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An accurate X-ray diffraction study at 20 K combined with DFT theoretical calculations has been performed for the estriol crystal with two conformationally different molecules in the asymmetric unit. The electron density has been modeled via a multipole expansion, using both experimental and theoretical structure factors, and a topological analysis has been performed. The experimental molecular geometry, hydrogen bonding, atomic charges, dipole moments, and other topological characteristics are compared with those calculated theoretically. In particular, the molecular electrostatic potential has been extracted and compared with those reported for other estrogen molecules exhibiting different binding affinities to the estrogen receptors (ERα and ERβ).

The charge density of 2,6-dinitrophenol has been carefully determined from low temperature (20 K) single crystal X-ray diffraction data and periodic ab initio theoretical calculations. The topological analysis performed on the refined densities within the framework of the Quantum Theory of Atoms in Molecules (QTAIM), allowed us to characterize, both qualitatively and quantitatively, the various intra- and intermolecular interactions existing in the crystal structure of this compound. Notably two strong intramolecular noncovalent interactions have been characterized (O···H, De > 60 kJ/mol; O···O, De ∼ 19 kJ/mol). In addition, a series of weaker intermolecular interactions (O···N, O···O, O···C, and C···C) with estimated dissociation energies of 1–9 kJ/mol have been identified.

This paper presents topological analysis of charge density from multipolar refinement and ab initio calculations for both melamine-barbital co-crystal and isolated molecules. The comparison of electron density properties for the isolated molecules and for the co-crystal allowed one to unravel the factors contributing to the formation of specific hydrogen bond patterns in the co-crystal structure. The inspection of charge density properties revealed that N1A of barbital and N3B of melamine compete for H1A atom in hydrogen bond formation. The hydrogen bond shows an intermediate character between closed and shared shell interaction. The tendency of barbital to adopt tautomeric forms in solution which can be correlated with the shift of electron density (mesomeric effect) was emphasized. Charge density studies confirmed the existence of the mesomeric form B of barbital in the studied co-crystal. The form B was also reported in polar co-crystals comprised of barbital and barbituric acid.

X-ray/neutron (X/N) diffraction data measured at very low temperature (15 K) in conjunction with ab initio theoretical calculations were used to model the crystal charge density (CD) of the host-guest complex of hydroquinone (HQ) and acetonitrile. Due to pseudosymmetry, information about the ordering of the acetonitrile molecules within the HQ cavities is present only in almost extinct, very weak diffraction data, which cannot be measured with sufficient accuracy even by using the brightest X-ray and neutron sources available, and the CD model of the guest molecule was ultimately based on theoretical calculations. On the other hand, the CD of the HQ host structure is well determined by the experimental data. The neutron diffraction data provide hydrogen anisotropic thermal parameters and positions, which are important to obtain a reliable CD for this light-atom-only crystal. Atomic displacement parameters obtained independently from the X-ray and neutron diffraction data show excellent agreement with a |ΔU| value of 0.00058 Å(2) indicating outstanding data quality. The CD and especially the derived electrostatic properties clearly reveal increased polarization of the HQ molecules in the host-guest complex compared with the HQ molecules in the empty HQ apohost crystal structure. It was found that the origin of the increased polarization is inclusion of the acetonitrile molecule, whereas the change in geometry of the HQ host structure following inclusion of the guest has very little effect on the electrostatic potential. The fact that guest inclusion has a profound effect on the electrostatic potential suggests that nonpolarizable force fields may be unsuitable for molecular dynamics simulations of host-guest interaction (e.g., in protein-drug complexes), at least for polar molecules.

The discovery of the ultra-high thermoelectric figure of merit of 2.6 in SnSe has drawn attention to other lead-free IV-VI orthorhombic semiconductors. GeSe has been predicted to possess thermoelectric performances comparable to SnSe. Here, a complete structural study is reported of GeSe with temperature by means of high-resolution synchrotron powder X-ray diffraction. In the orthorhombic phase, the evolution of the bond distances with temperature is shown to deviate significantly with respect to SnSe. Analysis of the chemical bonding within the Quantum Theory of Atoms in Molecules shows that GeSe is ionic with van der Waals interlayer interactions. The signature of the N shell lone pair of Ge is also evident from both the electron density Laplacian and the ELF topologies.

Octahedral monodisperse a-MnS and MnO nanoparticles have been synthesized by decomposing manganese oleate and elemental sulfur in octadecene at high (250-320 °C) temperature. The chemical composition of the obtained NPs depends on the Mn:S ratio in an unexpected way. Pure a-MnS NP samples are obtained when S:Mn >= 2:1, whereas pure MnO NPs require S:Mn

The electron density distributions in both polymorphs of the promising photovoltaic material iron disulfide have been determined by multipole modelling against state-of-the-art synchrotron X-ray diffraction data collected at 10 K using minute single crystals with dimensions less than 10 μm. Charge density analysis of FeS2 pyrite and marcasite offers a unique opportunity to relate local atomic properties, such as 2-center chemical bonding, atomic charges and d-orbital populations, to polymorphism in extended crystal structures. In combination with results from periodic calculations on the compounds in the experimental geometries using WIEN2k, the study provides unambiguous answers to a number of unsolved issues regarding the nature of the bonding in FeS2. The Fe–S bonds exhibit all the virtues of polar covalent bonds, with only minor charge accumulation but significantly negative energy densities at the bond critical points. Compared to a non-interacting model, the density is found to be concentrated along the Fe–S interaction line in support of a partial covalent bonding description. The homopolar covalent S–S interaction is seemingly stronger in pyrite than in marcasite, determined not only from the shorter distance but also from all topological indicators. The study also clarifies that the atomic charges are significantly smaller than the estimation based on crystal-field theory of Fe2+, S−1. The experimentally derived Fe d-orbital populations are found to deviate from the commonly assumed full t2g set, empty eg set, and they fit exceptionally well with the theoretical individual atomic orbitals projected density of states showing a higher dxy participation in the valence band in marcasite compared with pyrite. Thus, the differences between the two polymorphic compounds are directly reflected in their valence density distributions and d-orbital populations.

CoSb3is an example of a highly challenging case for experimental charge-density analysis due to the heavy elements (suitability factor of ∼0.01), the perfect crystallinity and the high symmetry of the compound. It is part of a family of host–guest structures that are potential candidates for use as high-performance thermoelectric materials. Obtaining and analysing accurate charge densities of the undoped host structure potentially can improve the understanding of the thermoelectric properties of this family of materials. In a previous study, analysis of the electron density gave a picture of covalent Co–Sb and Sb–Sb interactions together with relatively low atomic charges based on state-of-the-art experimental and theoretical data. In the current study, several experimental X-ray diffraction data sets collected on the empty CoSb3framework are compared in order to probe the experimental requirements for obtaining data of high enough quality for charge-density analysis even in the case of very unsuitable crystals. Furthermore, the quality of the experimental structure factors is tested by comparison with theoretical structure factors obtained from periodic DFT calculations. The results clearly show that, in the current study, the data collected on high-intensity, high-energy synchrotron sources and very small crystals are superior to data collected at conventional sources, and in fact necessary for a meaningful charge-density study, primarily due to greatly diminished effects of extinction and absorption which are difficult to correct for with sufficient accuracy.

One of the most basic concepts in chemical bonding theory is the octet rule, which was introduced by Lewis in 1916, but later challenged by Pauling to explain the bonding of third-row elements. In the third row, the central atom was assumed to exceed the octet by employing d orbitals in double bonding leading to hypervalency. Ever since, polyoxoanions such as SO(4)(2-), PO(4)(3-), and ClO(4)(-) have been paradigmatic examples for the concept of hypervalency in which the double bonds resonate among the oxygen atoms. Here, we examine S-O bonding by investigating the charge density of the sulfate group, SO(4)(2-), within a crystalline environment based both on experimental and theoretical methods. K(2)SO(4) is a high symmetry inorganic solid, where the crystals are strongly affected by extinction effects. Therefore, high quality, very low temperature single crystal X-ray diffraction data were collected using a small crystal (∼30 μm) and a high-energy (30 keV) synchrotron beam. The experimental charge density was determined by multipole modeling, whereas a theoretical density was obtained from periodic ab initio DFT calculations. The chemical bonding was jointly analyzed within the framework of the Quantum Theory of Atoms In Molecules only using quantities derived from an experimental observable (the charge density). The combined evidence suggests a bonding situation where the S-O interactions can be characterized as highly polarized, covalent bonds, with the "single bond" description significantly prevailing over the "double bond" picture. Thus, the study rules out the hypervalent description of the sulfur atom in the sulfate group.