Analytical theory of gamma-encoded double-quantum recoupling sequences in solid-state nuclear magnetic resonance

Many important double-quantum recoupling techniques in solid-state NMR are classified as being gamma-encoded. This means that the phase of the double-quantum effective Hamiltonian, but not its amplitude, depends on the third Euler angle defining the orientation of the molecular spin system in the frame of the magic-angle-spinning rotor. In this paper, we provide closed analytical solutions for the dependence of the powder-average double-quantum-filtered signal on the recoupling times, within the average Hamiltonian approximation for gamma-encoded pulse sequences. The validity of the analytical solutions is tested by numerical simulations. The internuclear distance in a (13)C(2)-labelled retinal is estimated by fitting the analytical curves to experimental double-quantum data.     

J-Stabilization of singlet states in the solution NMR of multiple-spin systems

Long-lived singlet states have been observed in the solution NMR of spin systems containing more than two coupled spins, despite the fact that the singlet state is expected to be quenched by small long-range J-couplings. We show that the stability of localized singlet states may be explained by taking into account the intra-pair J-coupling between the two spins which participate in the singlet state. The relatively strong intra-pair J-coupling protects the singlet state against quenching by weaker out-of-pair J-couplings.     

Accelerated formation of the boron–oxygen complex in p‐type Czochralski silicon

This Letter reports on the acceleration of the rate of formation of the boron–oxygen defect in p‐type Czochralski silicon with illumination intensities in excess of 2.1 × 10¹⁷ photons/cm²/s. It is observed that increased light intensities greatly enhance the rate of defect formation, without increasing the saturation concentration of the defect. These results suggest a dependence of the defect formation rate upon the total majority carrier concentration. Finally, a method using temperatures up to 475 K and an illumination intensity of 1.68 × 10¹⁹ photons/cm²/s is shown to result in near‐complete defect formation within seconds. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Batch versus Flow Photochemistry: A Revealing Comparison of Yield and Productivity

The use of flow photochemistry and its apparent superiority over batch has been reported by a number of groups in recent years. To rigorously determine whether flow does indeed have an advantage over batch, a broad range of synthetic photochemical transformations were optimized in both reactor modes and their yields and productivities compared. Surprisingly, yields were essentially identical in all comparative cases. Even more revealing was the observation that the productivity of flow reactors varied very little to that of their batch counterparts when the key reaction parameters were matched. Those with a single layer of fluorinated ethylene propylene (FEP) had an average productivity 20 % lower than that of batch, whereas three‐layer reactors were 20 % more productive. Finally, the utility of flow chemistry was demonstrated in the scale‐up of the ring‐opening reaction of a potentially explosive [1.1.1] propellane with butane‐2,3‐dione.     

Bis{2,6‐bis[3‐(2,4,6‐trimethylphenyl)pyrazol‐1‐yl‐κN2]pyridine‐κN}zinc(II) diperchlorate bis(nitromethane) solvate

The title compound, [Zn(C₂₉H₂₉N₅)₂](ClO₄)₂·2CH₃NO₂, contains a Zn^II ion showing only small deviations from local D_2d symmetry. The lower rhombicity exhibited by this complex compared with that of its Cu^II congener suggests that the highly rhombic stereochemistry exhibited by the latter is largely imposed by the stereoelectronic preferences of the Cu^II ion.     

Designing and using professional development resources for inquiry-based learning

This paper describes an attempt to design, analyse and refine professional development (PD) resources that encourage the implementation of inquiry-based learning (IBL). We describe the iterative development of the resources in England with over 100 mathematics teachers from secondary, tertiary and adult education and then analyse the impact these resources had on teachers’ beliefs and practices and the issues arising. This evaluation revealed that teachers had moved away from transmission-based orientations, encouraged by the use of less structured tasks and sample lesson plans, but some found it difficult to adopt IBL pedagogies. The most significant issues for teachers may be summarised as: confusing IBL with ‘discovery’ learning; developing and managing collaborative cultures within the classroom; and planning lessons that adapt to the emerging needs of learners.     

A notable conversion of a thiolacetate to its corresponding sulfonyl chloride

A direct conversion of a thiolacetate to its corresponding sulfonyl chloride in the presence of acid and base sensitive functional groups is described.     

Stretching polymer chains

This paper is based on a presentation given at the de Gennes discussion meeting held at Chamonix in February 2009. The paper gives a personal review of the way developments relating to the stretching of polymer chains has taken place over the last 40 years. de Gennes was very influential in relation to chain dynamics concepts at the University of Bristol, where many pioneering concepts relating to chain stretching were developed by the late Sir Charles Frank and Andrew Keller. The paper reviews basic concepts on extensional rheology, droplet deformation, chain extension from extensional flow and the achievement of high chain extension for high-modulus polyethylene. The paper also reviews recent developments concerning the influence of chain stretching on polymer melt processing.