Phosphoryl and Thiophosphoryl Transfer Reactions: Stereochemical Imperatives for Metaphosphate and Thiometaphosphate

Abstract

The stereochemical courses of phosphoryl transfer reactions in aprotic solvents and thiophosphoryl transfer reactions in protic solvent have been determined. The extensive racemisation observed in both instances is discussed in terms of metaphosphate and thiometaphosphate intermediates of significant life-times.     

Resonance behavior of electron scattering from nitrogen molecules adsorbed on metallic surfaces

We use the coupled angular modes theory of electron scattering from molecules to identify the important role played by substrate multiple scattering in determining the lifetime of transient negative ions in adsorbed molecules. For the ²Π_g negative ion resonance in N₂, we demonstrate the phenomenon of complete resonance quenchingwhich occurs when the resonance energy is such that the probe electron is strongly reflected from the substrate. This situation may occur when the resonance energy lies in a band-gap of the unoccupied states of a metallic substrate. It is proposed that the simple resonance energy shift and lifetime reduction observed in previous experimental and theoretical studies of resonance electron scattering from adsorbates is by no means a universal phenomenon. The importance of the unoccupied metallic band structure of the substrate in determining the properties of negative ions in adsorbed molecules is discussed.     

Visualisation of the fibrillar and pore morphology of cellulosic fibres applying transmission electron microscopy

Applying transmission electron microscopy (TEM) on ultra-thin cross-sections of fibres, the main characteristics of the internal morphology of cotton and the main man-made cellulosic fibres (modal, viscose and lyocell) could be visualised. To obtain an appropriate contrast for TEM, isoprene was polymerised into the swollen fibres after a stepwise solvent exchange from water to acetone. The included polymer is stainable with osmium tetraoxide. Significant differences in distribution of pore sizes and pore arrangements in the cellulosic fibres were seen. Cotton showed very small pores in the bulk of the fibre, but drying cracks and flat pores between the sheets of the secondary wall appear as larger pores. Lyocell contains only nanopores in the bulk of the fibre with a slight gradient in pore density, and a very porous skin layer. In viscose and modal, a very wide pore size distribution from nanometer to micrometer size can be seen.     

Biased surface fluctuations due to current stress

Direct correlation between temporal structural fluctuations and electron wind force is demonstrated, for the first time, by STM imaging and analysis of atomically resolved motion on a thin film surface under large applied current (10(5) A/cm2). The magnitude of the momentum transfer between current carriers and the geometrically constrained atoms in the fluctuating structure is at least 5x to 15x (+/-1sigma range) larger than for freely diffusing adatoms. Corresponding changes in surface resistivity will contribute significant fluctuation signature to nanoscale electronic properties.