The generalized invariom database (GID).

Invarioms are aspherical atomic scattering factors that enable structure refinement of more accurate and more precise geometries than refinements with the conventional independent atom model (IAM). The use of single-crystal X-ray diffraction data of a resolution better than sin θ/λ = 0.6 Å(-1) (or d = 0.83 Å) is recommended. The invariom scattering-factor database contains transferable pseudoatom parameters of the Hansen-Coppens multipole model and associated local atomic coordinate systems. Parameters were derived from geometry optimizations of suitable model compounds, whose IUPAC names are also contained in the database. Correct scattering-factor assignment and orientation reproduces molecular electron density to a good approximation. Molecular properties can hence be derived directly from the electron-density model. Coverage of chemical environments in the invariom database has been extended from the original amino acids, proteins and nucleic acid structures to many other environments encountered in organic chemistry. With over 2750 entries it now covers a wide sample of general organic chemistry involving the elements H, C, N and O, and to a lesser extent F, Si, S, P and Cl. With respect to the earlier version of the database, the main modification concerns scattering-factor notation. Modifications improve ease of use and success rates of automatic geometry-based scattering-factor assignment, especially in condensed hetero-aromatic ring systems, making the approach well suited to replace the IAM for structures of organic molecules.

Charge-density studies of energetic materials: CL-20 and FOX-7.

Experimental electron densities and derived properties have been determined for the two energetic materials CL-20 (3,5,9,11-tetraacetyl-14-oxo-1,3,5,7,9,11-hexaazapentacyclo-[5.5.3.02,6.04,10.08,12]pentadecane), and FOX-7 (1,1-diamino-2,2-dinitroethylene) from single-crystal diffraction. Synchrotron data extending to high scattering angles were measured at low temperature. Low figures-of-merit and excellent residuals were obtained. The Hansen & Coppens multipole-model electron density was compared with results from theoretical calculations via structure factors simulating an experiment. Chemical bonding in the molecules is discussed and a topological analysis gives insight especially into the character of those bonds that are thought to play a key role in the decomposition of the molecules. A comparison of theoretical and experimental electrostatic potentials shows no obvious evidence supporting earlier findings on other nitroheterocyclic molecules that electron-density maxima near the C-NO(2) bonds mapped on the electron-density isosurface can be correlated with impact sensitivities. For FOX-7 periodic Hartree-Fock calculations were performed to investigate the influence of the crystal field on the electron density distribution.

Invarioms for improved absolute structure determination of light-atom crystal structures.

The determination of molecular absolute configuration from an X-ray analysis for structures that contain only light elements is challenging owing to the weak anomalous dispersion signal. The achievable precision of the Flack x parameter for such structures is therefore limited, especially when the independent-atom model is employed. Invariom modelling can improve this situation. Invarioms are theoretically predicted pseudoatoms within the Hansen & Coppens multipole formalism. They are transferable from one molecule to another and provide generalized aspherical atomic form factors. It is shown that, by application of the invariom approach, the precision and standard uncertainty of the Flack x parameter and therefore the reliability of deducing molecular chirality in an absolute structure determination can be improved.

The invariom model and its application: refinement of D,L-serine at different temperatures and resolution.

Three X-ray data sets of the same D,L-serine crystal were measured at temperatures of 298, 100 and 20 K. These data were then evaluated using invarioms and the Hansen & Coppens aspherical-atom model. Multipole populations for invarioms, which are pseudoatoms that remain approximately invariant in an intermolecular transfer, were theoretically predicted using different density functional theorem (DFT) basis sets. The invariom parameters were kept fixed and positional and thermal parameters were refined to compare the fitting against the multi-temperature data at different resolutions. The deconvolution of thermal motion and electron density with respect to data resolution was studied by application of the Hirshfeld test. Above a resolution of sin theta/lambda approximately 0.55 A-1, or d approximately 0.9 A, this test was fulfilled. When the Hirshfeld test is fulfilled, a successful modeling of the aspherical electron density with invarioms is achieved, which was proven by Fourier methods. Molecular geometry improves, especially for H atoms, when using the invariom method compared to the independent-atom model, as a comparison with neutron data shows. Based on this example, the general applicability of the invariom concept to organic molecules is proven and the aspherical density modeling of a larger biomacromolecule is within reach.