Copolymerization of ethylene with polar monomers: chain propagation and side reactions. A DFT theoretical study using zwitterionic Ni(II) and Pd(II) catalysts

Calculations utilizing anionic substituted derivates of the cationic N(wedge)N--Ni(II) and Pd(II) diimine Brookhart complex have been carried out on the barriers of ethylene and acrylonitrile insertion into a M- methyl, propyl and CH(CN)Et bond for M = Ni, Pd. The possibility of side reactions such as chelate formation with the polar functionality and oligomerization of the active species after acrylonitrile insertion are explored. The diimine ring system N--N = -NR' 'CR(1)CR(2)NR' ' with R' ' = 2,6-C(6)H(3)(i-Pr)(2) and R(1),R(2) = Me was functionalized by adding one or two anionic groups (BF(3)(-), etc.) in place of i-Pr on the aryl rings or by replacing one Me diimine backbone group (R(1)) with BH(3)(-). The objective of this investigation is computationally to design catalysts for ethylene/acrylonitrile copolymerization that have activities that are comparable to that of the cationic Ni(II) diimine or at least the Pd(II) diimine Brookhart system for ethylene homopolymerization. Complexes that might meet this objective are discussed.     

Synthesis and reactivity of sulfido ruthenium carbonyl clusters

A survey of the structures and reactivity of ruthenium carbonyl cluster complexes containing sulfido ligands is presented and discussed.     

Circular dichroism spectroscopic studies reveal pH dependent binding of curcumin in the minor groove of natural and synthetic nucleic acids

For the first time, an interaction between the non-toxic, cancer chemopreventive agent curcumin and both natural and synthetic DNA duplexes has been demonstrated by using circular dichroism (CD) and absorption spectroscopy techniques. Upon addition of curcumin to calf thymus DNA, poly(dG-dC).poly(dG-dC) and poly(dA-dT).poly(dA-dT) solutions, an intense positive induced CD band centered around 460-470 nm was observed depending on the actual pH and Na+ ion concentration of the medium; no CD signal was obtained, however, with single stranded poly(dC). Interaction of curcumin with calf thymus DNA was observed already at pH 6.5 in contrast with poly(dG-dC).poly(dG-dC) which induces no extrinsic Cotton effect above a pH value of 5. The protonated, Hoogsteen base-paired structure of poly(dG-dC).poly(dG-dC) is necessary for curcumin binding while the alternating AT-rich polymer formed complexes with curcumin only at certain Na+ concentrations. Evaluation of the spectral data and molecular modeling calculations suggested that curcumin, this dietary polyphenolic compound binds in the minor groove of the double helix. The mechanism of the induced CD activity, the effects of the pH and Na+ ions on the ligand binding and conformation of the double helix are discussed in detail. As well as being an essentially new phenolic minor groove binder agent curcumin is also a promising molecular probe to study biologically important, pH and cation induced conformational polymorphisms of nucleic acids.     

Dimensional changes as a function of charge injection in single-walled carbon nanotubes

Motivated by the central importance of charge-induced dimensional changes for carbon nanotube electromechanical actuators, we here predict changes in nanotube length and diameter as a function of charge injection for armchair and zigzag nanotubes having different diameters. Density functional theory with periodic boundary conditions is used, which we show provides results consistent with experimental observations for intercalated graphites. Strain-versus-charge relationships are predicted from dimensional changes calculated with a uniform background charge ("jellium") for representing the counterions. These jellium calculations are consistent with presented calculations that include specific counterions for intercalated graphite, showing that hybridization between the ions and the graphite sheets is unimportant. The charge-strain relationships calculated with the jellium approximation for graphite and isolated single-walled nanotubes are asymmetric with respect to the sign of charge transfer. The dependence of nanotube strain on charge approaches that for a graphite sheet for intermediate-sized metallic nanotubes and for larger diameter semiconducting nanotubes. However, the strain-charge curves strongly depend on nanotube type when the nanotube diameter is small. This reflects both the dependence of the frontier orbitals for the semiconducting nanotubes on the nanotube type and the pi-sigma mixing when the nanotube diameter is small.     

Reversible Dimerization and Polymerization of a Janus Diradical To Produce Labile C−C Bonds and Large Chromic Effects

Conducting polymers can be synthesized by irreversible diradical monomer polymerization. A reversible version of this reaction consisting of the formation/dissociation of σ‐dimers and σ‐polymers from a stable quinonoidal diradical precursor is described. The reaction reversibility is made by a quinonoidal molecule which changes its structure to an aromatic species by forming weak and long intermolecular C?C single bonds. The reaction provokes a giant chromic effect of about 2.5 eV. The two opposite but complementary quinonoidal and aromatic tautomers provide the Janus faces of the reactants and products which produces the observed chromic effect. A reaction mechanism is proposed to explain the variety of final products starting with structurally very similar reactants. These reversible reactions, covering an unusual regime of weak covalent supramolecular bonding, yield products which might be envisaged as novel molecular and polymeric soft matter phases.     

Experiments on the propagation of plasma filaments

We investigate experimentally the motion and structure of isolated plasma filaments propagating through neutral gas. Plasma filaments, or "blobs," arise from turbulent fluctuations in a range of plasmas. Our experimental geometry is toroidally symmetric, and the blobs expand to a larger major radius under the influence of a vertical electric field. The electric field, which is caused by nabla B and curvature drifts in a 1/R magnetic field, is limited by collisional damping on the neutral gas. The blob's electrostatic potential structure and the resulting E x B flow field give rise to a vortex pair and a mushroom shape, which are consistent with nonlinear plasma simulations. We observe experimentally this characteristic mushroom shape for the first time. We also find that the blob propagation velocity is inversely proportional to the neutral density and decreases with time as the blob cools.     

10% Yb3+-Lu2O3 ceramic laser with 74% efficiency

We demonstrate laser oscillation at 1080 nm with more than 16 W of output power and with an optical-to-optical slope efficiency of up to 74% using a 10% Yb3+ doped Lu2O3 ceramic made by hot pressing. This represents the highest output power and efficiency obtained for a Yb3+ doped Lu2O3 ceramic and demonstrates the feasibility for power scaling.     

Gauge covariance of the Aharonov–Bohm phase in noncommutative quantum mechanics

The gauge covariance of the wave function phase factor in noncommutative quantum mechanics (NCQM) is discussed. We show that the naive path integral formulation and an approach where one shifts the coordinates of NCQM in the presence of a background vector potential leads to the gauge non-covariance of the phase factor. Due to this fact, the Aharonov–Bohm phase in NCQM which is evaluated through the path-integral or by shifting the coordinates is neither gauge invariant nor gauge covariant. We show that the gauge covariant Aharonov–Bohm effect should be described by using the noncommutative Wilson lines, what is consistent with the noncommutative Schrödinger equation. This approach can ultimately be used for deriving an analogue of the Dirac quantization condition for the magnetic monopole.