Resin dynamics contributes to the NMR line broadening of organic molecules grafted onto a polystyrene resin

Despite the use of high resolution magic angle spinning NMR, the NMR linewidth of anchored molecules on the commonly used Merrifield solid phase resins remains larger than that of the corresponding molecules in solution. We investigate the different mechanisms that might be at the origin of this line broadening. Experimentally, we use the CPMG method to determine the (15)N relaxation times of a tethered tripeptide and show that the slow resin dynamics significantly contributes to the transverse relaxation.     

Description of pulse propagation in a dispersive medium by use of a pulse quality factor

We investigated how the duration of short laser pulses evolves in a dispersive material, using rms widths and a propagation law based on a pulse quality factor. Experiments were carried out with femtosecond pulses (10 to 25fs at the temporal waist) propagating in bulk fused silica. Excellent agreement was found between theory and experiment. This approach does not require complete characterization of laser pulses and eliminates the need for any assumption regarding the interpretation of autocorrelation traces. The method is of general validity, and it can be applied to pulses of arbitrary shape.     

On the representations of quantum oscillator algebra

It is shown that for q<1, the quantum oscillator algebra has a supplementary family of representations inequivalent to the usual q-Fock representation, with no counterpart at the limit q=1. They are used to build representations of SU _q (1,1) and E(2) in Schwinger's way.     

Synchronized EPI phase contrast velocimetry in a mixing reactor

Notwithstanding its widespread use in cardiovascular and functional MRI studies, Echo Planar Imaging (EPI) has only recently been subjected to systematic validation studies. Most velocity measurement studies employing such ultrafast MRI methods involve the use of phantoms characterized by rigid or deformable solid motion. The current implementation involves a rotating phantom (angular velocity up to 10.5 rpm) with a superimposed swirling liquid flow (with axial velocities ranging between 0.145 and 0.27 cm/s) of water doped with copper sulfate. The standard implementation of single-shot EPI with phase contrast velocity encoding allows the complete mapping of the Eulerian velocity field in slices perpendicular to the rotation axis following a subtractive procedure requiring the synchronized acquisition of each velocity component on each selected transverse slice during two revolutions of the rotor. The image acquisition time is 100 ms (per velocity component) at each 64 x 64 slice. In addition to acquiring full-field velocity data for future direct comparisons with other techniques, EPI is employed here for the first time to reconstruct the three-dimensional flow field between the blades of a partitioned pipe mixer.     

Site segregation in size-mismatched nanoalloys: Application to Cu-Ag

In order to determine the energetic driving forces for surface segregation in bimetallic clusters, we use a combined approach coupling numerical simulations within an N-body interatomic potential and a lattice-gas model. This approach, which has been used successfully to study both the superficial segregation in semi-infinite alloys and the intergranular segregation, allows us to determine the relative contributions of the three elementary driving forces for the different sites of the cluster surface (vertices, edges and facets) in both dilute limits for the Cu-Ag system. We show that the segregation hierarchy based on broken-bond arguments (preferential segregation to the vertex sites, less to edge sites, and least to facet sites) is not at all universal. In particular, unusual hierarchies are predicted when the sizes of the constituents are strongly different. Furthermore, we compare the segregation driving forces for cubo-octahedral and icosahedral clusters. They are similar for the vertex sites and edge sites, whereas they differ significantly for the sites of the triangular facets. The segregation of the species with the largest atomic radius (Ag) is indeed largely enhanced in the icosahedral structure due to dilations of the orthoradial distances.     

Amplitude modulation sensitivity as a mechanism increment detection

Detectability of a tonal signal added to a tonal masker increases with increasing duration ("temporal integration"), up to some maximum duration. Initially assumed to be some form of energy integration over time, this phenomenon is now often described as the result of a statistical "multiple looks" process. For continuous maskers, listeners may also use a mechanism sensitive to changes in stimulus intensity, possibly a result of inherent sensitivity to amplitude modulation (AM). In order to examine this hypothesis, change detection was investigated in the presence of AM maskers presented at either the same carrier frequency as the target signal or at a distant frequency. The results are compatible with the hypothesis that listeners detect intensity increments by using change-detection mechanisms (modeled here as the outputs of a bank of modulation filters) sensitive to envelope modulation at both low (4-16 Hz) and high (around 100 Hz) rates. AM masking occurred even when the masker was at a carrier frequency more than two octaves above that of the signal to be detected. This finding is also compatible with the hypothesis that similar mechanisms underlie sensitivity to AM (where across-frequency masking is commonly shown) and detection of intensity increments.     

Detection of multicomponent signals: effect of difference in level between components

The detection of multicomponent signals for which the components are not equidetectable is precisely investigated as a function of the level difference ΔL(i∕j) between components. The detection thresholds are determined for a seven-tone complex signal with random starting phases masked by white noise. Level differences between the components are examined. A model for non-equidetectable conditions based on the statistical summation model is described. The improvement in detection is calculated from the level difference between components that is related to the thresholds for single components. The model predictions are in accordance with the experimental results.     

Glycerol and glycerol carbonate as ultraviscous solvents for mixture analysis by NMR

NMR of weakly polar analytes in an apolar ultraviscous solvent has recently been proposed for mixture analysis as a pertinent alternative to the DOSY experiment. The present article reports the first use of glycerol and glycerol carbonate as polar solvents for the NMR analysis of a model mixture of dipeptides. This work demonstrates the high potentiality of these solvents for the analysis of mixtures made of polar and potentially bioactive compounds. Medium-sized molecules slowly reorient in glycerol and glycerol carbonate under particular temperature conditions, so that solute resonances may show spin diffusion in NOESY spectra, thus opening the way to mixture analysis. Glycerol and glycerol carbonate have turned out to be ultraviscous solvents of choice for the individualization of four structurally close mixed dipeptides: Leu-Val, Leu-Tyr, Gly-Tyr and Ala-Tyr by means of 1D and 2D NOESY experiments. Selective sample excitation and signal detection were implemented to eliminate the intense proton signals of the non-deuterated solvents. Moreover, the recording of a multiplet selective 2D NOESY-TOCSY has shown that the analytical power of NMR in highly viscous solvents is not limited to the extraction of mixture component 1D subspectra but may also yield some supplementary information about atom connectivity within components.     

Backward particle production in neutrino neon interactions

Backward proton and pion production is studied in ν and \(\bar v\) charged current interactions in neon. The results are compared with other experiments and theory. The complete backward proton data is compatible with protons produced by reinteractions in the nucleus. However in events with only one proton, muon variables appear correlated to those for the backward proton, as expected by the two-nucleon correlation model.     

High-frequency ultrasound backscattering by blood: analytical and semianalytical models of the erythrocyte cross section

This paper proposes analytical and semianalytical models of the ultrasonic backscattering cross section (BCS) of various geometrical shapes mimicking a red blood cell (RBC) for frequencies varying from 0 to 90 MHz. By assuming the first-order Born approximation and by modeling the shape of a RBC by a realistic biconcave volume, different scattering behaviors were identified for increasing frequencies. For frequencies below 18 MHz, a RBC can be considered a Rayleigh scatterer. For frequencies less than 39 MHz, the general concept of acoustic inertia tensor is introduced to describe the variation of the BCS with the frequency and the incidence direction. For frequencies below 90 MHz, ultrasound backscattering by a RBC is equivalent to backscattering by a cylinder of height 2 microm and diameter 7.8 microm. These results lay the basis of ultrasonic characterization of RBC aggregation by proposing a method that distinguishes the contribution of the individual RBC acoustical characteristics from collective effects, on the global blood backscattering coefficient. A new method of data reduction that models the frequency dependence of the ultrasonic BCS of micron-sized weak scatterers is also proposed. Applications of this method are in tissue characterization as well as in hematology.