NMR investigations of proline and its derivatives: Part 5. A semi-empirical calculation of the endo-exo equilibrium of the pyrrolidine ring and its pH-dependence

Exo-endo interconversion of the pyrrolidine ring of l-proline is studied, using the correlation of vicinal interproton couplings to the molecular geometry parameters via the sum of direct carbon-proton couplings.     

An efficient semi‐implicit subgrid method for free‐surface flows on hierarchical grids

We present a new modelling strategy for improving the efficiency of computationally intensive flow problems in environmental free‐surface flows. The approach combines a recently developed semi‐implicit subgrid method with a hierarchical grid solution strategy. The method allows the incorporation of high‐resolution data on subgrid scale to obtain a more accurate and efficient hydrodynamic model. The subgrid method improves the efficiency of the hierarchical grid method by providing better solutions on coarse grids. The method is applicable to both steady and unsteady flows, but we particularly focus on river flows with steady boundary conditions. There, the combined hierarchical grid–subgrid method reduces the computational effort to obtain a steady state with factors up to 43. For unsteady models, the method can be used for efficiently generating accurate initial conditions on high‐resolution grids. Additionally, the method provides automatic insight in grid convergence. We demonstrate the efficiency and applicability of the method using a schematic test for the vortex shedding around a circular cylinder and a real‐world river case study. Copyright © 2015 John Wiley & Sons, Ltd.     

A New Technique for the Generation of Alkyl Metathiophosphates

Abstract

We have recently described a technique for the generation of alkyl metaphosphates, ROPO₂, by thermolysis of phosphoramidic acids of structure ROP(O)(OH)NHR (1) [1]. We have now successfully shown that alkyl metathiophosphates, ROP(S)(O), which have received but little previous consideration [2], can be generated by the same approach. Compounds 2 (Ad = 1-adamantyl) and 3 were synthesized for this study. Both were easily fragmented on heating in an inert solvent. When an alcohol trapping agent was present each gave EtO-P(S)(OR)(OH) consistent with EtOP(S)(O) as an intermediate. In the absence of a trap, 2 was cleanly converted to the pyrophosphate derivative 4 in a process established to follow first-order kinetics, thus proving that decomposition of 2 proceeded by elimination of EtOP(S)(O). Compound 3 decomposed by mixed first and second order kinetics, and gave a more complex mixture of products. EtOP(S)(O) generated from 2 phosphorylated the OH groups on the surface of silica gel, a process previously demonstrated for ROPO₂. With a monoester of phosphoric acid, the mixed monothiopyrophosphate EtOP(S)(OH)-O-P(O)(OH)(OR) was formed in a new synthetic approach to these valuable compounds.     

Room temperature ultralow threshold GaN nanowire polariton laser

We report ultralow threshold polariton lasing from a single GaN nanowire strongly coupled to a large-area dielectric microcavity. The threshold carrier density is 3 orders of magnitude lower than that of photon lasing observed in the same device, and 2 orders of magnitude lower than any existing room-temperature polariton devices. Spectral, polarization, and coherence properties of the emission were measured to confirm polariton lasing.     

Multi‐Electron Reactions Enabled by Anion‐Based Redox Chemistry for High‐Energy Multivalent Rechargeable Batteries

The development of multivalent metal (such as Mg and Ca) based battery systems is hindered by lack of suitable cathode chemistry that shows reversible multi‐electron redox reactions. Cationic redox centres in the classical cathodes can only afford stepwise single‐electron transfer, which are not ideal for multivalent‐ion storage. The charge imbalance during multivalent ion insertion might lead to an additional kinetic barrier for ion mobility. Therefore, multivalent battery cathodes only exhibit slope‐like voltage profiles with insertion/extraction redox of less than one electron. Taking VS₄ as a model material, reversible two‐electron redox with cationic–anionic contributions is verified in both rechargeable Mg batteries (RMBs) and rechargeable Ca batteries (RCBs). The corresponding cells exhibit high capacities of >300 mAh g^?1 at a current density of 100 mA g^?1 in both RMBs and RCBs, resulting in a high energy density of >300 Wh kg^?1 for RMBs and >500 Wh kg^?1 for RCBs. Mechanistic studies reveal a unique redox activity mainly at anionic sulfides moieties and fast Mg²⁺ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS₄.     

Mössbauer studies of ultrathin Fe/Pt multilayers

Summary A series of ultrathin Fe/Pt multilayers, prepared by magnetron sputtering, were studied by CEMS and transmission MS. The Fe-layer thickness varied from 3 to 12.5 ? and that of Pt from 5 ? to 39 ?. The 3 ?/9 ? Pt sample displays magnetic hyperfine structure at RT, while the 3 Fe/19 Pt sample is paramagnetic at RT, demonstrating the effect of interlayer interaction. Both samples display out-of-plane magnetic anisotropy with 39° angle with respect to the vertical for the former and nearly 0° angle for the latter. The analysis of the spectra of samples with thickness larger than one monolayer of Fe is done with components assigned to individual Fe monolayers. In all these cases a component appears with a hyperfine field larger by ∼ 10% at RT and 17% at L.He than the corresponding values of α-Fe. This component is attributed to the first monolayer below the Fe/Pt interface in accordance with similar results in the Fe/Pd system and with theoretical predictions. Paper presented at ICAME-95, Rimini, 10–16, September 1995     

Boundary Value Problems for Systems of Functional Differential Equations

Algorithms for finding an approximate solution of boundary value problems for systems of functional ordinary differential equations are studied. Sufficient conditions for consistency and convergence of these methods are given. In the last section, a construction of methods of arbitrary order is presented.     

Control of a coupled map lattice model for vortex shedding in the wake of a cylinder

The flow behind a vibrating flexible cable at low Reynolds numbers can exhibit complex wake structures such as lace-like patterns, vortex dislocations and frequency cells. These structures have been observed in experiments and numerical simulations, and are predicted by a previously developed low-order coupled map lattice (CML). The discrete (in time and space) CML models consist of a series of diffusively coupled circle map oscillators along the cable span. Motivated by a desire to modify the complex wake patterns behind flexible vibrating cables we have studied the addition of control terms into the highly efficient CML models and explored the resulting dynamics. Proportional, adaptive proportional and discontinuous non-linear (DNL) control methods were used to derive the control laws. The first method employed occasional proportional feedback. The adaptive method used spatio-temporal feedback control. The DNL method used a discontinuous feedback linearization procedure, and the controller was designed for the resulting linearized system using eigenvalue assignment. These techniques were applied to a modeled vortex dislocation structure in the wake of a vibrating cable in uniform freestream flow. Parallel shedding patterns were achieved for a range of forcing frequency-forcing amplitude combinations studied to validate the control theory. The adaptive proportional and DNL methods were found to be more effective than the proportional control method due to the incorporation of a spatially varying feedback gain across the cylinder span. The DNL method was found to be the most efficient controller of the low-order CML model. The required control level across the cable span was correlated to the 1/1 lock-on behavior of the temporal circle map.