Slightly attractive: The attractive and anisotropic nature of the ClCl interaction in C(6)Cl(6) is experimentally demonstrated from an expansion of the electron density rho(r) around the chlorine nuclei. The interaction is explained in a model in which there is a bonding attraction involving electron-deficient (see picture, blue) and electron-rich (red) regions of adjacent Cl atoms.
That non-directional pair-wise atom-atom potentials containing only attractive (r À6 dispersion term) and repulsive (inverse-power or exponential term) contributions are insufficient to model Cl···Cl interactions in molecular crystals was appreciated 40 years ago. [1] The layered orthorhombic crystal structures (space group Cmca) of the halogens (Cl 2 , Br 2 , I 2 ) cannot be anticipated with isotropic potentials (which would predict, for example, cubic packings, such as the Pa " 3 3 structure as seen for N 2 , NO, and CO), [2] and consideration of previously postulated quadrupole-quadrupole interactions [3] did not resolve this issue. Two possibilities have commonly been invoked to explain these anomalous halogen-atom contacts. Williams and Hsu proposed that halogen atoms in molecular crystals are weakly bonded (for Cl···Cl interactions, this bonding component was estimated to be around 3 % of the energy of a Cl À Cl covalent bond). [4] Such bonding was then considered to be the origin of the crystal anisotropy. Nyburg and Wong-Ng, on the other hand, proposed a model that assigned anisotropic non-bonded radii (Scheme 1 a) to Cl atoms in crystals. [5] The elliptically shaped atoms were then related to the origin of the anisotropy in the crystal packing. Over the years, all interpretations of Cl···Cl interactions in crystals invoked one or the other of these two models. [6] However, it is difficult to distinguish computationally between them: the Williams model proposes an increased attraction while the Nyburg model proposes a decreased repulsion between non-bonded atoms. Is there any real difference between these situations in an empirical or semiempirical computational approach? An experimental analysis Angewandte Chemie 3897
The projector augmented wave (PAW) methodology has been used to calculate a high precision electron density distribution ρ(r) for the hexachlorobenzene crystal phase. Implementing the calculation of the crystallographic structure factors in the VASP code has permitted one to obtain the theoretical multipolar ρ(r). This electron density is compared with both the DFT electron density and the experimental multipolar model obtained from high-resolution X-ray diffraction data. This comparison has been carried out in intra- and intermolecular regions within the framework of the quantum theory of atoms-in-molecules (QTAIM) developed by Bader and co-workers. The characterization of the electron density in both C-Cl and Cl···Cl regions, as well as within the atomic basins, shows similar features for the three models. As a consequence, the observation of charge depletion and charge concentration regions around the halogen nuclei (along the C-Cl bonding axis and in the perpendicular plane, respectively) underlines the nature of halogen bonding in terms of electrophilic and nucleophilic interactions.
3d-transition metal tungstates are interesting due to their scintillating detection properties and their use for the search of rare events in particle physics. Cuproscheelite (CuWO 4 ) has been also proposed as a material for the production of positive electrodes of lithium rechargeable batteries. Furthermore, from the fundamental point of view, CuWO 4 has attracted attention as a multiferroic material with an intriguing magnetic phase diagram showing low dimension antiferromagnetism at 90 K; quite above the Neel temperature T N = 23 K. The Jahn-Teller (JT) effect around the Cu 2+ ions is responsible for this antiferromagnetic phase. Due to the JT distortion of the CuO 6 octahedra the symmetry of cuproscheelite is lowered from the monoclinic wolframite structure (SG. P2/c) to a triclinic distorted structure (SG. Pī). Based on our optical absorption [1] and Raman spectroscopy [2] studies at high pressure (HP), it has been found that CuWO 4 undergoes a structural phase transition at 10 GPa. Otherwise, ab initio calculations [2] predict a wolframite-like structure for the HP phase. On top of that, according to calculations, the phase transformation involves an antiferromagnetic to ferromagnetic transition and a quenching of the JT distortion. In order to study the compressibility, solve the HP structure, and understand the mechanism of the phase transition, we have performed powder x-ray diffraction (PXRD) under different quasi-hydrostatic conditions up to 20 GPa. Experiments were performed at Diamond and APS synchrotrons. We also carried out single crystal x-ray diffraction (SXRD) at pressures smaller (7 GPa) and higher (13.4 GPa) than the transition pressure. These experiments were performed at HASYLAB synchrotron, Hamburg. Results showed that the compound's compressibility is affected by the presence of non-hydrostatic stresses. Depending upon the pressure-transmitting medium the bulk modulus of the low-pressure phase ranges from 139 to 171 GPa. We also found that only under nonhydrostatic conditions a second phase transition takes place at 17 GPa. On the other hand, SXRD experiments allowed us to solve the structure of the HP phase of CuWO 4 , which has a monoclinic-type structure with space group P2/c. Interestingly, the JT effect plays an important role in this phase transition and, in contrast to the theoretical predictions, it is reduced but not suppressed. We report on the electronic, structural, and thermodynamic properties of carbon dioxide phases up to 200 GPa and 10 000 K obtained from first-principles theory. The liquid region of the phase diagram is divided into several regimes -molecular and polymeric among others, based on a detailed analysis of local structure and molecular composition. The finite-temperature stability of several solid phases has been examined and the melting curve has been calculated up to 150 GPa. Liquid free energies were evaluated explicitly using a new and efficient method. The issue of carbon-oxygen demixing in the Earth's mantle will also be addressed. The second, or globa...
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