Langevin-schroedinger formulation of electronic tunneling through a molecular bridge with a dissipative acceptor

Modeling electronic tunneling through molecular bridges is desired in order to understand the mechanism of long-range electron transfer reactions in nature, as well as for the design of novel molecular electronics devices. Particularly interesting is the effect of the nuclear motion at the molecular bridge on the electron transfer mechanism and rate. In this work we study the effect of electronic nuclear coupling at the molecular bridge on a unidirectional electronic tunneling process from an electron donor into a dissipative acceptor, as may appear in controlled electron transfer reactions at biological membranes, or in heterogeneous electron transfer reactions. The model includes a collection of harmonic bath modes coupled to the dissipative acceptor site and a single mode at the molecular bridge. The parameters of the dissipative bath are tuned such that the electronic population decays from the donor to the acceptor. This process is simulated using a time-dependent nonlinear Langevin-Schroedinger equation, based on a mean-field approximation for the electronic-nuclear coupling at the acceptor site and a numerically exact treatment of the electronic-nuclear coupling at the molecular bridge. The simulations at zero temperature and weak electronic-nuclear coupling demonstrate that electronic tunneling is promoted by coupling to the nuclear mode at the bridge. This result is consistent with our previous studies of electronic tunneling oscillations in a symmetric donor-bridge-acceptor complex, and it emphasizes the importance of electronic nuclear coupling in analyzing long-range electron transfer processes through molecular bridges or wires.     

Statistical properties of the rWG(J) index of agreement.

L. R. James, R. G. Demaree, and G. Wolf (1984) introduced rwg(J) to estimate interrater agreement for a group. This index is calculated by comparing an observed group variance with an expected random variance. As researchers have gained experience using this index, several questions have arisen. What are the consequences of replacing values beyond the unit interval by 0? What is the dependence of rwg(J) on the group size? The authors' simulations show that a positive bias is caused by the truncation, but for large population values of rwg(J) it is negligible. Also, in this case, the group size has no effect on the expected value of rwg(J). For inference on rwg(J), researchers can exploit the availability of computers to simulate data from the hypothesized distribution and then compare the simulation results for rwg(J) with the actual values. In addition, it is shown how the bootstrap method can be used for comparing the indices of 2 groups. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Enantiomeric Control of Intrinsically Chiral Nanocrystals

The chiral aspect of inorganic crystals that crystallize in chiral space groups has been largely ignored until recently, partly due to difficulties in characterizing the chiroptical properties of bulk crystals, and also due to the difficulty in separating (sub)micrometer-scale chiral crystal enantiomers. In recent years, the colloidal synthesis of intrinsically chiral nanocrystals (NCs) of several chiral inorganic compounds with significant enantiomeric excess has been demonstrated. This is achieved through the use of chiral molecular ligands, which bind to the atomic/ionic components of the crystals, preferentially forming one crystal enantiomorph. Here, recent progress on several aspects of these NCs is described, including the connection between ligand structure and its ability to direct NC handedness, chiral amplification in the synthesis leading to enantiopure NC samples, spontaneous symmetry breaking, the formation of NCs with chiral shapes, the connection between lattice and shape chirality and mixed contributions of atomic-scale and shape chirality to the chiroptical properties.     

Disciplined and free-spirited: ‘Time-out behaviour’ at the Agile conference

In this article we observe and try to understand a peculiar duality in the agile community, whereby on the one hand, we see a serious professional community working hard to improve the quality of software products and submitting to the strictest discipline of high professional standards, while on the other hand, in its conferences, we see the same community adopting a playful free-spirited stance. Invoking an anthropological perspective, we propose that both the serious professional aspects and the playful free-spirited atmosphere at the conference, as well as the connection between the two, can all be seen to emerge from the fundamental principles of the agile community as expressed by its Manifesto.     

Case-based Sentencing Using a Tree of Legal Concepts

The Judge's Apprentice is a case-based decision support system implemented and intended for use in Israeli criminal law to aid sentencing in cases of either robbery or rape. The system uses a sentencing tree, which is a hierarchical classification of 371 legal concepts relevant to criminal sentencing. Each leaf in this tree represents an index, which can be input for any case. The indexes are important for determining index similarity in the retrieval process and for computation of the proposed sentence. After retrieval of suitable cases and selection of the best case, we use a case-based quantitative evaluation as a formula for computing a verdict for the case at hand.     

Modeling Hydrogen Sulfide Emission Rates in Gravity Sewage Collection Systems

The factors affecting sulfide buildup in gravity sewers are complex, consisting of biological and physical processes, both in the aqueous and the gas phases of the sewer. The rate of each of these processes varies (among other parameters) according to flow characteristics, temperature, and pH. Under fast and turbulent flow conditions, the stripping of hydrogen sulfide into the gas phase may become the dominant process. The paper presents a semiempirical approach to the problem of quantifying hydrogen sulfide emission rates in sewers. Kinetics of hydrogen sulfide emission as a function of hydraulic parameters was measured in the laboratory using methods adopted from flocculation theory. A flocculation unit was used to impart a selected velocity gradient (G) into the water, and sulfide concentration was measured with time. The process was repeated for a number of G values. Regression analysis was then used to fit the rate of hydrogen sulfide emission equation against G. An equation was developed linking G to HL (head loss) in sewers assuming plug flow conditions. The hydraulic model and the kinetic model were linked (via G) to give the desired rate equation for hydrogen sulfide emission along a sewer line. The model was used to predict H2S emission from a uniform flow sewer and the effect of parameters such as pH, sewer slope and degree of fullness was studied. As expected, results show that low pH, high slope, and low degree of fullness enhance emission rates. Reasonable agreement was attained when the model output was compared with measured results from a field test sewer in Virginia, South Africa, under conditions where sulfide stripping was the rate-dominant process.