A Counterion‐Directed Approach to the Diels–Alder Paradigm: Cascade Synthesis of Tricyclic Fused Cyclopropanes

An approach to the intramolecular Diels–Alder reaction has led to a cascade synthesis of complex carbocycles composed of three fused rings and up to five stereocenters with complete stereocontrol. Computational analysis reveals that the reaction proceeds by a Michael/Michael/cyclopropanation/epimerization cascade in which size and coordination of the counterion is key.     

Electrolyte-dependent pairwise particle motion near electrodes at frequencies below 1 kHz

A model incorporating a phase angle between an applied electric field and the motion of particles driven by it explains electrolyte-dependent pairwise particle motion near electrodes. The model, predicting that two particles aggregate when this phase angle is greater than 90 degrees but separate when the phase angle is less than 90 degrees , was based largely on contrasting behavior in two electrolytes (KOH and NaHCO3) used with indium tin oxide (ITO) electrodes. The present contribution expands the experimental evidence for this model to KOH, NaHCO3, NaOH, NH4OH, KCl, and H2CO3 solutions with Pt, as well as ITO electrodes. The phase angle correlation was verified in all cases. Comparisons of the model predictions to experimental data show that the sign and order of magnitude of rates of change in the separation distances between particle pairs are correctly predicted.     

A note on the integration of competing psychophysical theories

Virtually from its inception psychophysics has been caught on the horns of the dilemma as to the proper form of the fundamental psychophysical law. This paper illustrates an integration of two competing theories.     

Designing hierarchical molecular complexity: icosahedra of addressable icosahedra

ABSTRACT

Clusters and shells based on icosahedral symmetry are characterised, constructed from component particles that retain memory of their neighbours in a specific reference structure. This memory provides the particles with ‘addressable’ characteristics, with a ground state corresponding to the structure where all the components are correctly addressed in terms of the local environment. The interparticle potentials have separate attractive and repulsive components, defined by the reference structure. For a single target structure, the relaxation efficiency is mostly determined by the variation of the attractive component for correctly and incorrectly addressed particles. The effects of varying the addressability can be visualised directly in terms of the underlying energy landscape, and follow quantitative predictions from catastrophe theory. A doubly-addressable landscape can be designed within the same framework. In well-defined regions of parameter space, the predicted global minima for aggregates of the target cluster (target monomer) form ‘superclusters’, which can be described in terms of multiple interacting copies of local minima for the monomer. The predicted lowest energy superclusters formed from aggregates of addressable icosahedral clusters and shells are themselves based on icosahedral packing. These hierarchical icosahedral structures could be realised experimentally if particles can be synthesised to match the interactions encoded in the addressable potentials.     

Continuum percolation conductivity exponents in restricted domains

Conductivity behavior of continuum percolation in restricted two-dimensional domains is simulated by considering systems of randomly distributed disks. The domain is restricted in that conducting objects are permitted to lie in only a portion of the domain. Such a restricted domain might better approximate some natural systems. Simulations of two-dimensional systems, based on three distributions of local conductances, are examined and found to demonstrate a power-law behavior with conductivity exponents smaller than those arising in regular lattice and continuum percolation     

On the Modulations and Nonlinear Interactions of Waves and the Nonlinear Stability of a Near-Critical System

The nonlinear interactions and modulations of an n-dimensional wave and of a disturbance to a near-critical system governed by a general (n + 1)-dimensional system of equations are studied by perturbation methods. It is found that these modulations are governed by an evolution equation which is either by itself or coupled to a second equation, depending on the nature of the long wave solutions of the corresponding linearized system. When a single evolution equation exists, its leading terms are shown to give the nonlinear Schrödinger equation. Water waves and near-critical plane Poiseuille flow are discussed as examples.     

Voltammetric measurement of arsenic in natural waters

There are several U.S. EPA approved methodologies for the determination of arsenic in ground water. Such technologies are lab-based, time intensive and can lead to a large capital cost for multi-sample analysis. In light of the number of sites found to contain arsenic at levels higher than the maximum contaminant level (MCL), on-site screening and monitoring systems are an attractive alternative. This review article summarizes several examples in the recent literature to illustrate the breadth of work in voltammetric analysis of arsenic in environmental samples. Also, included are recent voltammetric results, obtained with a microfabricated gold array and a field portable potentiostat, at an arsenic contaminated site in southern New Jersey.     

Field-theoretic simulations of polymer solutions: finite-size and discretization effects

In this work we analyze the finite-size and discretization effects that occur in field-theoretic polymer simulations. Following our previous work, we study these effects for a polymer solution in the canonical ensemble confined to a slit (with nonadsorbing walls) of width L, and focus on the behavior of two quantities: the chemical potential mu, and the correlation length xi. Our results show that the finite-size effects disappear for both quantities once the lateral size of the system L is larger than approximately 2xi. On the other hand, the chemical potential is dominated by the lattice discretization Deltax. The origins of this dependence are discussed in detail, and a scheme is proposed in which this effect is avoided. Our results also show that the density profiles do not depend on the lattice discretization if Deltax < approximately xi/4. This implies that the correlation length xi, extracted from the density profiles, is free of lattice size and lattice discretization artifacts once L is > approximately 2xi and Deltax < approximately xi/4.     

Solvothermal synthesis of silver nanoparticles from thiolates

A new solvothermal method for the synthesis of thiol-protected silver nanoparticles starting from silver thiolates is reported. The method has been tried with thiols of different chain length, such as octane and octadecane thiols, and the particle size was found to be nearly the same for both molecules. The synthesis was dependent on heating conditions and the best results were obtained when the temperature was between 180 and 200 degrees C. Addition of complexation agents such as acetyl acetone or triethylamine to the solvent did not change the product distribution significantly.