Infra-red spectrum,structure and properties of the N2-Ar van der Waals molecule

The first spectroscopic observation of bound N₂-Ar van der Waals molecules has been achieved with a cryogenic long path cell maintained at 87 K. The infra-red spectrum exhibits prominent fine structure near the N₂ stretching frequency which is assigned to hindered internal rotation of N₂ within the weakly bound complex. An analysis of this fine structure yields a T-shaped equilibrium geometry in which the N₂ bond axis is perpendicular to the N₂-Ar van der Waals bond axis. The observed spectrum is shown to be consistent with an internal rotational barrier of 20 cm^-1 (57 cal/mole). Approximately 20 per cent of the bound species are trapped by this rotational barrier and acquire a locked semi-rigid structure. The remaining 80 per cent have ill-defined geometry and undergo hindered internal rotation. The rotational envelope of an infra-red fundamental is analysed to give an estimate of the N₂-Ar bond length as 3·9 Å.     

In situ measurement of the thermal and photo-assisted MOVPE of ZnTe using laser reflectometry

In situ laser reflectometry has been used to study the growth kinetics of ZnTe under pyrolytic and photo-assisted conditions from diisopropyltellurium (DIPTe) and dimethylzinc.triethylamine (DMZn.TEN). The growth rate of ZnTe was monitored as a function of VI:II ratio, temperature and laser power density. From the results a model for the growth involving the surface decomposition of DIPTe via a bimolecular reaction with methyl radicals homolytically released from the decomposition of DMZn.TEN is proposed. The activation energy under pyrolytic conditions in the low temperature regime for a 1:1 VI:II ratio was found to be 27.6 kcal/mol and under photo-assisted conditions this activation energy was seen to be lowered to 21.9 kcal/mol. The growth kinetics are explained in terms of two competitive processes. The decomposition of DMZn on the surface will be enhanced with increasing substrate temperature; however, the desorption rate of DIPTe will also be enhanced. The competitive nature of these two processes is seen to be particularly pronounced under laser illumination at high power densities, where a growth rate of 13 a.u./s was observed at 360°C, laser power density 67 mW/cm², whereas at 380°C for a similar power density the growth rate was seen to be reduced to 10.5 a.u./s.     

Wideband magneto-optic modulation in a bismuth-substituted yttrium iron garnet waveguide

We present a tunable wideband bismuth-substituted yttrium iron garnet (Bi-YIG) waveguide magneto-optic (MO) modulator. High-speed current transients are used to switch the in-plane magnetization of the film, which modulates an 800 nm optical beam. Large bandwidth optical modulation is achieved by driving the device in a non-resonant mode that is well below the ferromagnetic resonance frequency of the film. The MO modulator has the potential of operating at bandwidths higher than 1 GHz by tuning the applied static magnetic field.     

Magnetic,Spectroscopic and Thermal Study of [Cu(NO3)(PyTz)2](NO3). A Copper(II) Complex Containing Thiazine Derivative Ligand

The copper(II) complex [Cu(NO₃)(PyTz)₂](NO₃) has been previously characterized means X‐ray powder diffraction and now studied by IR spectroscopy, UV‐Vis‐NIR diffuse reflectance, magnetic susceptibility measurement, electronic spin resonance (ESR) and thermal analysis. The results are correlated with a distorted square pyramidal coordination around copper(II) ion rather than the cis‐distorted octahedral stereochemistry of a CuN₄OO′ chromophore in good concordance with their structure. Likewise, in order to indicate towards what square pyramidal isomer the complex is distorted, the method proposed by Carugo and Bisi has been applied to the structural data of [Cu(NO₃)(PyTz)₂](NO₃). It is deduced that there is a large distortion from the trigonal bipyramid geometry, close to a square pyramid geometry, being produced almost exclusively through the B route of the Berry mechanism.     

Allylic vs homoallylic control of stereospecificity in the epoxidation of 3(1′-hydroxyethyl)-5,8-dimethoxy-1,2-dihydronaphthalen-1-ol: implications for the synthesis of chiral anthracyclines

The stereochemistry of the epcxidation of the title compound with t-BuOOH catalysed by VO(acac)₂ is subject to exclusive homoallylic control. Secondary allylic alcohols in the side chain regain their normal controlling influence over diastereoselection only when the homoallylic group is blocked.     

Oxide ion transport in highly defective cubic stabilized zirconias

Ac impedance spectroscopy and neutron powder diffraction have been used to study the high temperature behaviour of defective fluorite solid electrolytes. In yttria-stabilised zirconia with an yttrium content of 15 mol% YO_1.5 there is a marked change in conductivity behaviour at around 650 °C, with a decrease in activation energy of 0.15 eV. Structural studies confirm that this is due to a change in the bulk of the sample with the disappearance of diffuse scattering peaks and marked changes in the behaviour of the isotropic temperature factors at the same temperature. These results indicate that the change in activation energy of yttria-stabilised zirconia at 650 °C is due to an order-disorder transition involving local defect clusters. In studies of zirconia co-doped with yttrium and niobium, activation energy for conduction is found to rapidly increase with the concentration of the trivalent yttrium Saturation doping is reached at about 20–30 % of yttrium and activation energy only increases slightly with doping. Introduction of pentavalent niobium at this level of doping serves to decrease activation energy, although it also decreases conductivity slightly. The low and high temperature activation energies converge as the saturation regime is approached. These observations seem to suggest that ordering of defect clusters into microdomains increases activation energy for ionic motion. At low defect concentrations and high temperatures, this local ordering breaks down and the activation energy for conduction decreases. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995