Dynamic collision-induced dissociation (DCID) in a quadrupole ion trap using a two-frequency excitation waveform: I. Effects of excitation frequency and phase angle

This study describes the application of a two-frequency excitation waveform to the end-cap electrodes of a quadrupole ion trap (QIT) during the mass acquisition period to deliberately fragment selected precursor ions. This approach obviates the need for a discrete excitation period and guarantees on-resonant excitation conditions without any requirement for resonant tuning; it is therefore faster than the conventional approach to collision-induced dissociation (CID) in QITs. The molecular ion of n-butylbenzene is used as thermometer molecule to determine the energetics of the new excitation procedure. The excitation waveform, consisting of two closely spaced sinusoidal frequencies, has an interference pattern that displays nodes and crests in the time domain. The energetics (determined by the product ion ratios of 91/92 Th) and CID efficiencies are highly dependent on the excitation amplitude, the relative position of the excitation frequencies in the Mathieu stability diagram, and whether the ions come into resonance during a node or crest of the excitation waveform. Under highly energetic conditions, ratios of 91/92 as large as 15 can be obtained at concomitant CID efficiencies of 10%, indicating internal energies in excess of 10 eV at the time of fragmentation. These extremely high internal energies far exceed the energetics achievable using conventional on-resonance excitation in QITs, indicating that the collisional heating rate is very fast in the new approach. Under less energetic conditions CID efficiencies as high as 70% are possible, which compares favorably with results obtained by conventional on-resonance excitation. Correlation analyses are used to determine the conditions that simultaneously optimize energetic and efficient fragmentation conditions.     

Theoretical and experimental evaluation of screen-printed tubular carbon ink disposable sensor well electrodes by dc and Fourier transformed ac voltammetry

Mass-produced, screen-printed, carbon-ink-based microtubular band (well) electrodes, suitable for routine sensing applications, have been fabricated and evaluated with respect to their theoretical and analytical performance. Microscopic examination of the electrode surface reveals they are inherently rough and could easily suffer from high and variable resistance, capacitance and area, unless care is taken to minimise these problems. Simulation models have been applied to analyse cyclic voltammetric responses obtained at the well electrodes. Results of these theoretical calculations further demonstrate the care needed with electrode design and resistance in carbon ink electrodes. Substantial differences in voltammetry when wells are produced by mechanically punching or laser drilling are considered. The application of multi- and single-frequency Fourier Transform ac voltammetry, previously applied to planar carbon ink disc electrodes for quality control purposes, is now demonstrated with respect to the microtubular band electrode geometry. Theoretical and practical limitations are discussed, as is the analytical application to the reversible redox couple in the presence of oxygen in aqueous solution. Dedicated to Keith on his 80th birthday, a good friend and colleague.     

meso-Arylporpholactones and their Reduction Products

The rational syntheses of meso-tetraaryl-3-oxo-2-oxaporphyrins 5, known as porpholactones, via MnO(4)(-)-mediated oxidations of the corresponding meso-tetraaryl-2,3-dihydroxychlorins (7) is detailed. Since chlorin 7 is prepared from the parent porphyrin 1, this amounts to a 2-step replacement of a pyrrole moiety in 1 by an oxazolone moiety. The stepwise reduction of the porpholactone 5 results in the formation of chlorin analogues, meso-tetraaryl-3-hydroxy-2-oxachlorin (11) and meso-tetraaryl-2-oxachlorins (12). The reactivity of 11 with respect to nucleophilic substitution by O-, N-, and S-nucleophiles is described. The profound photophysical consequences of the formal replacement of a pyrrole with an oxazolone (porphyrin-like chromophore) or (substituted) oxazole moiety (chlorin-like chromophore with, for the parent oxazolochlorin 12, red-shifted Q(x) band with enhanced oscillator strengths) are detailed and rationalized on the basis of SAC-CI and MNDO-PSDCI molecular orbital theory calculations. The single crystal X-ray structures of the porpholactones point at a minor steric interaction between the carbonyl oxygen and the flanking phenyl group. The essentially planar structures of all chromophores in all oxidation states prove that the observed optical properties originate from the intrinsic electronic properties of the chromophores and are not subject to conformational modulation.     

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6F5)3 Towards Carbene Formation

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N₂ release. Recently, tris(pentafluorophenyl)borane [B(C₆F₅)₃] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C₆F₅)₃ is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.     

Gold-catalyzed and N-iodosuccinimide-mediated cyclization of gamma-substituted allenamides

Chiral gamma-substituted allenamides have been shown to undergo efficient gold-catalyzed and N-iodosuccinimide-mediated cyclization to highly functionalized dihydrofurans. These reactions proceed rapidly and without loss of stereochemistry.     

Valence band XPS and FT-IR evaluation of thermal degradation of HVAF thermally sprayed PEEK coatings

Coatings of PEEK (poly-ether-ether-ketone) have been produced on stainless steel 304 using the High Velocity Air Fuel (HVAF) thermal spray technique. These coatings were produced using 50 and 100 mm nozzle lengths with 200, 300 and 400 mm gun to substrate distances. The thermal degradation of the PEEK during the production of the coatings was assessed with valence band X-ray Photoelectron Spectroscopy (XPS) and attenuated total reflectance Fourier Transform Infrared Spectroscopy (ATR FT-IR). Valence band XPS shows that in general there is minimal degradation of the PEEK during the HVAF thermal spraying process. The FT-IR results show that at the 200 mm gun to substrate (standoff) distance for both nozzle lengths there is more surface degradation of the PEEK coating than at the longer gun to substrate distances. Specifically absorption bands appeared at 2918 and 2850 cm⁻¹, which correspond to alkane -CH₂- asymmetric stretching modes. The reduction of the 1250 cm⁻¹ band as compared with the 1220 cm⁻¹ band, both representing the stretching modes of the ether bonds in PEEK suggests that the degradation occurs at only one of the ether bonds. The phenyl C-H vibration at 673 cm⁻¹ was split for coatings produced at gun-substrate distance of 200 and 300 mm. This indicates a structural change in the phenyl ring possibly indicating a change in the extent of crystallinity of the PEEK polymer.     

Boron-mediated stereoselective syntheses of gamma,gamma-disubstituted allenamides

Propargylic boranes generated by lithiation of optically active N-propargyloxazolidinones undergo reaction with a range of aldehydes to produce gamma,gamma-disubstituted allenamides with good diastereoselectivity.     

One-electron dispersion relations in metallic VO2

One-electron dispersion relations are presented for those bands in VO₂ involved in the metallic conduction which characterises the rutile phase. The density of states at the Fermi energy (0.56 eV) is found to be 2.89 eV^-1 per cation, and the metallic state to be stable against Mott-insulation.     

The 4He condensate: Experiment and theory reconciled

The extremely small values of the condensate fraction in superfluid ⁴He II (of order 1% or less), obtained from inelastic neutron scattering experiments involving large momentum transfers are shown to be consistent with the calculated zero temperature values (of order 10%).     

The Pd-catalysed asymmetric allylic alkylation reactions of sulfamidate imines

The Pd-catalysed asymmetric allylic alkylation (Pd-AAA) of prochiral enamide anions derived from 5H-oxathiazole 2,2-dioxides has been developed. Various 4,5-disubstituted and 4-substituted cyclic sulfamidate imines have participated in the transformation with a range of allyl carbonates—as well as 2-vinyl oxirane, 2-vinyl-N-tosylaziridine, and 2-vinyl-1,1-cyclopropane dicarboxylate—to furnish the desired C-allylated products in moderate to high yields, with high regioselectivites and generally high enantioselectivities. Conversion between N- and C-allyl products was observed, with the N-allylated products converting to the C-allylated products over time. The resulting high-value allylated heterocyclic products all bear a tetrasubstituted stereogenic centre and can be reduced to an allylated chiral sulfamidate or an amino alcohol.

The Pd-catalysed asymmetric allylic alkylation (Pd-AAA) of prochiral enamide anions derived from 5H-oxathiazole 2,2-dioxides has been developed.