On linear versus nonlinear flow rules in strain localization analysis

Summary This note contains some remarks on the analysis of bifurcation phenomena, specifically strain localization (onset of a strain rate discontinuity), in small-deformation elastoplasticity. Nonassociative flow rules are allowed for to cover constitutive models frequently adopted for frictional (and softening) materials such as concrete. The conventional derivation of the localization criterion resting on an incrementally linear comparison material is critically reviewed and compared to the criterion resulting from actual nonlinear plastic flow laws.

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Sommario Si svolgono alcune considerazioni sui fenomeni di biforcazione in solidi elastoplastici in regime di piccole deformazioni (di linearità geometrica) e precisamente sul manifestarsi di localizzazioni intese come discontinuità nel campo delle deformazioni incrementali. Si considerano leggi nonassociate. Vengono così inclusi nella trattazione modelli costitutivi frequentemente adottati per descrivere il comportamento di materiali ad attrito interno e soggetti a danneggiamento (nel senso di degrado di rigidezze elastiche in seguito a deformazioni anelastiche), oltre che soggetti a manifestazioni di instabilità per incrudimento negativo (softening). Si esamina criticamente il criterio, frequentemente adottato in letteratura, di localizzazione fondato su materiale di confronto incrementalmente lineare e si fornisce, corredato da osservazioni comparative, il criterio che risulta dall'assunzione del modello di materiale incrementale non lineare

     

Energy and radiation transfer in a finite convex body with general boundary conditions

Summary We consider a nonlinear system of equations describing the interaction of radiation with matter through a balance of the specific intensity of radiation and the specific internal energy in a convex bounded body.The boundary conditions are not homogeneous. However, suitable additive decompositions of the unknown functions lead to systems with homogeneous boundary conditions and known source terms. We give two such decompositions: one of these, suggested by physical considerations, arises from the study of a nonhomogeneous linear integral equation in a convenient Banach space. The system is studied by using the theory of semigroups of linear operators and nonlinear perturbations in an L ₁ setting. We prove the existence and uniqueness of a positive differentiable solution, which is global in time.

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Sommario Si considera un sistema non lineare di equazioni che descrivono l'interazione della radiazione con la materia e rappresentano il bilancio dell'intensità specifica di radiazione e dell'energia specifica internu in un mezzo finito convesso. Le condizioni al contorno sono di tipo generale e non omogenee; l'introduzione di opportune scomposizioni della funzione incognita permette di ricondurci a sistemi con condizioni al contorno omogenee; ciò comporta l'introduzione di termini di sorgente noti. Si presentano due esempi di scomposizione, uno dei quali, suggerito da considerazioni fisiche, scaturisce dallo studio di un'equazione integrale lineare non omogenea in un opportuno spazio di Banach. Si riduce quindi il problema ad un'equazione di evoluzione non lineare in uno spazio di Banach di norma L ₁.Si prova l'esistenza di un'unica soluzione forte non negativa, utilizzando la teoria dei semigruppi e delle perturbazioni non lineari; tale soluzione risulta inoltre globale nel tempo.

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Work performed under the auspices of C.N.R. (Gruppo Nazionale per la Fisica Matematica) and partially supported by M.P.I.     

A Versatile Organocatalytic Approach for the Synthesis of Enantioenriched gem‐Difluorinated Compounds

The combination of a practical and highly enantioselective organocatalytic reaction, which allows the stereoselective introduction of a benzodithiol group, with a fluorination step, gives a new and effective strategy for the stereoselective synthesis of difluorinated building blocks. The benzodithiol group is a versatile and chameleonic group that can be further functionalized before fluorination, giving customized and tailored useful synthetic strategies. As an example of the application of this facile strategy, the effective enantioselective synthesis of difluoroarundic acid is described.     

Determination of eight polyphenols and pantothenic acid in extra‐virgin olive oil samples by a simple,fast, high‐throughput and sensitive ultra high performance liquid chromatography with tandem mass spectrometry method

A new ultra high performance liquid chromatography coupled with tandem mass spectrometry method for a fast and sensitive determination of eight polyphenols (hydroxytyrosol, catechin, epicatechin, epigallocatechin gallate, oleuropein, quercetin, rutin, tyrosol) and panthotenic acid in extra‐virgin olive oil was developed. The method does not require long sample pre‐treatment and presents the lowest limit of detection and limit of quantitation values present in literature. Inter‐ and intra‐day variability, linear dynamic range of the calibration curve, recovery and matrix effect were also determined and investigated. The method was applied to several oil samples of different type and origin. Given its accuracy, precision and rapidity, the method is characterized by an interestingly high throughput, reliability, and sensitivity.     

Detection of Significant Aprotic Solvent Effects on the Conformational Distribution of Methyl 4‐Nitrophenyl Sulfoxide: From Gas‐Phase Rotational to Liquid‐Crystal NMR Spectroscopy

The conformational equilibrium of methyl 4‐nitrophenyl sulfoxide (MNPSO) was experimentally investigated in the gas phase by using microwave spectroscopy and in isotropic and nematic liquid‐crystal solutions, in which the solvents are nonaqueous and aprotic, by using NMR spectroscopy; moreover, it was theoretically studied in vacuo and in solution at different levels of theory. The overall set of results indicates a significant dependence of the solute conformational distribution on the solvent dielectric permittivity constant: when dissolved in low‐polarity media, the most stable conformation of MNPSO proved to be strongly twisted with respect to that in more polar solvents, in which the conformational distribution maximum essentially coincides with that obtained in the gas phase. We discuss a possible explanation of this behavior, which rests on electrostatic solute–solvent interactions and is supported by calculations of the solute electric dipole moment as a function of the torsional angle. This function shows that the least polar conformation of MNPSO is located at a twist angle close to that of the conformational distribution maximum found in less‐polar solvents. This fact, associated with a relatively flat torsional potential, can justify the stabilization of the twisted conformation by the less‐polar solvents.     

Ultrametricity in the Edwards-Anderson model

We test the property of ultrametricity for the spin-glass three-dimensional Edwards-Anderson model in zero magnetic field with numerical simulations up to 20(3) spins. We find an excellent agreement with the prediction of the mean field theory. Since ultrametricity is not compatible with a trivial structure of the overlap distribution, our result contradicts the droplet theory.     

Mass distribution, polydispersity and focusing properties of carrier ampholytes for IEF. III: pH 2.5-4 intervals

To study the molecular mass distribution and number of species in narrow-range (2-pH-unit wide, in the nominal pI 2-4 or 3-5 interval) carrier ampholytes from four commercial sources (Bio-Lyte, Servalyt, Ampholine and Pharmalyte), a 2-D technique was adopted, consisting of a preparative focusing step in a Rotofor instrument, followed by analysis of every other collected fraction (10 out of 20) by CE-MS. It was found that Ampholine pH 3.5-5 contains 105 different molecular mass (M(r)) compounds, in the M(r) interval 205-965 Da, for a total of 446 isoforms. Bio-Lyte pH 3-5 consists of 84 different M(r) species, in the M(r) range 216-965 Da, for a total of 383 isoforms. Servalyt pH 2-4 is made of 227 different M(r) compounds, in the M(r) interval 204-929 Da, for a total of 1201 isoforms. Pharmalyte pH 2.5-5 comprises 245 amphoteres, in the M(r) range 203-857 Da, for a total of 857 isoforms. Pharmalyte appears to be the best brand, with the vast majority of species focusing sharply at their pI position and almost no 'poor' species, distributed along the entire pH gradient, denoting an extremely shallow pH/mobility curve across the pI value. Due to some overlap with the adjacent acidic pH 4-6 interval, the species in common have been evaluated: the most extended overlaps are found in Ampholine (55% of the species appearing in the two neighbouring intervals) and in Servalyt (47% coincidence). The lowest overlaps are found in Pharmalyte (23%) and in Bio-Lyte (20%).     

Regioselectivity and diasteroselectivity in Pt(II)-mediated "green" catalytic epoxidation of terminal alkenes with hydrogen peroxide: mechanistic insight into a peculiar substrate selectivity

Recently developed electron-poor Pt(II) catalyst 1 with the "green" oxidant 35% hydrogen peroxide displays high activity and complete substrate selectivity in the epoxidation of terminal alkenes because of stringent steric and electronic requirements. In the presence of isolated dienes bearing terminal and internal double bonds, epoxidation is completely regioselective toward the production of terminal epoxides. Insight into the mechanism is gained by means of a reaction progress kinetic analysis approach that underlines the peculiar role of 1 in activating both the alkene and H2O2 in the rate-determining step providing a rare example of nucleophilic oxidation of alkenes by H2O2.