Phase Diagram Behaviors for Rod/Plate Liquid Crystal Mixtures

We present a simple theory of rod/plate liquid-crystal mixtures in which the angle-dependent pair interactions are assumed to be of second-rank form. Calculations are reported for varying relative anisotropies of the “rods” and “plates”. Temperature vs. mole-fraction phase diagrams show successive isotropic (I) → uniaxial ( U) and U → biaxial ( B) transitions which are first- and second-order, respectively. For each pair of species there exists a special composition ( x _rod*) for which cooling of the isotropic phase leads directly (and continuously) to a biaxial liquid. As X _rod approaches X _rod* from either side, the first-orderness of the I → U transition (i.e. discontinuities in volume, order parameter, etc.) becomes vanishingly small. Furthermore the transition temperature of the rod-(plate-) solvent is found to be depressed when “doped” by not-too-anisotropic plates (rods) and elevated when doped by sufficiently anisotropic plates (rods). These behaviors are explained in terms of simple excluded volume considerations and compared with recent experimental data on rod/plate mixtures.     

A novel approach towards intermolecular stabilization of para-quinone methides. First complexation of the elusive, simplest quinone methide, 4-methylene-2,5-cyclohexadien-1-one

A novel approach towards the intermolecular stabilization of "simple" (i.e. methylene-unsubstituted) p-quinone methides (QMs) by their coordination to a transition-metal center is described. 4-Bromomethyl phenols, protected by a silyl group, were employed as the QM precursors and cis-chelating diphosphine Pd0 complexes were chosen as the metal precursors, since they have strong back-bonding interactions with the electron-poor QM moiety. Removal of the silyl protecting-group from the corresponding [LPd(benzyl)Br] complex (L=bisphosphine) with fluoride results in the spontaneous rearrangement of the unobserved zwitterionic Pd(II) complex into the QM-Pd0 complex. The feasibility of this approach was demonstrated in the synthesis of the structurally characterized Pd0 complex of BHT-QM (4), a biologically relevant metabolite of 2,6-di-tert-butyl-p-cresol, and the synthesis of the complex of 4-methylene-2,5-cyclohexadien-1-one (11), the simplest, and so far unobserved QM molecule. These complexes exhibit a remarkable thermal stability and do not react with alcohol or water. In both cases, the use of an appropriate incoming ligand allowed the release of the coordinated QM into the reaction media in which it was effectively trapped by added nucleophiles.     

DNA capture into a nanopore: interplay of diffusion and electrohydrodynamics

We present a detailed analysis of the process of voltage driven capture of DNA molecules by nanopores. We show that ionic current generates a nonuniform electric field that acts on both the DNA and on its counterions and that the response of DNA to the electric field is affected by its electroosmotic coupling to the mobile counterions. We calculate the voltage and molecular mass dependence of the radius of capture and of the capture rate in the diffusion limited regime. We argue that electroosmotic flow through the DNA coil is suppressed in the vicinity of the pore and present a tentative estimate of the capture rate in the barrier limited regime.     

Pd/C-catalyzed alkynylation of β-chloroacroleins

The first Pd/C-mediated Sonogashira coupling of β-chloroacroleins with terminal alkynes is described here. Pd/C–CuI–PPh₃ was found to be an efficient catalyst system for this coupling reaction. Using this economic and general process a variety of 4-alkynyl-2H-chromene-3-carbaldehydes and 5-alkynyl-2,3-dihydro benzo[b]oxepine-4-carbaldehydes were prepared in good yields.     

Inclusion of poly[bis(methoxyethoxyethoxy)phosphazene] into layered graphite oxide

The highly ionically conducting polymer poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) has been complexed with lithium triflate and intercalated between the layers of graphite oxide (GO). The resulting composite was characterized by powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and ionic conductivity measurements.     

Application of ion-impact energy measurement to electrospray ionization mass spectrometry of proteins and protein mixtures

We have used a normal metal-insulator-superconductor (NIS) microcalorimeter to measure the impact energy of protein ions produced by electrospray ionization (ESI) in a magnetic-sector mass spectrometer (MS). We have used these measurements to resolve spectral ambiguities and to analyze protein mixtures. Energy measurement may be useful for the direct MS analysis of complex biopolymer mixtures that normally would confound ESI-MS deconvolution algorithms.     

Enhancement of weak anti-localization signatures in the magneto-resistance of bismuth anti-dot thin films

Anti-dot array thin films of bismuth were prepared by the e-beam deposition of this semi-metal on nano-porous substrates. The magneto-resistance measurements of bismuth thin films deposited under identical conditions on various substrates displayed signatures from both classical magneto-resistance and weak anti-localization effects. The relative intensity of the two effects could be altered by the choice of the substrate, with the anti-dot array morphology suppressing the classical magneto-resistance, and enhancing the weak anti-localization contribution to the measured magneto-resistance. As a result, the weak anti-localization effect can be traced to higher magnetic field strengths and higher temperatures than is possible in non-patterned films, improving the accuracy of the parameters extracted from the data. PACS 73.20.Fz; 73.23-b; 73.61.At