Photoelectron spectra,reactions and structures of some acyclic 2-azadiene systems

A series of substituted 2-azadiene systems has been prepared and characterized by reactivity, structure, and photoelectron-spectroscopic studies. An attempt has been made to employ photo-electron spectroscopy as a tool for predicting sites of chemical reactivity for these azadiene systems. Although such correlations can be tenuous, they seem to be useful in this limited study.     

Analysis of the IGLO bond contributions to the 13C shielding tensors in the local bond frame

A new method is presented to analyze the IGLO (individual gauge for localized orbitals) bond contributions in ¹³C chemical shielding. The IGLO bond contributions calculated in the molecular frame are rotated to a local bond frame, in which one component is selected along the bond. This procedure removes the explicit angular dependence of the IGLO bond contributions and allows a comparison of the bond contributions in different molecules. The results provide a new method to study the electronic basis of shielding interactions. The problems associated with the multiple gauge origins used in the IGLO method are discussed in their relationship to the bond contribution analysis.     

N-labelling studies of the reaction of 2,6-di-t-butyl-4-methyl-4-nitrocyclohexa-2,5-dienone with nitrogen dioxide; A revised mechanism

Reaction of the ¹⁵N-4-nitrodienone (1) with excess nitrogen dioxide in benzene for 16 h gives the 4,5,6-trinitro ketone (2), labelled (¹⁵NO₂) at C6 but not at C4, and recovered 4-nitrodienone (1) in which extensive loss of ¹⁵NO₂ has occurred. The mechanistic implications of these and related results are discussed.     

Development of a trace gas stable isotope capture system in a mobile laboratory for temporal and spatial sampling of field and laboratory experiments

We describe the development of a novel mobile field laboratory, purposely designed for the automated capture and subsequent stable isotopic analyses of multiple gas samples. The multiple capture system is integrated into a mobile laboratory that is fully capable of measuring the concentration of carbon dioxide, methane and nitrous oxide trace gases in a flow-through system connected to a gas chromatograph fitted with both electron capture and flame ionisation detectors. The capture of gases is achieved by routing samples through a series of 135 mL gas flasks that are sealed by micro-solenoid valves triggered by a timing system. Trace gas light stable isotope ratio mass spectrometry can then be carried out on gas samples collected by the system (NERC (15)N Stable Isotope Facility). The excitingly unique potential of the system to the ecological research field is that it will allow the collection of cyclical data for three different trace gases both in real-time and in situ. We present data arising from the validation of this mobile system as well as a preliminary experimental assessment of this technique. This technique was used to measure delta(13)C in CO(2) and CH(4) in soil gases released from waterlogged cores and delta(13)C-CH(4) values were significantly depleted in wet cores compared with dry ones (p < 0.001).     

Polar,non-coordinating ionic liquids as solvents for the alternating copolymerization of styrene and CO catalyzed by cationic palladium catalysts

The palladium-catalyzed copolymerization of styrene and CO in an ionic liquid solvent, 1-hexylpyridinium bis(trifluoromethanesulfonyl)imide, gave improved yields and increased molecular weights compared to polymerizations run in methanol.     

Removal of Solid Organic Films from Rotating Disks Using Emulsion Cleaners

Measurements have been made of the rate of removal of a solid organic film (phenanthrene) from the surface of a rotating disk using emulsions containing water, the nonionic surfactant Tween 20, and d-limonene as the organic phase. The results show that phenanthrene removal initially occurs by the uptake of phenanthrene into the emulsion drops as small aggregates. Simultaneously, the organic phase penetrates into the phenanthrene film, diminishing the adhesive force between the film and the substrate. After sufficient time, the phenanthrene film detaches from the rotating disk surface as a solid. This detachment mechanism accounts for the vast majority of the phenanthrene removal ( approximately 90%). Initial solubilization rates were analyzed using two solubilization models. Both models assume that phenanthrene removal occurs via a mass transfer limited removal of phenanthrene-laden emulsion drops from the phenanthrene film surface into the bulk solution. One model treats the emulsion as homogeneous while the other accounts for the finite size of the emulsion droplets. The latter model was also used to relate the flux of organic phase impacting the phenanthrene film to the detachment times. Copyright 2000 Academic Press.     

Comparative Study of Natural Terpenoid Precursors in Reactive Plasmas for Thin Film Deposition

If plasma polymer thin films are to be synthesised from sustainable and natural precursors of chemically heterogeneous composition, it is important to understand the extent to which this composition influences the mechanism of polymerisation. To this end, a well-studied monoterpene alcohol, terpinen-4-ol, has been targeted for a comparative study with the naturally occurring mix of terpenes (viz. Melaleuca alternifolia oil) from which it is commonly distilled. Positive ion mode mass spectra of both terpinen-4-ol and M. alternifolia oil showed a decrease in disparities between the type and abundance of cationic species formed in their respective plasma environments as applied plasma power was increased. Supplementary biological assay revealed the antibacterial action of both terpinen-4-ol and M. alternifolia derived coatings with respect to S. aureus bacteria, whilst cytocompatibility was demonstrated by comparable eukaryotic cell adhesion to both coatings. Elucidating the processes occurring within the reactive plasmas can enhance the economics of plasma polymer deposition by permitting use of the minimum power, time and precursor pre-processing required to control the extent of monomer fragmentation and fabricate a film of the desired thickness and functionality.     

Energetic and stereochemical effects of the protein environment on substrate: a theoretical study of methylmalonyl-CoA mutase

QM/MM methods were used to study the isomerization step from (2R)-methylmalonyl-CoA to succinyl-CoA. A pathway via a "fragmentation-recombination" mechanism is ruled out on energetic grounds. For the other radicalic pathway, involving an addition recombination step, geometries and vibrational contributions have been determined, and a barrier height of 11.70 kcal/mol was found. The effect of adjacent hydrogen-donating groups was found to reduce the energy barrier by 1-2 kcal/mol each and thus to provide a significant catalytic effect for this reaction. By means of molecular dynamics studies, the stereochemistry of the methylmalonyl-CoA mutase catalyzed reaction was examined. It is shown that TYR89 is essential for maintaining stereoselectivity of the abstraction of a hydrogen in the backreaction. The subsequent selective formation of one isomer of methylmalonyl-CoA is probably due to the presence of a bulky side chain.     

Solid-state synthesis of a conducting polythiophene via an unprecedented heterocyclic coupling reaction

Prolonged storage ( approximately 2 years) or gentle heating (50-80 degrees C) of crystalline 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) affords a highly conducting, bromine-doped poly(3,4-ethylenedioxythiophene) (PEDOT), as confirmed by solid-state NMR, FTIR, CV, and vis-NIR spectroscopies. The novel solid-state polymerization (SSP) does not occur for 2,5-dichloro-3,4-ethylenedioxythiophene (DCEDOT), and requires a much higher temperature (>130 degrees C) for 2,5-diiodo-3,4-ethylenedioxythiophene (DIEDOT). X-ray structural analysis of the above dihalothiophenes reveals short Hal.Hal distances between adjacent molecules in DBEDOT and DIEDOT, but not in DCEDOT. The polymerization may also occur in the melt but is significantly slower and leads to poorly conductive material. Detailed studies of the reaction were performed using ESR, DSC, microscopy, and gravimetric analyses. SSP starts on crystal defect sites; it is exothermic by 14 kcal/mol and requires activation energy of approximately 26 kcal/mol (for DBEDOT). The temperature dependence of the conductivity of SSP-PEDOT (sigma(rt) = 20-80 S/cm) reveals a slight thermal activation. It can be further increased by a factor of 2 by doping with iodine. Using this approach, thin films of PEDOT with conductivity as high as 20 S/cm were fabricated on insulating flexible plastic surfaces.