral angles with the ---Cl or ---CN derivatives.     

A survey on discriminatory processor sharing

The Discriminatory Processor Sharing (DPS) model is a multi-class generalization of the egalitarian Processor Sharing model. In the DPS model all jobs present in the system are served simultaneously at rates controlled by a vector of weights {g_k > 0; k = 1,..., K }. If there are N_k jobs of class k present in the system, k = 1,..., K, each class-k job is served at rate . The present article provides an overview of the analytical results for the DPS model. In particular, we focus on response times and numbers of jobs in the system. This work is part of a French-Dutch Van Gogh research project funded by NWO (The Netherlands Organization for Scientific Research) and EGIDE under grant VGP 61-520. We also acknowledge the support of EuroNGI Network of Excellence. This work was done while U. Ayesta was an ERCIM Postdoc fellow at CWI.     

Optimization of spin-valve parameters for magnetic bead trapping and manipulation

Magneto-optic Kerr effect (MOKE) and magnetoresistance (MR) measurements were used to measure the switching characteristics of spin-valve (SV) arrays currently being developed to trap and release superparamagnetic beads within a fluid medium. The effect of SV size on switching observed by MOKE showed that a 1 μm×8 μm SV element was found to have optimal switching characteristics. MR measurements on a single 1 μm×8 μm SV switched with either an external applied magnetic field or a local magnetic field generated by an integrated write wire (current density ranging from 10⁶ to 10⁷ A/cm²) confirmed the MOKE findings. The 1 μm×8 μm SV low field switching was observed to be +8 and −2 mT with two stable states at zero field; the high field switching was observed to be −18 mT. The low switching fields and the large magnetic moment of the SV trap along with our observation of minimal magnetostatic effects for dense arrays are necessary design characteristics for high-force, “switchable-magnet,” microfluidic bead trap applications.     

4,7-Dimethoxy-1,10-phenanthroline: an excellent ligand for the Cu-catalyzed N-arylation of imidazoles

[reaction: see text] 4,7-Dimethoxy-1,10-phenanthroline (L) was found to be an efficient ligand for the copper-catalyzed N-arylation of imidazole with aryl iodides and bromides under mild conditions. A variety of hindered and functionalized imidazoles and aryl halides were transformed in good to excellent yields.     

PRESSURE-VELOCITY EQUILIBRIUM HYDRODYNAMIC MODELS

This article describes mathematical models for phase separated mixtures of materials that are in pressure and velocity equilibrium but not necessarily temperature equilibrium. General conditions for constitutive models for such mixtures that exhibit a single mixture sound speed are discussed and specific examples are described.