Christel Gervais

Resolution enhancement in solid-state MQ-MAS experiments achieved by composite decoupling

It is shown that, in the presence of strong heteronuclear dipolar couplings, resolution can be significantly improved in solid‐state multiple quantum magic angle spinning NMR experiments on half integer quadrupolar nuclei by applying composite decoupling schemes during triple quantum evolution and acquisition. Reduction of the effects of heteronuclear dipolar coupling during the multiple quantum evolution period is shown to lead to improved resolution of 27Al sites in crystalline chiolite (Na5Al3F14) and 11B sites in a polyborazilene sample.

Interfacing a heteropolytungstate complex and gelatin through a coacervation process: design of bionanocomposite films as novel electrocatalysts

Bionanocomposite films on glassy carbon electrodes (GCEs) have been prepared by a very straightforward and reliable method based on a layer-by-layer deposition of polyoxometalate (POM) (i.e. [BW12O40]5−) and gelatin solutions. The strong immobilisation of [BW12O40]5− on the surface of the GCE results from electrostatic interactions between gelatin and POM, as evidenced in a hybrid hydrogel prepared by a coacervation process. The GCE has also been modified by imidazolium based ionic liquids in order to increase the charge transfer rate. Two modified electrodes with one or two POM layers have been prepared by this strategy. When compared to other POM-based modified electrodes, the novel electrodes with two POM layers exhibit excellent electrocatalytic performance for nitrite detection at pH 3 in terms of sensitivity (868 mA M−1 cm−2) and linear range of nitrite concentrations (50–600 μM).

Modeling the attenuated total reflectance infrared (ATR-FTIR) spectrum of apatite

Abstract Attenuated total reflectance (ATR) infrared spectra were measured on a synthetic and a natural fluorapatite sample. A modeling approach based on the computation of the Fresnel reflection coefficient between the ATR crystal and the powder sample was used to analyze the line shape of the spectra. The dielectric properties of the samples were related to those of pure fluorapatite using an effective medium approach, based on Maxwell–Garnett and Bruggeman models. The Bruggeman effective medium model leads to a very good agreement with the experimental data recorded on the synthetic fluorapatite sample. The poorer agreement observed on the natural sample suggests a more significant heterogeneity of the sample at a characteristic length scale larger than the mid-infrared characteristic wavelength, i.e., about 10 micrometers. The results demonstrate the prominent role of macroscopic electrostatic effects over fine details of the microscopic structure in determining the line shape of strong ATR bands.

Molecular Level Characterization of the Structure and Interactions in Peptide-Functionalized Metal-Organic Frameworks.

We use density functional theory, newly parameterized molecular dynamics simulations, and last generation 15 N dynamic nuclear polarization surface enhanced solid-state NMR spectroscopy (DNP SENS) to understand graft-host interactions and effects imposed by the metal-organic framework (MOF) host on peptide conformations in a peptide-functionalized MOF. Focusing on two grafts typified by MIL-68-proline (-Pro) and MIL-68-glycine-proline (-Gly-Pro), we identified the most likely peptide conformations adopted in the functionalized hybrid frameworks. We found that hydrogen bond interactions between the graft and the surface hydroxyl groups of the MOF are essential in determining the peptides conformation(s). DNP SENS methodology shows unprecedented signal enhancements when applied to these peptide-functionalized MOFs. The calculated chemical shifts of selected MIL-68-NH-Pro and MIL-68-NH-Gly-Pro conformations are in a good agreement with the experimentally obtained 15 Nu2005NMR signals. The study shows that the conformations of peptides when grafted in a MOF host are unlikely to be freely distributed, and conformational selection is directed by strong host-guest interactions.

Theoretical study of the local charge compensation and spectroscopic properties of B-type carbonate defects in apatite

AbstractnThe structure and spectroscopic properties of selected models of B-type carbonate defects in apatite locally compensated by fluoride or hydroxyl ions are investigated using first-principles quantum mechanical calculations. Theoretical infrared absorption spectra and 13C, and 19F nuclear magnetic resonance chemical shifts are determined. Among the investigated models, only the clumped (CO32−, F−) defect, with the carbonate group close to the sloping face of the tetrahedral site and the F− ion at the remaining apex, corresponds to previous experimental observations performed on carbonate-fluorapatite samples. Although the substitution of hydroxyl by fluoride ions is commonly observed in minerals, the clumped (CO32−, OH−) defects are unlikely to occur in apatite, considering both their theoretical spectroscopic properties and relative stability. Anionic F− for OH− exchange between channel and B sites displays a preference of ~20xa0kJ/mol for the local charge compensation by fluoride ions at the B-site, pointing to a significantly different behavior of F− and OH− ions in the charge compensation mechanism. This difference is ascribed to the poor H-bond acceptor character of available oxygen atoms surrounding the apex of the tetrahedral site. The explicit calculation of the infrared absorption spectra of the defect models is also used to interpret the significant difference observed in the linewidth of the ν2 and ν3 CO3 infrared powder absorption bands of carbonated apatite samples. It is shown that for a concentration of 4.4 wt% of CO2, long-range electrostatic effects already significantly contribute to the broadening of the ν3 CO3 bands in apatite.

Unleashing the Potential of 17O NMR Spectroscopy Using Mechanochemistry

17 Ou2005NMR spectroscopy has been the subject of vivid interest in recent years, because there is increasing evidence that it can provide unique insight into the structure and reactivity of many molecules and materials. However, due to the very poor natural abundance of oxygen-17, 17 Ou2005labeling is generally a prerequisite. This is a real obstacle for most research groups, because of the high costs and/or strong experimental constraints of the most frequently used 17 O-labeling schemes. Here, we show for the first time that mechanosynthesis offers unique opportunities for enriching in 17 O a variety of organic and inorganic precursors of synthetic interest. The protocols are fast, user-friendly, and low-cost, which makes them highly attractive for a broad research community, and their suitability for 17 Ou2005solid-state NMR applications is demonstrated.