Christopher G. Gianopoulos

Bonding in Uranium(V) Hexafluoride Based on the Experimental Electron Density Distribution Measured at 20 K

The electron density distribution of [PPh4][UF6] was obtained from high-resolution X-ray diffraction data measured at 20 K. The electron density was modeled with an augmented Hansen-Coppens multipolar formalism. Topological analysis reveals that the U-F bond is of incipient covalent nature. Theoretical calculations add further support to the bonding description gleaned from the experimental model. The impact of the uranium anomalous dispersion terms on the refinement is also discussed.

Revised structure of trans-resveratrol: Implications for its proposed antioxidant mechanism.

The crystal structure of trans-resveratrol has been redetermined by X-ray diffraction. The newly refined structure demonstrates that the previously reported, dynamically disordered hydrogen-bonding network is rather the superposition of two crystallographically independent molecules of trans-resveratrol. This latter arrangement possesses a well-defined hydrogen-bonding network in a unit cell of double the previously reported volume. While not meant as a criticism of the proposed antioxidant mechanism itself, the present studies clearly show that the X-ray diffraction data should no longer be used for its additional support.

Revisiting the charge density analysis of 2,5-dichloro-1,4-benzoquinone at 20 K

A high-resolution X-ray diffraction measurement of 2,5-dichloro-1,4-benzoquinone (DCBQ) at 20 K was carried out. The experimental charge density was modeled using the Hansen-Coppens multipolar expansion and the topology of the electron density was analyzed in terms of the quantum theory of atoms in molecules (QTAIM). Two different multipole models, predominantly differentiated by the treatment of the chlorine atom, were obtained. The experimental results have been compared to theoretical results in the form of a multipolar refinement against theoretical structure factors and through direct topological analysis of the electron density obtained from the optimized periodic wavefunction. The similarity of the properties of the total electron density in all cases demonstrates the robustness of the Hansen-Coppens formalism. All intra- and intermolecular interactions have been characterized.

Experimental and Theoretical Electron Density Determination for Two Norbornene Derivatives: Topological Analysis Provides Insights on Reactivity.

The electron density distribution of two substituted norbornene derivatives (cis-5-norbornene-endo-2,3-dicarboxylic anhydride (1) and 7-oxabicylo[2.2.1]hept-5-ene-exo-2,3-dicarboxylic anhydride (2) has been determined from low-temperature (20 K) X-ray diffraction data and from DFT calculations with periodic boundary conditions. Topological analysis of the electron density is discussed with respect to exo-selective additions, the partial retro-Diels-Alder (rDA) character of the ground state, and intermolecular interaction energies.

Insights on uranium halogen bonding derived from charge-density studies at 20 K

Accurate electron density distributions can be obtained from high-resolution X-ray diffraction data measured at low temperatures, even in the case of systems containing very heavy elements such as uranium. Modeling of the experimental electron density distribution with an augmented Hansen-Coppens multipolar formalism and subsequent topological analysis of the model density provides insight into the nature of U-X bonding in the electronic ground-state. The results of charge density studies measured at 20 K for systems containing uranium-halogen bonds are presented and compared to complementary results obtained from gas-phase theoretical calculations.