Influence of inter- and intramolecular hydrogen bonding on kemp decarboxylations from QM/MM simulations

The Kemp decarboxylation reaction for benzisoxazole-3-carboxylic acid derivatives has been investigated using QM/MM calculations in protic and dipolar aprotic solvents. Aprotic solvents have been shown to accelerate the rates of reaction by 7-8 orders of magnitude over water; however, the inclusion of an internal hydrogen bond effectively inhibits the reaction with near solvent independence. The effects of solvation and intramolecular hydrogen bonding on the reactants, transition structures, and the rate of reaction are elucidated using two-dimensional potentials of mean force (PMF) derived from free energy perturbation calculations in Monte Carlo simulations (MC/FEP). Free energies of activation in six solvents have been computed to be in close agreement with experiment. Solute-solvent interaction energies show that poorer solvation of the reactant anion in the dipolar aprotic solvents is primarily responsible for the observed rate enhancements over protic media. In addition, a discrepancy for the experimental rate in chloroform has been studied in detail with the conclusion that ion-pairing between the reactant anion and tetramethylguanidinium counterion is responsible for the anomalously slow reaction rate. The overall quantitative success of the computations supports the present QM/MM/MC approach, which features PDDG/PM3 as the QM method.     

KLM model for lossy piezoelectric transducers

A KLM model has been developed using equivalent circuits to analyze the effects of different loss mechanisms on the transducer performance in the time and frequency domain. Transducers from two different piezoelectric materials (lead-zirconate-titanate (PZT) and polyvinylidene fluoride) were constructed to validate our model. Experimental results are in good agreement with the theoretical simulation.     

A lattice dynamics model for M2XY6 systems in the Fm3m(Oh5) space group

 This work is about the development of a generalized dynamic model to account for both the short-range and the long-range interactions in M ₂ XY ₆-type crystals in cubic environments. The short-range vibrational interactions are handled using a mixed force field, whereas the long-range interaction terms are worked out by employing a revised version of the Ewald method. In both cases, new criteria are introduced so as to get a more realistic overall picture of the lattice dynamics for these crystals. As for the short-range vibrational terms, a criterion is included to minimize the cross terms in the potential-energy distribution and in this way to include the idea of a natural potential-energy distribution. The Coulombic interaction terms are worked out and a new convergence test is introduced to make sure that the series expansion in both the direct and the reciprocal spaces converge at the same speed and simultaneously, preserving the electroneutrality of the system. The current model has been applied to Cs₂UBr₆; we have fitted the k→=0→ vibrational frequencies as well as the generation of the phonon dispersion curves for different polarization directions. As a result, the model is shown to have some utility and flexibility to handle this kind of complex calculation and could be generalized to more complex systems, such as elpasolite-type crystals, to test the validity of the approximations and constrains involved. We anticipate that many questions are still open to discussion and the need for more complete and accurate data is emphasized throughout the course of the current research work. Received: 12 May 2001 / Accepted: 11 September 2001 / Published online: 22 March 2002     

The measurement of the spatial average temporal average intensity I(sata) and ultrasonic power W in composite ultrasonic transducers for medical application

Two possible methods to determine the spatial average temporal average intensity I(sata) and ultrasonic power W in composite ltrasonic transducers for medical application are described. Results showed that integrals using one method will yield accurate results but a vast amount of computational effort is required. On the other hand, using an alternative method these integrals may readily be solved at the expense of the accuracy of the results deduced.     

A Note on Distance Matrices Yielding Elementary Landscapes for the TSP

Symmetric and antisymmetric distance matrices in the single agent traveling salesman problem (TSP) are not the only distance matrices to generate elementary landscapes for swap and 2-opt neighborhoods.     

Isothermal Vapor–Liquid Equilibria for the Ternary System 2-Propanol + Tetrahydrofuran + 1-Chlorobutane at 25°C

Isothermal vapor–liquid equilibria (VLE) for mixtures containing 2-propanol + tetrahydrofuran + 1-chlorobutane have been measured using a modified version of a Boublik–Benson still at 25°C. A test of thermodynamic consistency, like the McDermott–Ellis method was applied to the activity coefficients. Excess molar Gibbs free energies were calculated over the entire range composition. Different expressions existing in the literature were used to predict activity coefficients.     

Control of cell growth direction by direct fabrication of periodic micro‐ and submicrometer arrays on polymers

In this work, we describe a laser‐assisted microstructuring technique called Direct Laser Interference Patterning to produce topographical cues for tumor cells in a one‐step process. Line‐like patterns with spatial periods ranging from 500 nm to 10 μm are fabricated on polyimide (PI) films. The resulting structures exhibit a well‐defined shape and quality even for patterns with small periodic distances. Subsequently, the behavior of mouse mammary adenocarcinoma cells over those structures is evaluated. The results show that cell growth is well aligned to the direction of the patterns (over 60% lying within 0° to 15° to either side of the surface lines) for all evaluated structure sizes. Moreover, cells grown on patterns with 500 nm spatial period are the most narrowly aligned (up to 80% found between 0° and 15°), showing the potential of the technique. The fabrication process of the PI patterns is supported by a mathematical model of the underlying photo‐chemical ablation process. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Synthesis of Benzo[b]thiophene Carboxamides Connected to 4‐Arylpiperazines through a Benzylic Spacer: Potential Ligands with 5‐HT1A Binding Affinity

New benzothiophene arylpiperazine derivatives 8 (a–f) were synthesized as potential serotoninergic agents with 5‐HT_1A receptor affinity. Preparation of the derivatives was performed by treating N‐[2‐(chloromethyl)phenyl]‐4,7‐dimethoxybenzo[b]thiophene‐2‐carboxamide (7) with a series of substituted 4‐arylpiperazines.     

Studies of Kinetics of Coalescence of Water‐in‐Furrial Crude Oil Emulsions by Confocal Microscopy

The kinetic of coalescence of water in Furrial crude oil emulsions (W/O) during the initial stage of demulsification process is showed through of high‐resolution image micrographics by using a confocal microscope. Furrial crude oil from Monagas state is a crude oil extra heavy with severe flocculation/aggregation problems. The kinetic of the initial stage in the coalescence process is critical and of great importance in the definition of the foreword behavior. This information allowed us to characterize the demulsification rate process in a Furrial crude oil. Total W/O emulsion separation was achieved about of 2 hours at 80ºC. The initial fast coalescence is characterized by a short binary coalescence time, which is followed by a large binary coalescence time. Our results demonstrate that the initial coalescence rate determinate the time necessary to achieve a total separation.