Ai Wang

Anisotropic displacement parameters from dispersion-corrected DFT methods and their experimental validation by temperature-dependent X-ray diffraction

In chemical crystallography, the thermal motion of scattering centres is commonly described by anisotropic displacement parameters (ADPs). Very recently, it has been shown that ADPs are not only accessible by diffraction experiments but also via theory: this emerging approach seems promising but must be thoroughly tested. In this study, we have performed specifically tailored X-ray diffraction (XRD) experiments in fine steps between 100 and 300 K which allow detailed comparison to ab initio data from dispersion-corrected density functional theory (DFT) combined with periodic lattice-dynamics. The compound chosen for this study, crystalline pentachloropyridine (C5NCl5), is well suited for this purpose: it represents a molecular crystal without H atoms, thus posing no challenge to XRD; its solid-state structure is controlled by dispersion and halogen-bonding interactions; and the ADPs associated with the peripheral Cl atoms show strong temperature dependence. Quality criteria in direct and in reciprocal space prove that ADPs are predicted with high confidence for the temperature range between 100 and 200 K, and that several economic dispersion corrections to DFT can be reliably employed for this purpose. Within the limits we have explored here, the ab initio prediction of ADPs appears to be a facile and complementary tool, especially in those cases where diffraction data cannot provide a straightforward model for thermal motion.

Lattice thermal expansion and anisotropic displacements in -sulfur from diffraction experiments and first-principles theory

Thermal properties of solid-state materials are a fundamental topic of study with important practical implications. For example, anisotropic displacement parameters (ADPs) are routinely used in physics, chemistry, and crystallography to quantify the thermal motion of atoms in crystals. ADPs are commonly derived from diffraction experiments, but recent developments have also enabled their first-principles prediction using periodic density-functional theory (DFT). Here, we combine experiments and dispersion-corrected DFT to quantify lattice thermal expansion and ADPs in crystalline α-sulfur (S8), a prototypical elemental solid that is controlled by the interplay of covalent and van der Waals interactions. We begin by reporting on single-crystal and powder X-ray diffraction measurements that provide new and improved reference data from 10 K up to room temperature. We then use several popular dispersion-corrected DFT methods to predict vibrational and thermal properties of α-sulfur, including the anisotropic lattice thermal expansion. Hereafter, ADPs are derived in the commonly used harmonic approximation (in the computed zero-Kelvin structure) and also in the quasi-harmonic approximation (QHA) which takes the predicted lattice thermal expansion into account. At the PPBE+D3(BJ) level, the QHA leads to excellent agreement with experiments. Finally, more general implications of this study for theory and experiment are discussed.

Interplay of ligand chirality and metal configuration in mononuclear complexes and in a coordination polymer of Cr(III)

In order to investigate the influence of ligand chirality on the configuration of the coordinated metal, five pseudooctahedral Cr(III) complexes with one or two chelating R,R-1,2-diaminocyclohexane ligands have been synthesized. The mononuclear complexes [Cr(R,R-chxn)2Cl(DMSO)]Cl2, [Cr(R,R-chxn)2Cl2]Cl, [Cr(acacCN)(R,R-chxn)2](NO3)2, [Cr(acacCN)2(R,R-chxn)]NO3, [Cr(acacCN)2(R,R-chxn)]PF6. (R,R-chxn = R,R-1,2-diaminocyclohexane; acacCN = deprotonated 3-cyanoacetylacetone and DMSO = dimethyl sulfoxide) have been obtained as crystalline solids, mostly solvates, and the potential chirality transfer from the enantiopure ligand to the configuration at the Cr(III) center has been investigated. The cationic complex [Cr(acacCN)2(R,R-chxn)]+ has been synthesized as exclusively Λ configured at the metal. In this complex, the dangling nitrile groups of the ditopic acacCN ligands may coordinate to Ag(I): the chiral-at-metal building block has thus been converted to the 2D network Ag[Cr(acacCN)2(R,R-chxn)]2(PF6)3 under retention of the stereochemistry at Cr(III). With respect to topology, the polycations in this mixed-metal coordination polymer correspond to two interpenetrated {4,4} nets.

Conformational dimorphism in o-nitrobenzoic acid: alternative ways to avoid the O...O clash.

Polymorphism is a challenging phenomenon and the competitive packing alternatives which are characteristic for polymorphs may be encountered for essentially rigid molecules. A second crystal form of the well known compound o-nitrobenzoic acid, C7H5NO4, an important intermediate in the production of dyes, pharmaceuticals and agrochemicals, is described. Although obtained serendipitously, its intra- and intermolecular features match expectations from database searches and theoretical calculations. O-H...O hydrogen-bonded carboxylic acid dimers represent the building blocks in both polymorphs. For steric reasons and in agreement with a calculated potential energy surface, the carboxylic acid and nitro groups cannot simultaneously be coplanar with the benzene ring but have to tilt. In the well established crystal form, this out-of-plane torsion is more pronounced for the nitro substituent. In contrast, the new polymorph is characterized by a major tilt of the carboxylic acid group. The molecules in both alternative crystal forms achieve a similar compromise with respect to acceptable intramolecular O...O contacts.

Charge density of the biologically active molecule (2-oxo-1,3-benzoxazol-3(2H)-yl)acetic acid

(2-Oxo-1,3-benzoxazol-3(2H)-yl)acetic acid is a member of a biologically active class of compounds. Its molecular structure in the crystal has been determined by X-ray diffraction, and its gas phase structure was obtained by quantum chemical calculations at the B3LYP/6-311++G(d,p) level of theory. In order to understand the dynamics of the molecule, two presumably soft degrees of freedom associated with the relative orientation of the planar benzoxazolone system and its substituent at the N atom were varied systematically. Five conformers have been identified as local minima on the resulting two-dimensional potential energy surface within an energy window of 27 kJ mol(-1). The energetically most favourable minimum closely matches the conformation observed in the crystal. Based on high-resolution diffraction data collected at low temperature, the experimental electron density of the compound was determined. Comparison with the electron density established by theory for the isolated molecule allowed the effect of intermolecular interactions to be addressed, in particular a moderately strong O-H...O hydrogen bond with a donor...acceptor distance of 2.6177 (9) Å: the oxygen acceptor is clearly polarized in the extended solid. The hydrogen bond connects consecutive molecules to chains, and the pronounced charge separation leads to stacking between neighburs with antiparallel dipole moments perpendicular to the chain direction.

Dimorphism of Hexaaquanickel(Ii) Bis(p-Nitrobenzoate) Dihydrate Salt: A New Triclinic Crystal Form

Room temperature crystallization from a water/ethanol solution of NiSO4, p-nitrobenzoic acid (HPNBA), and thiourea yields single crystals of [Ni(H2O)6](PNBA)2·2H2O. Crystal data are: a = 5.9626(8), b = 7.3552(11), c = 12.3459(17) Å, α = 97.885(12), β = 96.714(11), γ = 91.158(11)°, V = 532.28(13) Å3, space group Pī, Z = 1. An alternative monoclinic crystal form of the same compound with a slightly lower packing efficiency was previously obtained. The Ni(II) cation is located on a crystallographic inversion center and octahedrally coordinated by six water molecules. The PNBA moieties occupy an outer coordination sphere acting as counter-anions. An intricate system of H bonds between the complex cation, the PNBA anions, and the co-crystallized water molecules results in a three-dimensional network.