Effect of Two‐Dimensional Crystal Orbitals on Fermi Surfaces and Electron Transport in Three‐Dimensional Perovskite Oxides

Perovskite oxides are candidate materials in catalysis, fuel cells, thermoelectrics, and electronics, where electronic transport is vital to their use. While the fundamental transport properties of these materials have been heavily studied, there are still key features that are not well understood, including the temperature‐squared behavior of their resistivities. Standard transport models fail to account for this atypical property because Fermi surfaces of many perovskite oxides are low‐dimensional and distinct from traditional semiconductors. In this work, the low‐dimensional Fermi surfaces of perovskite oxides are chemically interpreted in terms of two‐dimensional crystal orbitals that form the conduction bands. Using SrTiO₃ as a case study, the d/p‐hybridization that creates these low‐dimensional electronic structures is reviewed and connected to its fundamentally different electronic properties. A low‐dimensional band model explains several experimental transport properties, including the temperature and carrier‐density dependence of the effective mass, the carrier‐density dependence of scattering, and the temperature dependence of resistivity. This work highlights how chemical bonding influences semiconductor transport.     

An integrated optical coupler used in a fibre interferometry system for on-line surface measurements

In this paper, an integrated optical (IO) coupler produced using the technique of silica-on-silicon was described. A fibre interferometry system using the IO coupler was then developed and its performance was tested. The environmental noise and disturbances in this system were reduced significantly after combining a reference interferometer and an all fibre polarisation scrambler so as to produce good stability and repeatability. Furthermore, the measurement mirror was mounted on a piezoelectric transducer (PZT) and disturbances were introduced in the system in order to simulate on-line surface measurements. Both a commercial fibre and the IO couplers were compared using this method. Better experimental results were achieved when the system using the IO coupler than using the fibre one.     

Shape‐directed platinum nanoparticle synthesis: nanoscale design of novel catalysts

Platinum‐based catalytic materials have received significant attention, particularly in the shape and size control of faceted materials for catalysis. More recently, there has been a rapid increase in the number of reports of successful preparations in this field; however, a fundamental understanding of controlled growth towards catalytic material design is essential for future implementation and broad application. In this review, we provide an overview of the recent findings reported since 2009, focusing on methods for shape control as well as the effects of exposed surface facets on select catalytic reactions. Copyright © 2013 John Wiley & Sons, Ltd.     

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Electronically varied quinazolinaps for asymmetric catalysis

The synthesis and resolution of electronically varied axially chiral Quinazolinaps is reported. These ligands bear different aryl groups on the donor phosphorus atom and were synthesised as part of our investigations into electronic effects within this ligand class. A diastereomerically pure palladacycle of one ligand was characterised by X-ray crystallography. The application of these Quinazolinaps to the rhodium-catalysed hydroboration of vinylarenes resulted in enantioselectivities of up to 92%. Their application to the palladium-catalysed allylic alkylation of 1,3-diphenylprop-2-enyl acetate resulted in conversions of up to 99% and enantioselectivities of up to 94%.     

Rapid method for determination of plutonium, americium and curium in large soil samples

The analysis of actinides in environmental soil and sediment samples is very important for environmental monitoring. There is a need to measure actinide isotopes with very low detection limits. A new, rapid actinide separation method has been developed and implemented that allows the measurement of plutonium, americium and curium isotopes in large soil samples (100–200 g) with high chemical recoveries and effective removal of matrix interferences. This method uses stacked TEVA Resin®, TRU Resin® and DGA-Resin® cartridges from Eichrom Technologies (Darien, IL, USA) that allows the rapid separation of plutonium (Pu), americium (Am), and curium (Cm) using a single multi-stage column combined with alpha-spectrometry. The method combines an acid leach step and innovative matrix removal using cerium fluoride precipitation to remove the difficult soil matrix. This method is unique in that it provides high tracer recoveries and effective removal of interferences with small extraction chromatography columns instead of large ion-exchange resin columns that generate large amounts of acid waste. By using vacuum box cartridge technology with rapid flow rates, sample preparation time is minimized.     

Regioselective reactivity of an asymmetric tetravalent di[dihydroxotin(IV)] bis-porphyrin host driven by hydrogen-bond templation

Local molecular environment effects on the rates of ligand exchange at an asymmetric di[dihydroxotin(IV)] bis-porphyrin 5 are examined. The host 5 possesses four non-equivalent tin(IV)-ligand binding sites that are distinguished by their position relative to a shallow cavity, by the steric environment at each binding site and by electronic-structure differences between the constituent porphyrin and quinoxalinoporphyrin macrocycles. These design features of the asymmetric host are confirmed by X-ray crystal structure analysis. Binding experiments with monodentate carboxylic acids and bidentate dicarboxylic acids show significant differences in the rate of ligand exchange at each of the four tin(IV) binding sites. For monodentate carboxylic acids, binding preferentially occurs at the exterior porphyrin site. Further addition of carboxylic acid results in sequential binding at the quinoxalinoporphyrin sites and lastly at the interior site on the porphyrin, with high regioselectivity. These selective binding outcomes are immediately apparent by NMR spectroscopy. A series of 2D NMR spectroscopy experiments allowed identification of the preferred binding sites at the host. This positively identifies that steric hindrance and electron-withdrawing functionality on the porphyrin macrocycle impede ligand exchange. However, these effects are overcome by dicarboxylic acid guests, which form ditopic hydrogen-bond interactions between the intracavity hydroxo ligands in the initial stage of ligand exchange, leading to regioselective binding between the tin(IV) sites within the cavity. It is envisaged that the factors identified herein that define regioselective ligand exchange at host 5 will find wider application in supramolecular systems incorporating tin(IV) porphyrins.     

Positioning a Carbon–Fluorine Bond over the π Cloud of an Aromatic Ring: A Different Type of Arene Activation

It is known that the fluoro group has only a small effect on the rates of electrophilic aromatic substitutions. Imagine instead a carbon–fluorine (C?F) bond positioned tightly over the π cloud of an aryl ring—such an orthogonal, noncovalent arrangement could instead stabilize a positively charged arene intermediate or transition state, giving rise to novel electrophilic aromatic substitution chemistry. Herein, we report the synthesis and study of molecule 1 , containing a rigid C?F???Ar interaction that plays a prominent role in both its reaction chemistry and spectroscopy. For example, we established that the C?F???Ar interaction can bring about a >1500 fold increase in the relative rate of an aromatic nitration reaction, affording functionalization on the activated ring exclusively. Overall, these results establish fluoro as a through‐space directing/activating group that complements the traditional role of fluorine as a slightly deactivating aryl substituent in nitrations.