Photoinduced cycloadditions of electron deficient nitrones to alkenes

Oxaziridines are proposed as intermediates in the photoinduced cycloadditions of electron deficient nitrones to dipolarophiles.     

Thermal Properties of Binary Mixtures of Liquid Crystals. A Review

Owing to their wide range of industrial utility, binary systems of liquid crystals are currently a topic of extensive studies. This paper is a comprehensive review of the literature from 1973, where the review of Hsu et al. ⁹⁵ ended, through mid-1981. For simplification, the organization used here is similar to that incorporated in the previous review.

Because of the volumes of literature on mesophase systems, this study includes only binary mesophase systems in which both components are thermotropic mesogens. Included here are only those systems for which phase diagrams have been constructed or systems which are similar to those whose phase diagrams have been constructed.

Much of the work on binary mixtures is done by industrial applications specialists and thus was not available for this review. Inevitably, there will be omissions from published work, although every attempt was made to include all of the available literature.     

Tolanes derivatives

The phase behavior for each of two linear thermotropic liquid crystalline polyesters with a corresponding low molecular weight liquid crystal of similar structure have been investigated. Two distinct types of phase diagrams have been obtained for binary mixtures of a polymer, one containing an in-chain azoxybenzene moiety mixed with para-azoxyanisole and one containing a naphthalene unit mixed with a dimethylester liquid crystal. These phase diagrams were studied by DSC, polarized light microscopy and x-ray diffraction. From this understanding of the physical mixtures a transesterification reaction incorporating the bifunctional low molecular weight liquid crystal by a solid-state reaction into the naphthalene-containing polyester was undertaken. This reaction may be viewed as a potential new processing technique for polymers. The thermal stability of an interreacted blend was shown to be superior to that of either individual component by thermogravimetric analysis. Thus, the potential new processing technique features a lower melting, lower viscosity blend followed by the incorporation of the dimethylester liquid crystal into the polyester main chain producing a more rigid, more stable polymer backbone.     

Defects and localized states in MBE-grown GaAs1−x N x solid solutions prepared by molecular-beam epitaxy

Defects and localized states have been studied for molecular-beam-epitaxy (MBE)-grown high-resistivity and undoped GaAs_1?x N _x films with a N concentration not exceeding approximately 1.0 at.%. The crystalline quality of the films and hence the defects and localized states were determined by high-resolution X-ray diffraction, photoluminescence spectra, capacitance versus voltage measurements and photoinduced current transient spectra of GaAs and GaAs_1?x N _x layers. It was concluded that incorporation of low concentrations of N into MBE-grown GaAs_1?x N _x films promotes the formation of high densities of deep centres similar to EL2 donors, leading to heavy compensation of the films by some unidentified acceptors. GaAs antisite acceptors were believed to be responsible for the said compensation. A prominent defect band near 1.33-1.38 eV also appeared to be associated with these defects. The most prominent centres in dilute GaAs_1?x N _x films with N content less than 0.35 at.% seem to be the EL2 donors and the hole traps located near E _v +0.3 eV.     

Modelling resonances of the standing body exposed to vertical whole-body vibration: Effects of posture

Lumped parameter mathematical models representing anatomical parts of the human body have been developed to represent body motions associated with resonances of the vertical apparent mass and the fore-and-aft cross-axis apparent mass of the human body standing in five different postures: ‘upright’, ‘lordotic’, ‘anterior lean’, ‘knees bent’, and ‘knees more bent’. The inertial and geometric parameters of the models were determined from published anthropometric data. Stiffness and damping parameters were obtained by comparing model responses with experimental data obtained previously.The principal resonance of the vertical apparent mass, and the first peak in the fore-and-aft cross-axis apparent mass, of the standing body in an upright posture (at 5–6 Hz) corresponded to vertical motion of the viscera in phase with the vertical motion of the entire body due to deformation of the tissues at the soles of the feet, with pitch motion of the pelvis out of phase with pitch motion of the upper body above the pelvis. Upward motion of the body was in phase with the forward pitch motion of the pelvis. Changing the posture of the upper body had minor effects on the mode associated with the principal resonances of the apparent mass and cross-axis apparent mass, but the mode changed significantly with bending of the legs. In legs-bent postures, the principal resonance (at about 3 Hz) was attributed to bending of the legs coupled with pitch motion of the pelvis in phase with pitch motion of the upper body. In this mode, extension of the legs was in phase with the forward pitch motion of the upper body and the upward vertical motion of the viscera.     

Synthesis of tetraarylmethanes via a Friedel-Crafts cyclization/desulfurization strategy

Tetraarylmethanes are an important class of molecules that contain four aryl groups bonded to a central carbon atom. The shape/three-dimensionality of these molecules makes them suitable for organic light-emitting diodes (OLEDs), organic solar cells, hydrogen storage, and even drug-delivery. Despite their importance, there are only a few methods available for their preparation. Herein, we report a simple procedure for the preparation of tetraarylmethanes that involves a bismuth-catalyzed Friedel-Crafts cyclization followed by a desulfurization reaction mediated by Raney nickel.