A. Alan Pinkerton

Energetic materials: variable-temperature crystal structures of γ- and ∊-HNIW polymorphs

Crystals of γ- and e-hexanitrohexaazaisowurtzitane (y- and e-HNIW, space group P2 1 /n for both crystals) have been investigated in the 100-298 K temperature range using single-crystal X-ray diffraction techniques. Temperature-dependent changes of their crystal lattices have been evaluated from the second-rank thermal expansion tensors. Both lattices undergo anisotropic thermal expansion, that of γ-HNIW being more anisotropic than that of the e phase. Comparison with previously reported predictions from molecular dynamics calculations indicates significant differences. Although there are many short (less than van der Waals) intermolecular interactions in both polymorphs, there is no obvious relationship between the short distances and the difference in thermal expansion behavior. Non-linear temperature dependence of the atomic displacement parameters is indicative of anharmonicity of the crystal mean field potential.

Optimization and evaluation of data quality for charge density studies

Criteria for the evaluation of hardware and software for the collection of charge density diffraction data are presented with examples. Evaluation of charge density studies using area detectors (based on 61 data sets) indicates that, although it is possible to obtain excellent data using such detectors, there are significant shortcomings which need to be addressed.

Lanthanide-induced contact shifts. the average electron spin polarization, theory and experiment

Abstract The reduced value of the average spin polarization, z >, for the tervalent paramagnetic lanthanide ions has been recalculated using both relativistic Hartree-Fork and pseudofree-ion values for the spin orbit coupling, t. For the ions Sm 3+ and Eu 3+ , z > has been determined at various temperatures from analysis of the |su1|H and 3 1 P NMR spectra of the complexes [Ln(S 2 PR 2 ) 4 ] − where R = Me, OMe, OEt, OPr i .

Cooperative Jahn-Teller induced phase transition of TbVO4: single crystal structure analyses of the tetragonal high temperature phase and the twinned orthorhombic phase below 33 K

The rare earth vanadate TbVO4, undergoes a crystallographic phase transition below 33 K induced by a cooperative Jahn-Teller effect. Twinning of the crystal occurs upon this transition from the tetragonal high temperature phase to the orthorhombic low temperature phase resulting in a domain structure. Single crystal x-ray analyses of the tetragonal and the twinned orthorhombic phase verify a reduction in the site symmetry of the Tb ion from D2d (m2-dodecahedral) for the high temperature phase to D2 (222-distorted dodecahedral) for the low temperature phase. Concomitantly, the space group symmetry is lowered from D194h (I41/amd) to D242h (Fddd). The twinned orthorhombic phase is described as domains related to each other by a 180? rotation about the orthorhombic [110] axis.

Importance of the consideration of anharmonic motion in charge‐density studies: a comparison of variable‐temperature studies on two explosives, RDX and HMX

Extremely accurate X-ray data were obtained for the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) at three different temperatures (20, 120 and 298 K). Collected reflections were integrated using the latest version of the program VIIPP which uses separate Kα(1)/Kα(2) contributions to the profile fitting during integration. For each temperature both anharmonic and harmonic descriptions of the atomic thermal motion were utilized in the model refinements along with the multipole expansion of the electron density. H atoms were refined anisotropically and agree well with a previous neutron study. Topological analysis [Bader (1990). Atoms in Molecules: A Quantum Theory. The International Series of Monographs of Chemistry, edited by J. Halpern & M. L. H. Green. Oxford: Clarendon Press] of the attained electron density followed. For 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), old data collected at 20 and 120 K were re-integrated with the new version of VIIPP and refined in the same manner as for RDX. In both cases theoretical structure factors were also calculated based on the 20 K structures, and employed in comparison multipole refinements for the atoms at rest. Limiting the refinement to a harmonic model of the atomic displacements may result in a biased and erroneous electron density, especially when atomic vibrations are significant (as in RDX) and at temperatures higher than obtained by using liquid helium. Given the similarity of the two compounds the effects of anharmonic motion are strikingly more severe in the case of RDX. Our study reinforces the conclusion of Meindl et al. [Acta Cryst. (2010), A66, 362-371] that in certain cases it is necessary to include anharmonic term(s) of the probability density function (or temperature factor) in order to obtain a meaningful electron density suitable for topological analysis, even for compact (high-density) light-atom structures. For RDX it was observed that the oxygen lone-pair concentrations of electrons are located close to perpendicular to the N-O bond vectors, which is typical for explosive materials. Conjugation of the electron density in the -N-NO(2) fragment has been established based on the topological bond orders. Nine moderately strong hydrogen bonds and nine N-N, O-N and O-O bonding interactions were found and described. The RDX molecular electronic energy per mole is 4.02-4.04 a.u., very close to the reported value for HMX.

Charge transfer vibronic transitions in uranyl tetrachloride compounds

The electronic and vibronic interactions of uranyl (UO(2))(2+) in three tetrachloride crystals have been investigated with spectroscopic experiments and theoretical modeling. Analysis and simulation of the absorption and photoluminescence spectra have resulted in a quantitative understanding of the charge transfer vibronic transitions of uranyl in the crystals. The spectra obtained at liquid helium temperature consist of extremely narrow zero-phonon lines (ZPL) and vibronic bands. The observed ZPLs are assigned to the first group of the excited states formed by electronic excitation from the 3σ ground state into the f(δ,ϕ) orbitals of uranyl. The Huang-Rhys theory of vibronic coupling is modified successfully for simulating both the absorption and luminescence spectra. It is shown that only vibronic coupling to the axially symmetric stretching mode is Franck-Condon allowed, whereas other modes are involved through coupling with the symmetric stretching mode. The energies of electronic transitions, vibration frequencies of various local modes, and changes in the O═U═O bond length of uranyl in different electronic states and in different coordination geometries are evaluated in empirical simulations of the optical spectra. Multiple uranyl sites derived from the resolution of a superlattice at low temperature are resolved by crystallographic characterization and time- and energy-resolved spectroscopic studies. The present empirical simulation provides insights into fundamental understanding of uranyl electronic interactions and is useful for quantitative characterization of uranyl coordination.

Characterization of bonding in cesium uranyl chloride: topological analysis of the experimental charge density.

The topological analysis of the charge density distribution in Cs(2)UO(2)Cl(4) obtained from an accurate X-ray diffraction experiment at 20K is reported. Details of the techniques applied during data collection and data refinement are discussed. A split Hansen-Coppens multipole model for uranium and cesium atoms has been used to describe the charge density features associated with valence electrons and core deformations. The analysis of the deformation density distribution, QTAIM space partitioning, the Laplacian of the electron density, and electron localization function are discussed. Local QTAIM descriptors for bonds to uranium and cesium are reported, as well as integrated properties of each individual atom. U(5f), U(6s), U(6p), U(6d), and U(7s) shells were required to describe the aspherical charge density of the uranium pseudoatom. Observed deformation of the cesium atom core was described by applying the multipole model to Cs(5s) and Cs(5p) shells.

Molecular and crystal properties of ethyl 4,6‐dimethyl‐2‐thioxo‐1,2,3,4‐tetrahydropyrimidine‐5‐carboxylate from experimental and theoretical electron densities

The electron density and electronic energy densities in ethyl 4,6-dimethyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate have been studied from accurate X-ray diffraction measurements at 110 K and theoretical single-molecule and periodic crystal calculations. The Quantum Theory of Atoms in Molecules and Crystals (QTAMC) was applied to analyze the electron-density and electronic energy-density features to estimate their reproducibility in molecules and crystals. It was found that the local electron-density values at the bond critical points derived by different methods are in reasonable agreement, while the Laplacian of the electron density computed from wavefunctions, and electron densities derived from experimental or theoretical structure factors in terms of the Hansen-Coppens multipole model differ significantly. This disagreement results from insufficient flexibility of the multipole model to describe the longitudinal electron-density curvature in the case of shared atomic interactions. This deficiency runs through all the existing QTAMC bonding descriptors which contain the Laplacian term. The integrated atomic characteristics, however, suffer noticeably less from the aforementioned shortcoming. We conclude that the electron-density and electronic energy QTAMC characteristics derived from wavefunctions, especially the integrated quantities, are nowadays the most suitable candidates for analysis of the transferability of atoms and atomic groups in similar compounds.

High order stacking of a perfluoro ‘Y-enyne’

Abstract The X-ray structure of perfluorophenyl substituted Y-enyne 1 reveals a face-to-face stacking interaction between phenyl and perfluorophenyl groups of different molecules resulting in a high order in the solid state. Acetylene–arene and acetylene–fluorine interactions also contribute to the high order stacking. The X-ray structure suggests a distorted conjugation of the chromophore.

Energetic materials: The preparation and structural characterization of melaminium dinitramide and melaminium nitrate

Two energetic salts of the melaminium cation have been prepared and structurally characterized from room temperature X-ray single crystal diffraction data. Melaminium dinitramide (I), triclinic, P1¯, a = 6.6861(11), b = 6.9638(16), c = 10.447(2) Å , α = 99.07(3), β = 98.30(3), γ = 108.50(3)°, V = 445.6(2) Å3, and Z = 2. Melaminium nitrate (II), monoclinic, P21/c, a = 3.5789(7), b = 20.466(4), c = 10.060(2) Å, β = 94.01(2)°, V = 735.0(3) Å3, and Z = 4. The crystal structures of both salts show distinct monoprotonated melaminium cations and dinitramide- or nitrate anions, respectively. Efficient packing in the solid state is achieved by extensive hydrogen bonding between two-dimensional zigzag ribbons of the melaminium cations and the respective anions resulting in high densities of the solid state structures of 1.74 (I) and 1.71 g/cm3 (II).