Solvent isotope effects on the hydration of alkaline cations: H/D secondary isotope effects on electrostatic interactions

We investigate H/D secondary isotope effects on the binding energies of water alkaline cation complexes, Alk⁺(X₂O)_n (X = H, D; Alk = Li, Na, K; n = 1 ? 4), using the any particle molecular orbital approach. Our results reveal that deuteration reduces water’s capacity to solvate alkaline cations. An explanation to this behaviour is proposed in terms of the observed changes in distances, partial charges, electrostatic potentials and polarisation induced by deuteration.     

High dynamic range imaging method for interferometry

We demonstrate a method to easily and quickly extend the dynamic range imaging capabilities of the camera in a typical interferometric approach. The camera dynamic range is usually low and limited to 256 gray levels. Also, it is well known that one may have over or under-exposed regions in the interferogram (due to non-uniform illumination) which makes these image regions not reliable. In our proposed method it is not necessary to obtain or use the non-linear camera response curve in order to extend the camera dynamic range. We obtain a sequence of differently exposed interferograms, typically five or six; after that, we compute the corresponding normalized fringe patterns and modulation maps using a typical normalization method. These normalized patterns are combined through a temporal weighted average using as weights the corresponding modulation maps. We show a set of experimental results that prove the effectiveness of the proposed method.     

Predicting criticality and dynamic range in complex networks: effects of topology

The collective dynamics of a network of coupled excitable systems in response to an external stimulus depends on the topology of the connections in the network. Here we develop a general theoretical approach to study the effects of network topology on dynamic range, which quantifies the range of stimulus intensities resulting in distinguishable network responses. We find that the largest eigenvalue of the weighted network adjacency matrix governs the network dynamic range. When the largest eigenvalue is exactly one, the system is in a critical state and its dynamic range is maximized. Further, we examine higher order behavior of the steady state system, which predicts that networks with more homogeneous degree distributions should have higher dynamic range. Our analysis, confirmed by numerical simulations, generalizes previous studies in terms of the largest eigenvalue of the adjacency matrix.     

Simultaneous effects of hydrostatic pressure and applied electric field on the impurity-related self-polarization in GaAs/Ga1−xAlxAs multiple quantum wells

A detailed theoretical study of the combined effects of hydrostatic pressure and in-growth direction applied electric field on the binding energy and self-polarization of a donor impurity in a system of GaAs-(Ga,Al)As coupled square quantum wells is presented. The study is performed in the framework of the effective mass and parabolic band approximations and using a variational procedure. The electron effective mass, the dielectric constant, the barrier height, the well sizes, all them varying with the hydrostatic pressure are taken into account within the study. The results obtained show that the impurity binding energy and its self-polarization bear strong dependencies with the hydrostatic pressure, the strength of the applied electric field, the width of the confining potential barriers, and the impurity position.     

Magnetic phase diagram simulation of La1−xCaxMnO3 system by using Monte Carlo, Metropolis algorithm and Heisenberg model

This work presents a critical temperature study of La_1−xCa_xMnO₃ manganites in bulk by means of Monte Carlo method thermal activated magnetic properties. The analysis was carried out for stoichiometries in the range of 0≤x≤1. The model is based on a three-dimensional classical Heisenberg-Hamiltonian involving the presence of Mn^3+eg, Mn^3+eg′ and Mn⁴⁺ ions, and their nearest neighbor interaction. For this modeling, simple cubic lattice samples of size L³, with L=6, 15 and 30 were used. The values of exchange parameters were determined by using LaMnO₃ (x=0), La_0.5Ca_0.5MnO₃ and CaMnO₃ (x=1) phases. Relationships between exchange parameters and anisotropy constants for different hole densities were found. Results of transition temperatures for each phase showed good agreement with experimental reports, especially for L=30 and L→∞.     

Weighted percolation on directed networks

We present and numerically test an analysis of the percolation transition for general node removal strategies valid for locally treelike directed networks. On the basis of heuristic arguments we predict that, if the probability of removing node i is p(i), the network disintegrates if p(i) is such that the largest eigenvalue of the matrix with entries A(ij)(1-p(i)) is less than 1, where A is the adjacency matrix of the network. The knowledge or applicability of a Markov network model is not required by our theory, thus making it applicable to situations not covered by previous works.     

ChemInform Abstract: First-Principles Study of Defective and Nonstoichiometric Sr2FeMoO6.

The influence of disorder and stoichiometry-breaking point defects on the structural and magnetic properties of Sr₂FeMoO₆ is studied by electronic structure calculations within the spin-polarized GGA+U approach.     

Theoretical study of the mechanism of the Wittig reaction: Ab initio and MNDO-PM3 treatment of the reaction of unstabilized,semistabilized, and stabilized ylides with acetaldehyde

In this study, we describe the results of ab initio (HF and MP2) and MNDO-PM3 calculations on the model reactions of unstabilized (Me₃P=CH–CH₃), semi-stabilized (Me₃P=CH–C≡CH), and stabilized (Me₃P=CH–C≡N) ylides with acetaldehyde to form their respective Z and E olefins and trimethylphosphine oxide. These reactions occur in three stages: oxaphosphetane formation, oxaphosphetane pseudorotation, and oxaphosphetane decomposition. The calculated barriers for these processes vary considerably depending on the level of theory employed (ab initio vs. MNDO-PM3 or HF vs. MP2 at the ab initio level). However, self-consistent geometries of reactants, intermediates, transition states and products are obtained at all levels. Oxaphosphetane formation is best described as very asynchronous cycloaddition (borderline two-step mechanisms). The geometries of the transition states are near planar with respect to P, C, C, and O atoms. Analysis of the bond indices of these reactions shows that the C–C bonds are between 44% (unstabilized case) and 60% (stabilized case) formed, whereas the corresponding P–O bonds have not been formed to any significant degree. Oxaphosphetane decomposition can be described as a very asynchronous retrocycloaddition where P–C bond breakage runs ahead of C–O bond breakage. These results are compared with experimental findings for the Wittig reaction, and its relevance to the overall mechanism of the olefination is discussed. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 557–569, 1997