Investigation of Lewis Acid versus Lewis Base Catalysis in Asymmetric Cyanohydrin Synthesis

The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}₂] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO₃ or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}₂O] and triphenylphosphine oxide, a non‐linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron‐deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case.     

The three-body dissociation dynamics of Cl2O at 248 and 193 nm

The photodissociation of Cl₂O has been studied at 248 and 193 nm using photofragment translational spectroscopy (PTS) experiments with tunable VUV photoionization detection. The sole products observed were Cl, O and ClO fragments. Based on the derived translational energy distributions for the ClO and Cl photofragments we conclude that at 248 nm 15% of Cl₂O excitation results in three-body dissociation. At 193 nm no Cl₂ fragments are observed and we conclude that the oxygen atoms arise solely from three-body dissociation. Dissociation geometries derived from forward convolution fitting suggest two qualitatively distinct three-body dissociation pathways: asymmetric concerted dissociation and symmetric concerted dissociation in agreement with recent theoretical predictions.     

Simultaneous velocity and temperature measurements in gaseous flow fields using the VENOM technique

We present an initial demonstration of simultaneous velocity and temperature mapping in gaseous flow fields using a new nitric oxide planar laser-induced fluorescence-based method. The vibrationally excited NO monitoring (VENOM) technique is an extension of two-component velocimetry using vibrationally excited NO generated from the photodissociation of seeded NO(2) [Appl. Opt. 48, 4414 (2009)], where the two sequential fluorescence images are obtained probing two different rotational states to provide both velocity and temperature maps. Comparisons to computational fluid dynamics simulations show that the initial VENOM measurements provide good velocity and temperature maps in the relatively high-density regions of the flow, where the rms uncertainties are approximately 5% for velocity and 9% for temperature.     

Preliminary shielding analysis in support of the CSNS target station shutter neutron beam stop design

The construction of China Spallation Neutron Source (CSNS) has been initiated in Dongguan, Guangdong, China. Thus a detailed radiation transport analysis of the shutter neutron beam stop is of vital importance. The analyses are performed using the coupled Monte Carlo and multi-dimensional discrete ordinates method. The target of calculations is to optimize the neutron beamline shielding design to guarantee personal safety and minimize cost. Successful elimination of the primary ray effects via the two-dimensional uncollided flux and the first collision source methodology is also illustrated. Two-dimensional dose distribution is calculated. The dose at the end of the neutron beam line is less than 2.5 μSv/h. The models have ensured that the doses received by the hall staff members are below the standard limit required.     

Active TLCGD control of plane asymmetric buildings under earthquake excitation

The sealed,tuned liquid column gas damper (TLCGD) with gas-spring effect extends the frequency range of application and efficiently increases the modal structural damping.Active tuned liquid column gas damper (ATLCGD) is developed for the vibration control of plane asymmetric buildings subjected to earthquake excitation,improving the performance of the passive control scheme.The active behaviour is obtained by adjusting the pressure at the end of the liquid column using a pressurised reservoir.The classical linear quadratic regulator (LQR) design is presented as a straightforward approach to optimal control.Numerical simulations indicate a significant vibration reduction of plane asymmetric buildings by active control within the strong motion of the dynamic response.     

Enantio- and diastereoselective Darzens condensations

A cobalt(salen) complex has been shown to catalyse the asymmetric Darzens condensation between α-haloamides and aldehydes, allowing both the relative and absolute stereochemistry of the epoxy-amide products to be controlled. Under optimal conditions, cis-epoxides can be obtained diastereoselectively with up to 50% enantiomeric excess, whilst by changing the leaving group and base, trans-epoxides can be produced diastereoselectively with up to 43% enantiomeric excess.     

THERMODYNAMIC AND PARTICLE-DYNAMIC STUDIES ON SYNTHESIS OF SILICA NANOPARTICLES USING MICROWAVE-INDUCED PLASMA CVD

1. Introduction1.1 Silica nanoparticles and synthesis methods Silica (SiO2) nanoparticles are widely used in industry asan active filler for polymer reinforcement, a rheologicaladditive in fluids, a free flow agent in powders, and anagent for chemical mechanical polishing during IC (inte-grated circuit) fabrication (Sniegowski & de Boer, 2000).Silica powder is also used for producing silicon carbide(Koc & Cattamanchi, 1998) or opaque silica aerosols (Leeet al., 1995). Many methods can …     

Dynamic-stress concentration using photoelasticity and a laser light source

Since experimental techniques using dynamic photoelasticity are, in general, limited by suitably intense monochromatic light sources, it was of primary importance to describe a ruby-laser system which would completely remove this particular aspect of the problem. Modulation of the system, in this case by a Kerr cell, provides the necessary control of the light output such that a complete series of results can be obtained by putting together the results of many individual tests recorded on a conventional still camera. In this case, the dynamic-stress-concentration factor in a strut with a symmetrically located circular discontinuity was determined to establish definitely the potential of the system.     

The diffractographic strain gage

A method for measuring strain using diffraction of light from a single aperture is described, and results of a comparison tensile test with an electrical-resistance strain gage are presented. The “diffractographic strain gage” is shown to have high sensitivity, linearity, accuracy and temperature compensation and the ability to operate in a variety of hostile environments. It is furthermore simple, inexpensive, and the data can be collected by eye without assistance from further instrumentation.