Structural and magnetic properties of “one-dimensional” barium vanadium triselenide

BaVSe₃ has been synthesized and its crystal structure determined at 293(2)°K. The structure was solved in the hexagonal space group P6₃/mmc (D⁴_6h), with a = 6.9990(11) and c = 5.8621(13) Å. Scans (2 Θ) of a polycrystalline sample revealed that BaVSe₃ undergoes a transition to an orthorhombic unit cell (b′ 3^1/2 a, a′ a, c′ c) at 303(5)°K. Magnetic susceptibility measurements between 4 and 300°K indicate that BaVSe₃ is paramagnetic down to 41(1)°K, where magnetic ordering occurs, with a magnetic moment in the ordered phase of 0.2 μ_B per vanadium atom. The orthorhombic lattice distortion may be caused by the “freezing in” of “soft” vibrational modes.     

Detection of “memory” effects in polyethylene by magnetic susceptibility

The relevance of diamagnetic susceptibility as a tool for the structure analysis of solid high polymers is stressed in the light of some new examples. The present results complement previous data and offer new aspects on the diamagnetic investigations of longchain hydrocarbons, especially polyethylene (PE). The molecular susceptibility is proportional to the average number of repeat units in the chain. The proportionality factor defines an intermolecular constant μ_k which characterizes different physical states. This was found to be 2.5 × 10^?6 for the liquid and 3.5 × 10^?6 cgs for the crystalline state of paraffins and polyethylene (solution-crystallized). For melt-crystallized material, μ_k, approaches the typical value of the liquid paraffin in agreement with previous results. Such a low μ_k is probably related to the increased disorder of the paracrystalline lattice domains, in contrast to the more ordered microparacrystallites in the so-called “single crystals,” where μ_k = 3.5 × 10^?6. In single crystals of branched PE, μ_k approaches 2.5 × 10^?6 with increasing branching ratio. Like paraffins in the gaseous state, molten PE, with chains longer than 1000 Å, has μ_k = 0. If the solution-crystallized material is molten for 10 min and thereafter cooled, μ_k retains the original value 3.5 × 10^?6 cgs characteristic of the crystalline state. Hence, solution-crystallized polyethylene apparently possesses a kind of “memory.” Such a “memory” can, nevertheless, be partly destroyed when molten PE is stirred for 10 min and then quenched. Aggregates of solution-precipitated crystals with 3% branching concentration give μ_k = 2.9 ± 0.2 × 10^?6 in good agreement with x-ray diffraction data. Finally, experimental details on the magnetic measurements are critically discussed, and various aspects of improvements for further investigations are also described.     

Electrode “edge effects” analyzed by the Green function method

An exact method based on Green's equation is used to find the diffusion-controlled faradaic current for certain electrode geometries that incorporate edges and vertices. Thereby the magnitudes of the time-independent current density associated with angled electrode/electrode and electrode/insulator junctions are calculated. As well, the square-root-of-time-dependent currents associated with vertices, receive attention. These terms extend to longer times, the Cottrell formulation appropriate for short times. Though most of the problems solved here have been tackled previously, the novel Green function approach is shown to be straightforward and intuitive.     

Viscoelastic properties of blends of “entangled” polymers

The effect of molecular weight distribution on the viscoelastic properties of “entangled” polymers has been examined with blends of narrowly distributed polystyrene and broadly distributed polydimethylsiloxane. It is shown that blending laws established for nonentangled polymers do not apply to high molecular weight systems. The steady-state shear compliance of a blend is examined as a function of its molecular weight and the molecular weight of its components, and an approximation is given for the longtime viscoelastic response of entangled blends.     

Ab initio calculation of molecular chiroptical properties

This review describes the first-principles calculation of chiroptical properties such as optical rotation, electronic and vibrational circular dichroism, and Raman optical activity. Recent years have witnessed a flurry of activity in this area, especially in the advancement of density-functional and coupled cluster methods, with two ultimate goals: the elucidation of the fundamental relationship between chiroptical properties and detailed molecular structure, and the development of a suite of computational tools for the assignment of the absolute configurations of chiral molecules. The underlying theory and the basic principles of such calculations are given for each property, and a number of representative applications are discussed.