Electronic Coupling Parameter Research Articles

Bloch effective hamiltonian for the possibility of molecular switching in the ruthenium-bipyridylbutadiene-ruthenium system

Molecular compounds of topology S 1  φ  S 2 where S 1 and S 2 are interfacing sites and φ a connecting chain are interesting systems for molecular electronics. We show how the concept of Bloch effective hamiltonian can be used to extract the electronic coupling parameter between S 1 and S 2 sites. The application is performed on the model compound (NH 3) 5Ru-bipyridylbutadiene-Ru(NH 3) 5 in order to investigate theoretically the possibility of molecular switching.

Ammonium sulfate induced uncouplings of crayfish septate axons with and without increased junctional resistance

Cytoplasmic pH (pH i) of crayfish lateral giant axons was monitored using antimony microelectrodes placed near septate junctions and variations of internal pH was induced by short applications of ammonium sulfate in the perfusing bath of the preparation. This treatment produced a rapid cell alkalinization followed, after wash, by acidification rebound. Simultaneously, two successive phases of uncoupling of the septate junction occurred; they had the same time course as those of their associated pH i movements. Calculation of the electronic coupling parameters indicated that, during alkalinization, coupling was accompanied by an increased axonal membrane conductance (the intimate origin of which was beyond the scope of this study) and resulted from a shunt of the gap junctions; the resistance proper of the latter was unaffected; thus involvement of Ca 2+ was ruled out and uncoupling was only an indirect consequence of the electrotonic junction's network configuration. In contrast, and as expected from previous investigations, the junctional membrane resistance was increased during the second phase of cytoplamic acidification. Evidence that uncoupling can be brought about by a non-junctional membrane increased permeability raises questions about some of the criteria commonly used during investigations of electrotonic transmission.

Model for primary charge separation in reaction centers of photosynthetic bacteria

We present model calculations of the dynamics of primary electron transfer (ET) in reaction centers of photosynthetic bacteria. We obtain half times of [unk]1 ps and approximately 5 ps for the first two ET processes, in excellent agreement with experimental observations. Our model is based on (i) a theoretical framework capable of describing ET in the presence of strong electronic interstate resonance coupling and (ii) energy parameters extracted from recent experimental data and molecular orbital calculations. Our analysis suggests that (i) strong electronic interstate mixing is crucial to the rapidity and efficiency of irreversible ET; (ii) possibly five rather than three electronic states participate in the transient ET prior to the reduction in vivo of the quinone complex; and (iii) conventional ET theories, which rely on weak electronic interstate mixing, are unfit for describing ET in reaction centers of photosynthetic bacteria.

A new nodal expansion for classical plasmas

In calculating the equation of state for plasmas we find that diagrammatic expansions for the free energy become unwieldy at high density. At best, many terms must be retained in order to obtain meaningful results. We present a new expansion technique which can be applied to plasmas in the interiors of Jupiter and white dwarf stars. In such cases the older techniques are unsatisfactory because of the size of the ion coupling parameter. Our work yields expansions for which this parameter is supplanted by ion correlation functions, which can be supplied by external computations. In this paper we assume a two-species plasma of classical particles, thereby focusing on combinatorial techniques. The final result is a new nodal expansion in terms of ion correlation functions and an electron coupling parameter.

Effect of pressure on the Curie temperature of [formula omitted]Fe, [formula omitted]Co, [formula omitted]Fe and [formula omitted]Co alloys

The Curie temperatures of Pd Fe, Pd Co, Pt Fe and Pt Co alloys with Fe or Co concentrations between 1 and 10 at. % have been measured by thermal scanning of the 57Fe Mössbauer resonance under pressures up to 170 kbar. Systematically, the largest (positive) values of the logarithmic derivative, dln T e /dln V, are found in the most dilute alloys. The data are compared with models, which relate the variation of T c under pressure to changes in the impurity spin-conduction electron coupling parameter, J i , and to changes of the pure host metal susceptibility, χ h . The discrepancies between the experimental data and these models are discussed.

Zur Molekülbewegung in Flüssigkeiten und Lösungen I

The magnetic proton relaxation behaviour of free radical solutions of diethylether and diethoxyethane has been studied by a nuclear-electron double resonance method. The dielectric relaxation in the solvents has also been determined from a study of the frequency dependance of the complex dielectric constant. The motional properties of the spin-carrying molecules which govern the magnetic interactions between different spin species are described by the model of randomly diffusing hard spheres. The time dependence is assumed to result from a combination of the relative translational diffusion of the spheres and the random rotational motions of complexes of spheres. Expressions and curves are given for the nuclear electron coupling parameter of dynamic nuclear polarisation and its frequency dependence. The comparison with experimental data taken at four different frequencies corresponding to magnetic fields of 15, 175, 1070 and 3230 gauss confirms the absence of scalar interactions and yields limits for the predominating translational diffusion. Correlation times at different temperatures and distances of closest approach of spins are evaluated. The dielectric studies in ether yield a single relaxation time which is attributed to the orientation of the molecule as a whole. Diethoxyethane has a spectrum of dielectric relaxation times. The dielectric relaxation times and also their activation energies are smaller than the correlation times obtained by magnetic measurements. The viscosity is more closely related to the translational diffusion of molecules than to the dielectric relaxation time. The results are discussed in terms of the different mechanisms of motion.