95Mo nuclear magnetic resonance parameters of molybdenum hexacarbonyl from density functional theory: appraisal of computational and geometrical parameters.

International audienceSolid-state (95)Mo nuclear magnetic resonance (NMR) properties of molybdenum hexacarbonyl have been computed using density functional theory (DFT) based methods. Both quadrupolar coupling and chemical shift parameters were evaluated and compared with parameters of high precision determined using single-crystal (95)Mo NMR experiments. Within a molecular approach, the effects of major computational parameters, i.e. basis set, exchange-correlation functional, treatment of relativity, have been evaluated. Except for the isotropic parameter of both chemical shift and chemical shielding, computed NMR parameters are more sensitive to geometrical variations than computational details. Relativistic effects do not play a crucial part in the calculations of such parameters for the 4d transition metal, in particular isotropic chemical shift. Periodic DFT calculations were tackled to measure the influence of neighbouring molecules on the crystal structure. These effects have to be taken into account to compute accurate solid-state (95)Mo NMR parameters even for such an inorganic molecular compound

Interview with Yann Françoise on climate action plans

Climate change is happening on a faster pace than ever recorded before. It has become visible in multiple wildfires on nearly all continents, heat waves, melting glacier, thawing of permafrost, water shortage, the reduction of the biodiversity and other occurrences. The Paris agreement, based on the negotiations of the 21st Conference of the Parties in Paris (COP21) in December 2015, focus to contain global warming well below 2°C compared to pre-industrial levels by 2100, and strives to limit the increase to +1.5°C. To implement the objectives of the Paris agreement on their territories, many European cities developed their municipal climate action plans with the objective of becoming carbon neutral by 2050. In the interview with Yann Françoise, Climate-Energies and Circular Economy Manager of the City of Paris, he explains different issues on the development of climate action plans

Network connectivity and extended Se chains in the atomic structure of glassy GeSe4

International audienceIn order to contribute to the ongoing debate on the atomic structure of Se-rich GexSe1−x glasses, we have studied the structure of glassy GeSe4 by first-principles molecular dynamics. The Perdew-Burke-Ernzerhof generalized gradient approximation has been employed. The nearest-neighbor Se environment is found to be compatible with the coexistence of a network of GeSe4 tetrahedra and Sen chains, thereby confirming previous Perdew-Wang results. When comparing PW and PBE results, changes are found in the Ge-Ge coordination environment and in the number of GeSe4 tetrahedra. In addition, very extended Sen chains (n > 3, up to n = 12) are present (PBE case)

Role of the van der Waals interactions and impact of the exchange-correlation functional in determining the structure of glassy GeTe4

International audienceThe structural properties of amorphous GeTe4 are studied within the framework of first-principles molecular dynamics combined with density functional theory. Four different theoretical schemes are considered, each one intended to correspond to a distinct structural model. These are obtained by selecting either the PBE (Perdew, Burke, and Ernzerhof) or the BLYP (Becke, Lee, Yang, and Parr) exchange-correlation functionals and, for each one of the two, by disregarding or including van der Waals dispersion forces. Based on the comparison with experimental total structure factor S(k) and total pair correlation function g(r), one can infer the quantitative character of such models, thereby extracting information on the underlying structures. We found that the inclusion of the vdW forces improves the predictive power of our approach for the PBE and (to a smaller extent) the BLYP exchange-correlation functionals. Overall, BLYP performs better than PBE in reproducing the available experimental quantities, providing a tetrahedral atomic-scale network profoundly different from the one predicted by PBE, in which tetrahedral and octahedral motifs do coexist. Our work demonstrates that a careful choice of the exchange-correlation functional, combined with the account of van der Waals forces, is crucial to achieve realistic structural predictions for glassy GeTe

A combined 77Se NMR and molecular dynamics contribution to the structural understanding of the chalcogenide glasses

International audienceSolid-​state 77Se NMR measurements, first-​principles mol. dynamics and DFT calcns. of NMR parameters were performed to gain insight into the structure of selenium-​rich GexSe(1-​x)​ glasses. We recorded the fully-​relaxed NMR spectra on natural abundance and 100​% isotopically enriched GeSe4 samples, which led us to reconsider the level of structural heterogeneity in this material. In this paper, we propose an alternative procedure to initialize mol. dynamics runs for the chalcogenide glasses. The 77Se NMR spectra calcd. on the basis of the structural models deduced from these simulations are consistent with the exptl. spectrum

(77)Se solid-state NMR of As2Se3, As4Se4 and As4Se3 crystals: a combined experimental and computational study.

International audience(77)Se NMR parameters for three prototypical crystalline compounds (As2Se3, As4Se4 and As4Se3) have been determined from solid-state NMR spectra in the framework of an investigation concerning AsxSe(1-x) glass structure understanding. Density functional NMR calculations using the gauge including projector augmented wave methodology have been performed on X-ray and optimized crystal structures for a set of selenium-based crystals. These theoretical results have been combined with the experimental data in order to achieve a precise assignment of the spectral lines. This work and the high sensitivity of solid-state NMR to local order show that the structure of As4Se3 should be reinvestigated using state-of-the-art diffraction techniques. Calculations performed on several molecules derived from the crystal structures have demonstrated the limited effect of interlayer or intermolecular interactions on the isotropic chemical shifts. These interactions are therefore not responsible for the unexpected large chemical shift difference observed between these three systems that could mostly be attributed to the presence of short rings