On the significance of the anchoring group in the design of antenna materials based on phthalocyanine stopcocks and zeolite L

The synthesis of stopcocks based on zinc phthalocyanine for selective adsorption at the channel entrances of zeolite L is reported. The introduction of either an inert SiMe? moiety, an imidazolium cation or a reactive isothiocyanate (NCS) group allows attachment to the channel entrances of zeolite L through van der Waals interactions, electrostatic interactions, or covalent binding, respectively. Stopcocks that rely on van-der-Waals-driven adsorption require careful selection of the solvent used for the deposition onto the zeolite surface to avoid a nonspecific distribution of the molecules. Regarding the design of photonic antenna systems, a stopcock with a cationic tail was found to be the most convenient, based on the observation that efficient energy transfer from molecules located in the zeolite nanochannels is more readily obtained than in the other cases.     

Measurement and statistical analysis of single-molecule current-voltage characteristics, transition voltage spectroscopy, and tunneling barrier height

We report on the measurement and statistical study of thousands of current-voltage characteristics and transition voltage spectra (TVS) of single-molecule junctions with different contact geometries that are rapidly acquired using a new break junction method at room temperature. This capability allows one to obtain current-voltage, conductance voltage, and transition voltage histograms, thus adding a new dimension to the previous conductance histogram analysis at a fixed low-bias voltage for single molecules. This method confirms the low-bias conductance values of alkanedithiols and biphenyldithiol reported in literature. However, at high biases the current shows large nonlinearity and asymmetry, and TVS allows for the determination of a critically important parameter, the tunneling barrier height or energy level alignment between the molecule and the electrodes of single-molecule junctions. The energy level alignment is found to depend on the molecule and also on the contact geometry, revealing the role of contact geometry in both the contact resistance and energy level alignment of a molecular junction. Detailed statistical analysis further reveals that, despite the dependence of the energy level alignment on contact geometry, the variation in single-molecule conductance is primarily due to contact resistance rather than variations in the energy level alignment.     

Free Ca<Superscript>2+</Superscript> as an early intracellular biomarker of exposure of cyanobacteria to environmental pollution

Calcium functions as a versatile messenger in a wide variety of eukaryotic and prokaryotic cells. Cyanobacteria are photoautotrophs which have a great ecological impact as primary producers. Our research group has presented solid evidence of a role of calcium in the perception of environmental changes by cyanobacteria and their acclimation to these changes. We constructed a recombinant strain of the freshwater cyanobacterium Anabaena sp. PCC 7120 that constitutively expresses the calcium-binding photoprotein apoaequorin, enabling in-vivo monitoring of any fluctuation in the intracellular free calcium concentration of the cyanobacterium in response to any environmental stimulus. The “Ca²⁺ signature” is the combination of changes in all Ca²⁺ signal properties (magnitude, duration, frequency, source of the signal) produced by a specific stimulus. We recorded and analyzed the Ca²⁺ signatures generated by exposure of the cyanobacterium to different groups of environmental pollutants, for example cations, anions, organic solvents, naphthalene, and pharmaceuticals. We found that, in general, each group of tested chemicals triggered a specific calcium signature in a reproducible and dose-dependent manner. We hypothesize that these Ca²⁺ signals may be related to the cellular mechanisms of pollutant perception and ultimately to their toxic mode of action. We recorded Ca²⁺ signals triggered by binary mixtures of pollutants and a signal induced by a real wastewater sample which could be mimicked by mixing its main constituents. Because Ca²⁺ signatures were induced before toxicity was evident, we propose that intracellular free Ca²⁺ may serve as an early biomarker of exposure to environmental pollution.     

Measuring acetic acid dimer modes by ultrafast time-domain Raman spectroscopy

Acetic acid is capable of forming strong multiple hydrogen bonds and therefore different dimeric H-bonded structures in neat liquid phase and in solutions. The low frequency Raman spectra of acetic acid (neat, in aqueous solution and as a function of temperature) were obtained by ultrafast time and polarization resolved optical Kerr effect (OKE) measurements. Isotropic OKE measurements clearly reveal a specific totally symmetric mode related to the dimeric structure H-bond stretching mode. The effects of isotope substitution, water dilution and temperature on this mode were investigated. These results together with anisotropic OKE measurements and density functional theory calculations for a number of possible dimers provide strong evidence for the cyclic dimer structure being the main structure in liquid phase persisting down to acetic acid concentrations of 10 M. Some information about the dimer structure and concentration dependence was inferred.     

Alliance free and alliance cover sets

A defensive (offensive) k-alliance in Γ = (V,E) is a set S ⊆ V such that every υ in S (in the boundary of S) has at least k more neighbors in S than it has in V / S. A set X ⊆ V is defensive (offensive) k-alliance free, if for all defensive (offensive) k-alliance S, S/X ≠ ∅, i.e., X does not contain any defensive (offensive) k-alliance as a subset. A set Y ⊆ V is a defensive (offensive) k-alliance cover, if for all defensive (offensive) k-alliance S, S ∩ Y ≠ ∅, i.e., Y contains at least one vertex from each defensive (offensive) k-alliance of Γ. In this paper we show several mathematical properties of defensive (offensive) k-alliance free sets and defensive (offensive) k-alliance cover sets, including tight bounds on their cardinality.     

Eulerian-Lagrangian localized adjoint method: The theoretical framework

This is the second of a sequence of papers devoted to applying the localized adjoint method (LAM), in space-time, to problems of advective-diffusive transport. We refer to the resulting methodology as the Eulerian-Lagrangian localized adjoint method (ELLAM). The ELLAM approach yields a general formulation that subsumes many specific methods based on combined Lagrangian and Eulerian approaches, so-called characteristic methods (CM). In the first paper of this series the emphasis was placed in the numerical implementation and a careful treatment of implementation of boundary conditions was presented for one-dimensional problems. The final ELLAM approximation was shown to possess the conservation of mass property, unlike typical characteristic methods. The emphasis of the present paper is on the theoretical aspects of the method. The theory, based on Herrera's algebraic theory of boundary value problems, is presented for advection-diffusion equations in both one-dimensional and multidimensional systems. This provides a generalized ELLAM formulation. The generality of the method is also demonstrated by a treatment of systems of equations as well as a derivation of mixed methods. © 1993 John Wiley & Sons, Inc.     

Solution of general ordinary differential equations using the algebraic theory approach

The algebraic theory for numerical methods, as developed by Herrera [3–7], provides a broad theoretical framework for the development and analysis of numerical approximations. To this point, the technique has only been applied to ordinary differential equations with constant coefficients. The present work extends the theory by developing a methodology for equations with variable coefficients. Approximation of the coefficients by piecewise polynomials forms the foundation of the approach. Analysis of the method provides firm error estimates. Furthermore, the analysis points to particular procedures that produce optimal accuracy. Example calculations illustrate the computational procedure and verify the theoretical convergence rates.     

On the structure and growth rate of unstable modes to the Rossby‐Haurwitz wave

The normal mode instability study of a steady Rossby‐Haurwitz wave is considered both theoretically and numerically. This wave is exact solution of the nonlinear barotropic vorticity equation describing the dynamics of an ideal fluid on a rotating sphere, as well as the large‐scale barotropic dynamics of the atmosphere. In this connection, the stability of the Rossby‐Haurwitz wave is of considerable mathematical and meteorological interest. The structure of the spectrum of the linearized operator in case of an ideal fluid is studied. A conservation law for perturbations to the Rossby‐Haurwitz wave is obtained and used to get a necessary condition for its exponential instability. The maximum growth rate of unstable modes is estimated. The orthogonality of the amplitude of a non‐neutral or non‐stationary mode to the Rossby‐Haurwitz wave is shown in two different inner products. The analytical results obtained are used to test and discuss the accuracy of a numerical spectral method used for the normal mode stability study of arbitrary flow on a sphere. The comparison of the numerical and theoretical results shows that the numerical instability study method works well in case of such smooth solutions as the zonal flows and Rossby‐Haurwitz waves. © 2004 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2005     

Dielectric behavior of hexaferrites BaCo2−xZnxFe16O27

The dielectric constant (′) and dielectric loss (tan δ) for hexaferrites BaCo_2−xZn_xFe₁₆O₂₇ have been studied as a function of frequency (f), temperature (T) and composition (x). The experimental results indicate that ′ and tan δ above the relaxation frequency only decrease as the frequency increases and as the temperature decreases. Tan δ shows the dielectric relaxation at certain critical frequencies which rise as temperature increases. The activation energy for the dielectric relaxation (E_D), ′, and tan δ are found to be minimum for x = 0.8.