The Forgotten Pyrazines: Exploring the Dakin–West Reaction

Pyrazines are an underreported class of N-heterocycles available from nitrogen-rich biomass presenting an interesting functional alternative for current aromatics. In this work, access to pyrazines obtained from amino acids by using the 90 year old Dakin–West reaction was explored. After a qualitative screening several functional proteinogenic amino acids proved good substrates for this reaction, which were successfully scaled to multigram scale synthesis of the corresponding intermediate α-acetamido ketones. Subsequently, the conditions towards pyrazine formation using δ-amino-levulinic acid were optimized, and these were employed to synthesize a relevant set of five functional dimethylpyrazines in high purity. These pyrazines can be considered a versatile toolbox of aromatic building blocks for a wide range of applications, such as in the synthesis of polymers or metal–organic frameworks.     

A comparison between parallelization approaches in molecular dynamics simulations on GPUs

We test the relative performances of two different approaches to the computation of forces for molecular dynamics simulations on graphics processing units. A “vertex‐based” approach, where a computing thread is started per particle, is compared to an “edge‐based” approach, where a thread is started per each potentially non‐zero interaction. We find that the former is more efficient for systems with many simple interactions per particle while the latter is more efficient if the system has more complicated interactions or fewer of them. By comparing computation times on more and less recent graphics processing unit technology, we predict that, if the current trend of increasing the number of processing cores—as opposed to their computing power—remains, the “edge‐based” approach will gradually become the most efficient choice in an increasing number of cases. © 2014 Wiley Periodicals, Inc.     

Investigation on time scales in a low Reynolds number jet flow using particle-tracking velocimetry

The near flow field of an axisymmetric water jet at Reynolds numbers between 2000 and 5000 is investigated using Particle-Tracking Velocimetry. Measurements are taken in the longitudinal section (along the mean flow) and in cross-sections (orthogonal to the mean flow). From the former, correlation coefficients of the two in-plane velocity components in a Lagrangian framework are obtained: thus Lagrangian integral scales can be computed. Those of the streamwise velocity (axial) component increase on moving away from the centreline, whereas the opposite happens for the vertical velocity (radial) component: integral time scales of the two components are almost equal at the interface between jet and ambient fluids. On the other hand, integral scales are almost constant or increase slightly with the axial direction. In cross-sections, fluid ejection and injection from the jet centreline are observed to be connected to counter-rotating vortices (mushroom): their number and size change with Reynolds number in agreement with results from other authors. The maximum ejection velocity (orthogonal to the mean jet flow), at 3 nozzle diameters downstream of the outlet, is found to be one half of the mean outlet velocity.     

The effect of boundary conditions by the side of the nozzle of a low Reynolds number jet

The near flow field of an axially symmetric water jet at Reynolds numbers from about 1,000 to 10,000 is investigated using laser-induced fluorescence (LIF), laser Doppler anemometry and particle tracking velocimetry. Spanwise and streamwise vortices are detected on the longitudinal plane and on cross-sections. Attention is focused onto the effects of rigid or free boundaries sideways to the nozzle outlet (no-slip or free-slip conditions), and particularly on the start up, growth and interaction of large vortical structures. On average, for the free-slip jet these structures develop more gradually and closer to the nozzle than for the no-slip jet; the local mixedness (derived from LIF measurements) is also higher for the free-slip case. Moreover, the measured velocity field decreases more slowly, with a longer potential core and a higher shear layer (momentum) thickness for the free-slip rather than for the no-slip conditions. The relation between spanwise and streamwise large-scale vortices is clarified by the observation that the ejection of fluid in cross-sections through streamwise vortices is coupled to the pairing of spanwise vortices on the longitudinal section.     

On Leon's criterion

Summary The theory of limit surfaces of resistance for isotropic materials in the space of the principal stress components is recalled. This resistance is viewed in terms of the phenomena of ductile or brittle failure that may arise in alternation for a given material, depending on the type of stress state. A transformation of coordinates is introduced that proves to be very useful for comparing the various limit conditions with that deriving from intrinsic curve theory. Reference is made to the criterion of Stassi, certainly the most reliable of the criteria that allow for dependence of the limit condition on the value of the principal intermediate stress. The theory of the intrinsic curve is generalised by means of Leon's modification, which interprets the phenomenon of tensile fracture. It is shown how Griffith's theory of brittle fracture leads to a limit condition that may be regarded as a special case of the one deriving from Leon's theory. Comparison with some experimental data shows that Leon's theory is well suitable for the study of brittle fracture. A generalisation that takes account of the influence of the principal intermediate stress and so permits good agreement with the experimental data in the field of ductile failure too, whilst conserving the characteristics of Leon's theory in brittle fracture, is proposed.

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Sommario Si richiama la teoria delle superfici limiti di resistenza per materiali isotropi nello spazio delle componenti principali di tensione. Tale resistenza è intesa nei riguardi dei fenomeni di rottura duttile o fragile che possono intervenire in alternativa per uno stesso materiale in dipendenza dei diversi tipi di stato tensionale. Si introduce una trasformazione di coordinate che risulta particolarmente utile per il confronto delle varie condizioni limiti con quella derivante dalla teoria della curva intrinseca. In particolare si fa riferimento al criterio di Stassi, certo il più attendibile fra quelli che prendono in considerazione la dipendenza della condizione limite dal valore della tensione principale intermedia. La teoria della curva intrinseca viene generalizzata attraverso la modifica di Leon, che interpreta il fenomeno della crisi per distacco. Si mostra come la teoria di Griffith della rottura fragile conduca ad una condizione limite che si può considerare un caso particolare di quella derivante dalla teoria di Leon. Per confronto con alcuni dati sperimentali si verifica che questa ultima teoria bene si presta allo studio dei fenomeni di rottura fragile. Si propone una generalizzazione che, tenendo conto dell'influenza della tensione principale intermedia, consente di ottenere un buon accordo con l'esperienza anche nel campo di rottura duttile, pur conservando le caratteristiche della teoria di Leon in quello della rottura fragile.

     

Oxidation of α-Acetoxy Acetals with Trichloroisocyanuric Acid

α-Acetoxy acid methyl esters are prepared in excellent yields by treating aliphatic α-acetoxy dimethyl acetals with trichloroisocyanuric acid in DMF.