Large Hexagonal Bi‐ and Trilayer Graphene Single Crystals with Varied Interlayer Rotations

Bi‐ and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the synthesis of high‐quality large‐size bi‐ and trilayer graphene single crystals still remains a challenge. Here, the synthesis of 100 μm pyramid‐like hexagonal bi‐ and trilayer graphene single‐crystal domains on Cu foils using chemical vapor deposition is reported. The as‐produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier‐transformed infrared spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first‐principle calculations, it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the experimental observations.     

BIMP‐Catalyzed 1,3‐Prototropic Shift for the Highly Enantioselective Synthesis of Conjugated Cyclohexenones

A bifunctional iminophosphorane (BIMP)‐catalysed enantioselective synthesis of α,β‐unsaturated cyclohexenones through a facially selective 1,3‐prototropic shift of β,γ‐unsaturated prochiral isomers, under mild reaction conditions and in short reaction times, on a range of structurally diverse substrates, is reported. α,β‐Unsaturated cyclohexenone products primed for downstream derivatisation were obtained in high yields (up to 99 %) and consistently high enantioselectivity (up to 99 % ee). Computational studies into the reaction mechanism and origins of enantioselectivity, including multivariate linear regression of TS energy, were carried out and the obtained data were found to be in good agreement with experimental findings.     

Error vector choice in direct inversion in the iterative subspace method

Based on Banach's principle, we formally obtain possible choices for an error vector in the direct inversion in the iterative subspace (DIIS) method. These choices not only include all previously proposed error vectors, but also a new type of error vector which is computationally efficient and applicable to much wider range of problems. The error vector analysis also reveals a strong connection between DIIS and damping, thus adding to understanding of the reasons behind DIIS's effect on convergence. We illustrate our conclusions with several examples. © 1996 by John Wiley & Sons, Inc.     

Development and evaluation of a detailed mechanism for the atmospheric reactions of isoprene and NOx

A detailed photochemical mechanism for the atmospheric reactions of isoprene and its major oxidation products in the presence of NO_x, which incorporates the most recent laboratory results and our current understanding of the system, is described. It is evaluated by comparing its predictions against results of NO_x-air irradiations of isoprene and its two major products, methacrolein, and methyl vinyl ketone (MVK), in five different types of environmental chambers at two different laboratories. In most cases it simulated experimental results within the uncertainty of the data and the chamber and run characterization model. However, the photodecomposition quantum yields of methacrolein and MVK and the organic nitrate yield from the OH + isoprene reaction had to be adjusted to obtain satisfactory simulations of the data. The major discrepancy observed was that the model tended to underpredict PAN by ca. 40% in the isoprene experiments, despite the fact that the model predicted PAN from methacrolein and MVK reasonably well. The uncertainties and additional data needed to completely characterize the isoprene atmospheric photooxidation system are discussed. © 1996 John Wiley & Sons, Inc.     

Efficacy and Limitations of Chemically Diverse Small-Molecule Enzyme-Inhibitors against the Synergistic Coagulotoxic Activities of Bitis Viper Venoms

Snakebite remains a significant public health burden globally, disproportionately affecting low-income and impoverished regions of the world. Recently, researchers have begun to focus on the use of small-molecule inhibitors as potential candidates for the neutralisation of key snake venom toxins and as potential field therapies. Bitis vipers represent some of the most medically important as well as frequently encountered snake species in Africa, with a number of species possessing anticoagulant phospholipase A₂ (PLA₂) toxins that prevent the prothrombinase complex from inducing clot formation. Additionally, species within the genus are known to exert pseudo-procoagulant activity, whereby kallikrein enzymatic toxins cleave fibrinogen to form a weak fibrin clot that rapidly degrades, thereby depleting fibrinogen levels and contributing to the net anticoagulant state. Utilising well-validated coagulation assays measuring time until clot formation, this study addresses the in vitro efficacy of three small molecule enzyme inhibitors (marimastat, prinomastat and varespladib) in neutralising these aforementioned activities. The PLA₂ inhibitor varespladib showed the greatest efficacy for the neutralisation of PLA₂-driven anticoagulant venom activity, with the metalloproteinase inhibitors prinomastat and marimastat both showing low and highly variable degrees of cross-neutralisation with PLA₂ anticoagulant toxicity. However, none of the inhibitors showed efficacy in neutralising the pseudo-procoagulant venom activity exerted by the venom of B. caudalis. Our results highlight the complex nature of snake venoms, for which single-compound treatments will not be universally effective, but combinations might prove highly effective. Despite the limitations of these inhibitors with regards to in vitro kallikrein enzyme pseudo-procoagulant venom activity, our results further support the growing body of literature indicating the potential use of small molecule inhibitors to enhance first-aid treatment of snakebite envenoming, particularly in cases where hospital and thus antivenom treatment is either unavailable or far away.     

A novel piezo-optical styrene sensor incorporating polymer-supported tribromide ion

Encapsulation of Amberlyst A-26 supported tribromide in a 10,000 MW polyethylene glycol matrix gives a robust colour-sensitive reagent matrix which can be deposited on indium-tin oxide coated polyvinylidene fluoride (PVDF) piezoelectric film for the detection of styrene (and other alkene) vapours.     

Pyrolysis-gas chromatography/mass spectrometry of a coal extract and its fractions separated by planar chromatography: correlation of structural features with molecular mass

The structural characterisation of a coal liquefaction extract and its three fractions separated by planar chromatography has been described. Size exclusion chromatography showed the molecular mass distributions to become progressively larger with decreasing mobility on the plate. UV-fluorescence spectroscopy of the fractions indicated parallel increases in the sizes of polynuclear aromatic ring systems. Analysis by probe-mass spectrometry of the 'whole' coal extract showed the expected array of small polynuclear aromatic groups extending to m/z 450. The probe mass spectra of the lightest fraction ('mobile in pyridine and acetonitrile') showed similar features, except for effects due to vacuum drying to remove solvent. In sharp contrast, the two heaviest fractions ('mobile in pyridine and immobile in acetonitrile' and 'immobile in pyridine') showed no significant ions other than those from residual NMP solvent (m/z 98 and 99). Pyrolysis-gas chromatography/mass spectrometry of these two heaviest fractions showed only traces of aromatic compounds or fragments. The aromatic pyrolysis products of these fractions were too large and involatile to pass through the GC column. The major components observed in the pyrolysis-gas chromatography/mass spectrometry of the two heavy fractions were alkanes and alkenes, ranging between C10-C25. Since none of the samples contained free alkanes, alkenes or cycloalkanes before pyrolysis, they were generated during the pyrolysis step. The shifts of UV-fluorescence spectral intensity to shorter wavelengths with decreasing size indicated by size exclusion chromatography (SEC) provide direct evidence of differences in structure with changing molecular mass. This evidence strongly suggests that species identified as being of large molecular mass in this extract sample are not composed of molecular aggregates. It remains difficult to establish whether and when it would be legitimate to invoke molecular aggregates to explain the large molecular masses (MMs) identified here and in other work. Copyright 2000 John Wiley & Sons, Ltd.     

Barriers to internal rotation in 1,3,5-trineopentylbenzenes—II: Unsymmetrically halogen-substituted compounds

The temperature dependence of the three different methylene AB spectra due to the 1-, 3- and 5-neopentyl groups in 2-chloro-4-iodo, 2-bromo-4-iodo and 2-bromo-4-chloro-1,3,5-trineopentyl-benzene was studied by proton magenetic resonance measurements at 60 MHz. The identification of the lines of the AB quartets at low temperature was carried out on an HA-100 spectrometer by means of the INDOR technique. The activation parameters for all three barriers in each compound were found to be approximately the same, which perhaps reflects the interdependence of the rotations, with the smaller of the two halogens determining the size of the barrier. An explanation in terms of magnetic nonequivalence induced in the 1- and 5-methylenes by the 3-neopentyl group is considered to be most plausible.