Linear and nonlinear circular dichroism of R-(+)-3-methylcyclopentanone

Linear and nonlinear circular dichroism of R-(+)-3-methylcyclopentanone (R-3MCP) is reported in the gas and liquid phases. Measurements of (2+1) resonance-enhanced multiphoton ionization circular dichroism (REMPICD) for nozzle-jet expanded molecular beams of the equatorial conformer of R-3MCP are presented. Monitoring either mass-selected cations or photoelectrons produced via (2+1) REMPI through the n --> 3s Rydberg transition yielded a REMPICD of +1.5+/-0.5% [REMPICD identical with 2(I(L)-I(R))(I(L)+I(R))], where I(L/R) refers to the ion/electron signal for left/right circularly polarized light. A racemic mixture of 3-methylcyclopentanone showed no significant CD; however, the signal fluctuations were much larger than that observed for the resolved R-(+)-3-methylcyclopentanone as might be expected for the small number of ions produced from slightly unequal numbers of enantiomers in each laser shot. Gas phase, vibrationally resolved, one-photon CD for vapor phase R-(+)-3-methylcyclopentanone (i.e., admixture of five axial and equatorial forms) was measured to be approximately 0 and -0.004 at photon energies corresponding to the one- (nonresonant) and two-(3s resonance) photon energy levels. The one-photon CD (of the room temperature population of conformers) at an energy corresponding to the ionization step was measured previously to be approximately +0.0011 which is of the same sign as the REMPICD. The first step is also near a positive CD region. This suggests that the (2+1) REMPICD is determined primarily by both the initial and continuum steps. The one-photon CDs for the equatorial and axial forms of 3MCP are calculated, using GAUSSIAN03, to be approximately equal but having opposite sign for the transitions of interest. The CD for 3MCP in cyclohexane is found to be strongly temperature dependent as a result of the presence of both the axial and equatorial conformers. The energy difference between the two conformers is determined from a van't Hoff plot of these data to be 3.50+/-0.05 kJ/mole in cyclohexane and is approximately 1 kJ/mole smaller than measurements employing other methods.     

Electroanalytical applications of boron-doped diamond microelectrode arrays

The electrochemical characteristics of a novel all diamond fabricated boron-doped diamond microelectrode array (BDD-MEA) are critically appraised. The voltammetric response of simple electron transfer processes has been investigated and found to generate sigmoidal voltammetric curves. Furthermore, the device has been utilized for various analytical applications including, the direct detection of 4-nitrophenol over the concentration range 1.8-9.2 μM, manganese over the range 0.1-4.8 μM and the indirect determination of sulfide producing a limit of detection of 23 μM.     

Apparent 'electrocatalytic' activity of multiwalled carbon nanotubes in the detection of the anaesthetic halothane: occluded copper nanoparticles

The electrocatalytic detection of the anaesthetic halothane on a multiwalled carbon nanotube modified glassy carbon electrode is reported with a low limit of detection of 4.6 microM. A thorough investigation of the underlying cause of this apparent catalytic effect is undertaken by comparing the response of various carbon electrodes including glassy carbon, basal- and edge-plane pyrolytic graphite electrodes (bppg and eppg respectively) to increasing additions of halothane. The reduction of halothane is shifted by 250-300 mV to more negative potentials at an eppg electrode than that observed at the GC-CNT electrode. Therefore the results of this investigation show that, surprisingly, the electrocatalysis is not solely due to the introduction of edge-plane-like defect sites on the carbon nanotubes as is commonly found for many other substrates showing favourable voltammetry at nanotube modified electrodes. Instead, we reveal that in this unusual case the electroactive sites for the reduction of halothane are due to the presence of copper nanoparticles occluded within the carbon nanotubes during their production, which are never completely removed by standard purification techniques such as acid washing. This is only the third known case where apparent electrocatalysis by carbon nanotube modified electrodes is due to occluded metal-related nanoparticles within the nanotube structure, rather than the active sites being the edge-plane-like defect sites on the nanotubes. Furthermore this is the first case where the active sites are nanoparticles of copper metal, rather than metal oxide nanoparticles (namely oxides of iron(II)/(III)) as was found to be the case in the previous examples.     

Micromechanical models to guide the development of synthetic ‘brick and mortar’ composites

This paper describes a micromechanical analysis of the uniaxial response of composites comprising elastic platelets (bricks) bonded together with thin elastic perfectly plastic layers (mortar). The model yields closed-form results for the spatial variation of displacements in the bricks as a function of constituent properties, which can be used to calculate the effective properties of the composite, including elastic modulus, strength and work-to-failure. Regime maps are presented which indicate critical stresses for failure of the bricks and mortar as a function of constituent properties and brick architecture. The solution illustrates trade-offs between elastic modulus, strength and dissipated work that are a result of transitions between various failure mechanisms associated with brick rupture and rupture of the interfaces. Detailed scaling relationships are presented with the goal of providing material developers with a straightforward means to identify synthesis targets that balance competing mechanical behaviors and optimize material response. Ashby maps are presented to compare potential brick and mortar composites with existing materials, and identify future directions for material development.     

Single Nanoparticle Voltammetry: Contact Modulation of the Mediated Current

The cyclic voltammetric responses of individual palladium‐coated carbon nanotubes are reported. Upon impact—from the solution phase—with the electrified interface, the nanoparticles act as individual nanoelectrodes catalyzing the hydrogen‐oxidation reaction. At high overpotentials the current is shown to reach a quasi‐steady‐state diffusion limit, allowing determination of the tube length. The electrochemical response of the individual nanotubes also reveals the system to be modulated by the electrical contact between the electrode and carbon nanotube. This modulation presents itself as fluctuations in the recorded Faradaic current.     

Ultrasonically induced phthalocyanine degradation: decolouration vs. metal release

The ultrasonically induced degradations of nickel(II) 2,9,16,23-tetraphenoxy-29H, 31H-phthalocyanine and vanadyl 2,9,16,23-tetraphenoxy-29H, 31H-phthalocyanine in a biphasic system consisting of chloroform and water are investigated. Decolourisation of the organic phase containing the metallo-phthalocyanines is found to occur rapidly (approximately minutes for 8 ml of ca. 5 microM solution). Analysis of the aqueous solute via ICP-OES, reveals significant amounts of the released nickel ultimately transfers into the aqueous phase but at a rate much slower than that of decolourisation, whereas the vanadium remains within the organic phase suggesting only partial degradation despite efficient decolourisation.     

The adsorption of quinizarin on boron-doped diamond

The voltammetric response of the quinone species 'quinizarin' (QZ) and its electrocatalytic reduction of oxygen are studied at a boron doped diamond electrode (BDD). It is demonstrated that, contrary to the widespread belief that adsorption of organic molecules on BDD is minimal, not only does QZ readily adsorb to the electrodes surface but this adsorption is also influenced at low surface coverages by the pre-exposure of the electrode to organic solvents. Furthermore, the nature of this adsorbed QZ species is investigated and a potential dependent phase transition is observed. This is to the authors knowledge the first system to exhibit a phase transition of an adsorbed species on a boron doped diamond surface. At low scan rates the system is found to oscillate; these oscillations are ascribed to the presence of a 'negative differential resistance'.     

Electrochemical determination of glutathione: a review

The physiological importance of glutathione and glutathione disulfide is evident from their implications in an array of medical conditions including diabetes, Parkinson's disease and cancer. As such the need for simple, rapid and cheap assays to aid clinical diagnostics and treatment is clear. These requirements are, in principle at least, ideally suited to electrochemical detection. Accordingly a large array of voltammetric methods ultimately aimed at making cheap and most likely disposable electrodes have been reported. This critical review analyses the context in which physiological glutathione measurement can be undertaken electrochemically and compares it to current assay approaches, while also covering the current literature for glutathione disulfide detection. The various characteristics and limitations of the methodologies are compared and contrasted, with the analytical parameters (matrix, pH, limit of detection, etc.) tabulated to aid comparison.     

Formic acid electro-synthesis from carbon dioxide in a room temperature ionic liquid

The novel synthesis of formic acid has been achieved in a room temperature ionic liquid via the reaction of electro-activated carbon dioxide and protons on pre-anodised platinum. Only mild reaction conditions of room temperature and 1 atm CO(2) were used. This work highlights the effect of pre-anodisation on Pt surfaces.     

The optical activity of carvone: a theoretical and experimental investigation

The optical rotatory dispersion (ORD) and circular dichroism of the conformationally flexible carvone molecule has been investigated in 17 solvents and compared with results from calculations for the "free" (gas phase) molecule. The G3 method was used to determine the relative energies of the six conformers. The optical rotation of (R)-(-)-carvone at 589 nm was calculated using coupled cluster and density functional methods, including temperature-dependent vibrational corrections. Vibrational corrections are significant and are primarily associated with normal modes involving the stereogenic carbon atom and the carbonyl group, whose n → π? excitation plays a significant role in the chiroptical response of carvone. Without the inclusion of vibrational corrections the optical rotation calculated with CCSD and DFT has the opposite sign of experimental data. Calculations of optical rotation performed in solution using the polarizable continuum model were also opposite in sign when compared to that of the experiment.