Reactions of solute species at an electrode modified with titanocene functionalized polypyrrole film: ferrocene and titanocene dichloride

We have studied for the first time the ability of a conducting polymer film, p(Tc3Py), representing a polypyrrole matrix with covalently attached titanocene dichloride (TcCl₂) centers, to serve as an intermediator for the electron charge transport between the electrode and the reaction sites of solute reactants. The standard potential of the first of these electroactive species, ferrocene (Fc), is in the range where the polymer matrix is in its slightly oxidized state so that solute Fc species give a reversible response at the surface of this modified electrode. Another solute reactant, TcCl₂, was studied in solutions in which it demonstrates a (quasi)reversible behavior at bare electrode surfaces, THF+TBAPF₆ and AN+TEACl. The standard redox potential of this species belongs to the range of the electroactivity of immobilized TcCl₂ centers (where the matrix is in its non-conducting state) so that the electron charge has to be transported via stepwise redox reactions between neighboring centers inside the film. The combination, solute reactant+film, results in a greater CV current compared to the response of the film in background solution or of the solute species at the bare electrode surface. This current for THF solution even exceeds the sum of separate currents for the film and the reactant. This finding is attributed to a catalytic effect of solute species as redox intermediators for the transformation of immobilized electroactive centers leading to a greater degree of the film reduction. The presence of solute TcCl₂ species results in a much greater stability of immobilized centers (compared to the corresponding reactant-free solution), both in the course of CV with the passage of the range of their response and in experiments with the film holding at the potential within this range. This holding leads to an almost constant current related to the reaction of solute species at the film/solution interface. Our estimate shows that immobilized centers undergo above 10,000 reversible transformations (without an observed tendency to the degradation) to ensure the passage of this current. The conclusion has been drawn that immobilized TcCl₂ centers are able to serve as sufficiently stable redox intermediators for the electron charge transport across the film, a prerequisite for the catalytic applications of such films.Abbreviations AN acetonitrile - THF tetrahydrofuran - Cp cyclopentadienyl, C₅H₅ - Cp cyclopentadienyl radical, C₅H₄ - Fc ferrocene, Cp₂Fe - TcCl₂ titanocene=bis(cyclopentadienyl)titanium dichloride, Cp₂TiCl₂ or its radical CpCpTiCl₂ - PPy polypyrrole - Tc3Py titanocene-propyl-pyrrole, Cl₂TiCpCp(CH₂)₃NC₄H₄ - p(Tc3Py) polymer obtained from Tc3Py - TBAPF₆ tetrabuthylammonium hexafluorophosphate - TEACl tetraethylammonium chlorideDedicated to Zbigniew Galus on the occasion of his 70th birthday.     

Nuclear magnetic resonance-compatible furnace for high temperature MR imaging and spectroscopy in situ

A high temperature magnetic resonance compatible furnace for real time in situ monitoring of materials, processes, and chemical reactions with magnetic resonance imaging and spectroscopy is described. Design issues are analyzed. Example applications are demonstrated with a time sequence of proton images of the binder burnout in a porous green ceramic cylinder containing polyethylene glycol binder at 200 degrees C, and 7Li images of the molten salt LiCl at 700 degrees C.     

Classical force field parameters for the heme prosthetic group of cytochrome c

Accurate force fields are essential for describing biological systems in a molecular dynamics simulation. To analyze the docking of the small redox protein cytochrome c (cyt c) requires simulation parameters for the heme in both the reduced and oxidized states. This work presents parameters for the partial charges and geometries for the heme in both redox states with ligands appropriate to cyt c. The parameters are based on both protein X-ray structures and ab initio density functional theory (DFT) geometry optimizations at the B3LYP/6-31G* level. The simulations with the new parameter set reproduce the geometries of the X-ray structures and the interaction energies between water and heme prosthetic group obtained from B3LYP/6-31G* calculations. The parameter set developed here will provide new insights into docking processes of heme containing redox proteins.     

Non-Newtonian behavior in simple fluids

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheology of a microscopic sample of simple fluid. The calculations were performed using a configurational thermostat which unlike previous nonequilibrium molecular dynamics or nonequilibrium Brownian dynamics methods does not exert any additional constraint on the flow profile. Our findings are in agreement with experimental results on concentrated "hard sphere"-like colloidal suspensions. We observe: (i) a shear thickening regime under steady shear; (ii) a strain thickening regime under oscillatory shear at low frequencies; and (iii) shear-induced ordering under oscillatory shear at higher frequencies. These results significantly differ from previous simulation results which showed systematically a strong ordering for all frequencies. They also indicate that shear thickening can occur even in the absence of a solvent.     

Efficient synthesis of tris(4-imidazolyl)methanol derivatives

Biomimetic tris(4-imidazolyl)carbinol derivatives are prepared from imidazole in a short, high-yielding sequence via sulfonamide 1, which is converted to the 2-silylated carbinol 2 by one-pot, sequential 2-functionalization and then 4(5)-functionalization. Alcohol 2 can be transformed either to the parent carbinol 3 or to a desilylated sulfonamide derivative 4. The tripodal alcohol 3 is a convenient precursor to ethers by solvolysis and to metal complexes, as illustrated by the preparation of a bis-tripod complex with iron(III).     

Self-healing femtosecond light filaments

A recent experiment [Appl. Phys. Lett. 83, 213 (2003)] indicated that filaments created in femtosecond high-power pulses propagating in air are surprisingly robust when interacting with microscopic water droplets. We present numerical modeling of the dynamics of the filament-droplet interaction. Our simulation results provide further insight into the interplay between the filament's core and the wide transverse pedestal of the pulse. It is shown that the robustness of the filament comes from the transverse low-intensity pedestal that controls the formation of the central hot spot. Implications for penetration of wide, high-power beams through obscurants are discussed.     

Characterization and optimization of magnetron sputtered Sc/Si multilayers for extreme ultraviolet optics

Scandium/silicon multilayers have been deposited by magnetron sputtering and characterized by several techniques. Experimental peak reflectances of 0.22 and 0.37 have been measured respectively at wavelengths of 40 nm and 46 nm, for 10° incidence angle. The corresponding theoretical values for a perfect Sc/Si structure are respectively 0.38 and 0.57. In order to explain these differences between calculated and measured reflectivity, thin film and multilayer characterizations have been done. Effects of multilayer imperfections on the reflectivity have been estimated independently by means of simulation. Based on these results, a new design of Sc/Si multilayer is proposed with top layer thickness optimization. With this design, the experimental peak reflectance reaches 0.46 at a wavelength of 46 nm. PACS 78.67.Pt; 78.66.-W; 81.15.Cd