Modelling the experimental electron density: only the synergy of various approaches can tackle the new challenges

This is an open-access article distributed under the terms of the Creative Commons Attribution Licence.Electron density is a fundamental quantity that enables understanding of the chemical bonding in a molecule or in a solid and the chemical/physical property of a material. Because electrons have a charge and a spin, two kinds of electron densities are available. Moreover, because electron distribution can be described in momentum or in position space, charge and spin density have two definitions and they can be observed through Bragg (for the position space) or Compton (for the momentum space) diffraction experiments, using X-rays (charge density) or polarized neutrons (spin density). In recent years, we have witnessed many advances in this field, stimulated by the increased power of experimental techniques. However, an accurate modelling is still necessary to determine the desired functions from the acquired data. The improved accuracy of measurements and the possibility to combine information from different experimental techniques require even more flexibility of the models. In this short review, we analyse some of the most important topics that have emerged in the recent literature, especially the most thought-provoking at the recent IUCr general meeting in Montreal.PM thanks the Swiss National Science foundation (Project 160157) for financial support. CL and NC are grateful to Universite de Lorraine, Agence Nationale de la recherche and CNRS, for instrumental and financial support. JMG thanks CentraleSupélec, ANR and CNRS for financial support.Peer Reviewe

Osmium-Nitrosyl Oxalato-Bridged Lanthanide-Centered Pentanuclear Complexes - Synthesis, Crystal Structures and Magnetic Properties

International audienceA series of pentanuclear heterometallic coordination compounds of the general formula (Bu 4 N) 5 [Ln{Os(NO)(μ-ox)-Cl 3 } 4 (H 2 O) n ] [Ln = Y (for 2) and Dy (for 3) when n = 0; Ln = Dy (for 3), Tb (for 4), and Gd (for 5) when n = 1] were synthesized by the reaction of the precursor (Bu 4 N) 2 [Os(NO)(ox)-Cl 3 ] (1) with the respective lanthanide(III) (Gd, Tb, Dy) or yttrium(III) chloride. For the five new complexes, the coordination numbers eight or nine are found for the central metal ion. The compounds were fully characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction analysis, magnetic susceptibility measurements, and ESI mass spectrometry. In addition, compound 1 was studied by UV/Vis spectroscopy and cyclic voltammetry. The X-ray dif-fraction analyses revealed that the anionic complexes consist of a lanthanide or yttrium core bridged through oxalato li-gands to four octahedral osmium-nitrosyl moieties. This picture , in which the central ion is eight-coordinate, holds fo

Densités électroniques et densités de spin de matériaux magnétiques moléculaires

Nous avons étudié la densité électronique de matériaux magnétiques moléculaires purement organiques et de complexes d'atomes lourds. Grâce à l'analyse topologique obtenue par diffraction de rayons X à haute résolution, nous avons caractérisé les interactions intra et intermoléculaires susceptibles de contrôler les propriétés macroscopiques. Pour des radicaux nitronyle-nitroxyde, nous avons détaillé les relations étroites reliant la densité électronique aux propriétés magnétiques. L'étude des complexes de semiquinone, basée sur la densité électronique et la densité de spin (obtenue par diffraction de neutrons polarisés) et appuyée par des calculs théoriques ab initio, a aboutit pour celui d'yttrium à la description précise des ligands semiquinone et hydrotrispyrazolylborate et à la caractérisation de transferts de charge et de spin (mécanismes d'échange). Un problème fondamental sur le calcul des facteurs de diffusion atomiques, mis en évidence sur le complexe de gadolinium, est discuté.We have studied the electron density of molecular magnetic materials, either purely organic (nitronyle-nitroxyde radicals) or heavy atom complexes (Y and Gd, semiquinone radical). Using the topological analysis obtained by high resolution X-rays diffraction, we characterised the intra and intermolecular interactions involved in the control of the macroscopic properties. For the nitronyle-nitroxyde radicals, we have demonstrated the close relationship between the electron density distributions and the magnetic properties. For the heavy atom complexes, the electron and spin densities (obtained by polarised neutron diffraction) completed by ab initio calculations allow a precise description of the semiquinone and hydrotrispyrazolylborate ligands and the determination of the charges and spin transfers, which are fundamental in the understanding of the exchange mechanisms. A fundamental problem on the calculation of the atomic form factors, pointed out on the gadolinium complex, is discussed.NANCY1-SCD Sciences & Techniques (545782101) / SudocSudocFranceF

Modélisation expériementale des matériaux magnétiques moléculaires (études combinées par diffraction X, neutrons et neutrons polarisés)

Nous avons développé un modèle et un programme d'affinement joint des densités de charge et spin. Lors des premiers tests plusieurs difficultés sont apparues et ont été étudiées puis résolues notamment par la mise en place de contraintes. Après la mise en place d'un programme stable d'affinement joint nous avons testé celui-ci sur le complexe MnCu(pba)...(H2O)3...2H2O, ou pba représente le 1,3-propylenbis(oxamato) en réutilisant les données provenant d'une expérience de diffraction de neutrons polarisés et en effectuant une nouvelle expérience de diffraction des rayons X à 10K, température à laquelle l'expérience de diffraction des neutrons polarisés a été conduite. Cette étude a permis de tester trois schémas de pondération, ainsi que les contraintes. Ces tests ont montré que l'affinement joint permet de retrouver les résultats des différents affinements séparés mais aussi d'aller plus loin en autorisant un affinement de la densité de spin avec plus de paramètres pertinents. Suite à ces premiers tests nous nous sommes intéressés à un complexe azido cuivre (Cu2L2(N3)2 avec L=1,1,1-trifluoro-7-(dimethylamino)-4-méthyle-5-aza-3-heptène-2-onato). L'affinement joint a permis d'avoir accès, pour la première fois, à la densité de valence expérimentale résolue en spin et d'affiner également des paramètres de contraction/dilatation différents pour la valence avec un spin up ou un spin down. Dans le dernier chapitre nous avons étudié un complexe de cobalt qui présentait des propriétés magnétiques intéressantes. Cependant la particularité magnétique du composé venant d'une forte anisotropie magnétique a rendu l'étude par affinement joint délicate dans un premier temps, c'est pourquoi nous avons étudié ce composé uniquement d'un point de vue de la densité de charge. Cette étude a tout de même permis de mettre en évidence expérimentalement à 100K un angle de torsion de 39 entre les axes principaux des atomes de cobalt, prédit par la théorieWe developed a model and a refinement program for charge and spin densities. During the first tests several difficulties have arisen and have been investigated and solved by implementation of constraints. After the establishment of stable joint refinement program, we tested it on the MnCu(pba)...(H2O)3...2H2O, with pba = 1,3-propylenbis(oxamato) complex reusing data from an experiment of polarized neutron diffraction and making a new experience of X-ray diffraction at 10K. This study tested three weighting schemes and constraints. These tests showed that the joint refinement give access to the same results as the separated refinements but also allow us to go further by refining the spin density with more pertinent parameters. Following these initial tests, we were interested in a copper azido complex (Cu2L2(N3)2 with L=1,1,1-trifluoro-7-(dimethylamino)-4-methyl-5-aza-3-hepten-2-onato). The joint refinement give us access for the first time to the experimental spin-resolved valence density and also to refine the parameters of contraction / expansion for spin up or spin down separately. In the last chapter we studied a cobalt complex which shows interesting magnetic properties. However, the magnetic properties of the compound come from a high magnetic anisotropy which complicates a study by joint refinement. That is why we studied only the charge density of this compound. This study still allowed to show experimentally a torsion angle of 39 between the principal axes of the cobalt atoms, which was predicted by a previous theoretical studyMETZ-SCD (574632105) / SudocNANCY1-Bib. numérique (543959902) / SudocNANCY2-Bibliotheque electronique (543959901) / SudocNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Joint Refinement of Charge and Spin Densities

International audienceA new charge and spin density model and the corresponding refinement software were recently developed to combine X- ray and polarized neutron diffraction experiments [1]. This joint refinement procedure allows getting access to both charge and spin densitiy distributions by refining both spin up and down parameters for magnetic atoms. The paper is focused on the refinement procedure and its application to the case of an end-to-end azido double bridged copper(II) complex. The results of this joint refinement of X-ray and polarized neutron diffraction data are presented

Crystal structure of 3,5-bis(4-chlorophenyl)-1-propyl-1,3,5-triazacyclohexane

International audienceIn the title mol­ecule, C18H21Cl2N3, the tri­aza­cyclo­hexane ring adopts a chair conformation with both 4-chloro­phenyl substituents in axial positions and the propyl group in an equatorial site. The dihedral angle between the planes of the benzene rings is 49.5 (1)°. In the crystal, mol­ecules are arranged in a head-to-tail fashion, forming columns along [010], and pairs of weak C—H⋯π inter­actions form inversion dimers between columns

Utilization of solid-state NMR to determine the local magnetic susceptibility

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Utilisation de la RMN du solide paramagnétique pour déterminer le tenseur de susceptibilité magnétique local

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Spin-resolved charge density and wavefunction refinements using MOLLYNX : a review

International audienceMOLLYNX is a new crystallographic tool developed to access a more precise description of the spin-dependent electron density of magnetic crystals, taking advantage of the richness of experimental information from high-resolution X-ray diffraction (XRD), unpolarized neutron (UND) and polarized neutron diffraction (PND). This new program is based either on the well known Hansen–Coppens multipolar model ( MOLLYNX -mult) or on a new expansion over a set of atomic orbitals ( MOLLYNX -orb). The main difference between the two models is the basis of the expansion: in MOLLYNX -mult the expansion is over atom centered real spherical harmonics, in MOLLYNX -orb the expansion is over a set of atomic orbitals with which mono and bicentric contributions are calculated. This new approach of MOLLYNX -orb can also be applied to nonmagnetic crystals. This paper summarizes the theoretical ground of two models and describes the first applications to organic, organometallic and inorganic magnetic material