Anharmonic Thermal Motion Modelling in the Experimental XRD Charge Density Determination of 1-Methyluracil at T = 23 K.

The experimental electron density distribution (EDD) of 1-methyluracil (1-MUR) was obtained by single crystal X-ray diffraction (XRD) experiments at 23 K. Four different structural models fitting an extensive set of XRD data to a resolution of (sinθ/λ) = 1.143 Å are compared.

Anharmonic motions versus dynamic disorder at the Mg ion from the charge densities in pyrope (MgAlSiO) crystals at 30 K: six of one, half a dozen of the other.

The possible occurrence of static/dynamic disorder at the Mg site in pyrope (MgAlSiO), with or without anharmonic contribution to the thermal vibrations even at low temperatures, has been largely debated but conclusions were contrasting. Here a report is given on the experimental charge density distribution, ρ, of synthetic pyrope at T = 30 K, built through a Stewart multipolar expansion up to l = 5 and based on a very precise and accurate set of in-home measured single-crystal X-ray diffraction amplitudes with a maximum resolution of 0.44 Å.

Progress in the understanding of the key pharmacophoric features of the antimalarial drug dihydroartemisinin: an experimental and theoretical charge density study.

The accurate, experimental charge density distribution, ρ(r), of the potent antimalarial drug dihydroartemisinin (DHA) has been derived for the first time from single-crystal X-ray diffraction data at T=100(2) K. Gas-phase and solid-state DFT simulations have also been performed to provide a firm basis of comparison with experimental results. The quantum theory of atoms in molecules (QTAIM) has been employed to analyse the ρ(r) scalar field, with the aim of classifying and quantifying the key real-space elements responsible for the known pharmacophoric features of DHA.

Rationalizing the effect of halogenation on the molecular structure of simple cyclobutene derivatives by topological real-space analysis of their electron density.

The accurate gas-phase equilibrium structures on the ground-state potential energy surface of the complete series of fluorinated and chlorinated cyclobutene derivatives with C(2v) symmetry have been evaluated at DFT PBE0/6-311++G(d,p) theory level. The optimized geometries have been compared with all the available experimental data reported in the literature, as obtained by microwave spectroscopy (MW) and gas-phase electron diffraction (GED) techniques. For hexafluorocyclobutene and 1,2-dichloro-3,3',4,4'-tetrafluorocyclobut-1-ene, the results of accurate low-temperature single-crystal X-ray diffraction experiments have also been considered.

Detection and kinetics of the single-crystal to single-crystal complete transformation of a thiiranium ion into thietanium ion.

The conversion of a di-tert-butyl-methylthiiranium ion into thietanium ion, that is reported in the literature as taking place spontaneously at 25 degrees C in a CD(2)Cl(2) solution, has been discovered to occur quantitatively at room temperature (RT) also in the crystalline state. The ring enlargement reaction is accompanied, in the solid phase, by a modest deterioration of the quality of the sample under investigation, and all three specimens here studied by in situ crystallography maintained their single-crystal nature up to 100% conversion. The rearrangement reaction implies the breaking of a C-S bond and the formation of a new bond of the same type, together with the migration of a methyl group.

Effect of methoxy substituents on the structural and electronic properties of fluorinated cyclobutenes: a study of hexafluorocyclobutene and its vinyl methoxy derivatives by XRD and periodic DFT calculations.

The effect of the methoxy substituent on the structure, crystal packing, and electrostatic properties of hexafluorocyclobutene (C(4)F(6)) was investigated in the solid-state with DFT-B3LYP calculations. Full geometry optimizations were done for the parent compound and its two vinyl methoxy derivatives C(4)F(5)OCH(3) and C(4)F(4)(OCH(3))(2), starting from the structures obtained by single-crystal X-ray diffraction at low temperature. A full topological analysis, followed by the calculation of several electrostatic properties, was performed on the periodic electron density using the quantum theory of atoms in molecules.

Estimated H-atom anisotropic displacement parameters: a comparison between different methods and with neutron diffraction results.

Anisotropic displacement parameters (ADPs) are compared for H atoms estimated using three recently described procedures, both among themselves and with neutron diffraction results. The results convincingly demonstrate that all methods are capable of giving excellent results for several benchmark systems and identify systematic discrepancies for several atom types. A revised and extended library of internal H-atom mean-square displacements is presented for use with Madsen's SHADE web server [J.

Physicochemical properties of zwitterionic L- and DL-alanine crystals from their experimental and theoretical charge densities.

The total experimental electron density distributions rho(r) of zwitterionic L- and DL-alanine crystals, as derived from extensive sets of X-ray diffracted intensities collected at 23 and 19 K, are compared to gain an insight into the different physical properties of the two related chiral compounds in the solid state and to explore the extent of the rho(r) transferability. Relevant parameters that characterize the two crystal forms are obtained, showing differences and similarities in terms of (i) geometric descriptors, (ii) topological indexes, (iii) molecular electrostatic potential Phi(r) distributions, (iv) atomic volumes and charges, (v) molecular electric moments, and (vi) electrostatic interaction energies. To assess the relative stability of the racemate with respect to the pure enantiomer, the crystal lattice energies, as obtained through DFT fully periodic calculations, are also discussed and compared with the experimental sublimation enthalpies after correction for the proton-transfer energies.

Experimental and theoretical charge density distribution of the colossal magnetoresistive transition metal sulfide FeCr2S4.

The total charge density distribution rho(r) of the colossal magnetoresistive transition metal sulfide FeCr(2)S(4) was evaluated through a multipole formalism from a set of structure factors obtained both experimentally, by means of single crystal high-quality x-ray diffraction data collected at T=23 K, and theoretically, with an extended-basis unrestricted Hartree-Fock periodic calculation on the experimental geometry. A full topological analysis, followed by the calculation of local energy density values and net atomic charges, was performed using the quantum theory of atoms in molecules. The experimental and theoretical results were compared.

Progress in the understanding of drug-receptor interactions, part 2: experimental and theoretical electrostatic moments and interaction energies of an angiotensin II receptor antagonist (C30H30N6(O)3S).

A combined experimental and theoretical charge density study of an angiotensin II receptor antagonist (1) is presented focusing on electrostatic properties such as atomic charges, molecular electric moments up to the fourth rank and energies of the intermolecular interactions, to gain an insight into the physical nature of the drug-receptor interaction. Electrostatic properties were derived from both the experimental electron density (multipole refinement of X-ray data collected at T=17 K) and the ab initio wavefunction (single molecule and fully periodic calculations at the DFT level). The relevance of SO and SN intramolecular interactions on the activity of 1 is highlighted by using both the crystal and gas-phase geometries and their electrostatic nature is documented by means of QTAIM atomic charges.

On the interplay between CH...O and OH...O interactions in determining crystal packing and molecular conformation: an experimental and theoretical charge density study of the fungal secondary metabolite austdiol (C12H12O5).

The total experimental electron density rho(r), its Laplacian inverted delta(2)rho(r), the molecular dipole moment, the electrostatic potential phi(r), and the intermolecular interaction energies have been obtained from an extensive set of single-crystal X-ray diffracted intensities, collected at T = 70(1) K, for the fungal metabolite austdiol (1). The experimental results have been compared with theoretical densities from DFT calculations on the isolated molecule and with fully periodic calculations. The crystal structure of (1) consists of zigzag ribbons extended along one cell axis and formed by molecules connected by both OH.

Progress in the understanding of drug-receptor interactions, Part 1: experimental charge-density study of an angiotensin II receptor antagonist (C30H30N6O3S) at T = 17 K.

An experimental study of the electron-density distribution rho(r) in an angiotensin II receptor antagonist 1 has been made on the basis of single-crystal X-ray diffraction data collected at a low temperature. The crystal structure of 1 consists of infinite ribbons in which molecules are connected by an N-H..

On the role of data quality in experimental charge-density studies.

High-resolution X-ray diffraction data were collected at T = 19 K from two similar spherical crystals of the fungal metabolite citrinin, C(13)H(14)O(5). The two data sets were of markedly different quality, one data set (MQ, medium quality) comprising a single octant of the reciprocal lattice and including reflections with obviously peculiar intensity profiles while the second (HQ, high quality) comprised a hemisphere of reflections and showed no flawed profiles. Parallel multipolar refinements were carried out for both.

Electron density investigation of a push-pull ethylene (C14H24N2O2.H2O) by X-ray diffraction at T = 21 K.

The electron charge distribution in a strongly twisted push-pull ethylene [PPE, 3-(1,3-diisopropyl-2-imidazolidinylidene)-2,4-pentanedione] has been determined by low temperature (T = 21 K) single-crystal X-ray diffraction analysis. The derived electronic properties are consistent with a zwitterionic molecule, as indicated by a charge transfer of 0.82(16) e from the push to the pull moieties and a charge polarization of 0.

The fungal metabolite austdiol.

The title compound, (7R,8S)-7,8-dihydroxy-3,7-dimethyl-6-oxo-7,8-dihydro-6H-isochromene-5-carbaldehyde, C(12)H(12)O(5), is a trans-vicinal diol. Of the two fused rings, which lie approximately in the same plane, the pyran ring is almost perfectly planar, while the cyclohexenone ring adopts a slightly distorted half-chair conformation. The crystal packing is dictated by two strong intermolecular O-H.

tert-Butyl (3S,6R,9R)-(5-oxo-3-[[1(R)-phenylethyl]carbamoyl]-1,2,3,5,6,7,8,8a-octahydroindolizin-6-yl)carbamate monohydrate at 95 K.

The asymmetric unit of the title compound, C(22)H(31)N(3)O(4).H(2)O, incorporates one water molecule, which is hydrogen bonded to the 3-oxo O atom of the indolizidinone system. The two rings of the peptidomimetic molecule are trans-fused, with the six-membered ring having a slightly distorted half-chair conformation and the five-membered ring having a perfect envelope conformation.