Assessing covalency in equatorial U-N bonds:density based measures of bonding in BTP and isoamethyrin complexes of uranyl

Calculations performed at the density functional level of theory have been used to investigate complexes of uranyl with the expanded porphyrin isoamethyrin and the bis-triazinyl-pyridine (BTP) ligands, the latter of which is well-known to be effective in the separation of trivalent lanthanides and actinides. Analysis has been performed using a range of density-based techniques, including the Quantum Theory of Atoms in Molecules (QTAIM), the Electron Localisation Function (ELF) and the reduced density gradient (RDG). The effects of peripheral alkyl substituents on UO2-isoamethyrin, known to be vital for proper replication of the experimental geometry, are considered. Evidence for comparable amounts of covalent character has been found in the largely ionic U–N bonds of UO2-isoamethyrin and [UO2(BTP)2]2+ and examination of the variation in the electronic characteristics of the uranyl unit upon complexation in both of these cases reveal striking similarities in the nature of the U–N bonding and the effect of this bonding on the U–Oyl interaction, as well as evidence of donation into the U–N bonding region from the uranyl unit itself

U−Oyl stretching vibrations as a quantitative measure of the equatorial bond covalency in uranyl complexes:a quantum-chemical investigation

The molecular structures of a series of uranyl (UO22+) complexes in which the uranium center is equatorially coordinated by a first-row species are calculated at the density functional theory level and binding energies deduced. The resulting electronic structures are investigated using a variety of density-based analysis techniques in order to quantify the degree of covalency in the equatorial bonds. It is shown that a consideration of the properties of both the one-electron and electron-pair densities is required to understand and rationalize the variation in axial bonding effected by equatorial complexation. Strong correlations are found between density-based measures of the covalency and equatorial binding energies, implying a stabilizing effect due to covalent interaction, and it is proposed that uranyl U–Oyl stretching vibrational frequencies can serve as an experimental probe of equatorial covalency

Neptunyl(VI) centred visible LMCT emission directly observable in the presence of uranyl(VI)

Room temperature detection of neptunyl(VI) LMCT emission in a coordination compound and in the presence of uranyl(VI) is reported for the first time. Differences in the excitation profiles of the complexes enable spectral editing so either exclusively neptunyl(VI) or uranyl(VI) emission is observed or a sum of the two

Ligand size dependence of U–N and U–O bond character in a series of uranyl hexaphyrin complexes:quantum chemical simulation and density based analysis

A series of uranyl complexes with hexaphyrin ligands are investigated at the density functional level of theory and analysed using a variety of density-based techniques. A relationship is identified between the size of the ligand and the stability of the complex, controlled by the presence of meso-carbon centres in the porphyrin ring. The complex with the smallest ligand, cyclo[6]pyrrole, is found to have enhanced covalent character in equatorial U–N bonds as defined by the quantum theory of atoms in molecules (QTAIM), as well as enhanced stability, compared to the larger complexes. QTAIM data are supported by electron density difference distributions, integrated electronic properties and analysis of the reduced density gradient (RDG), which all show unambiguous evidence of electron sharing in all U–N bonds. In all complexes, a weakening of the covalent axial U–Oyl interaction in comparison to free uranyl is found, with evidence for a separation of electronic charge resulting in a more ionic interaction. A relationship between covalent character in the U–N bonds and the magnitude of uranyl charge redistribution is identified, where the greater the covalent character of the U–N interaction, the more ionic the U–Oyl interaction appears. The complex with the largest ligand, hexaphyrin(1.1.1.1.1.1), is found to have additional interactions with the uranyl oxygen centres, perturbing the U–Oyl interaction

U–O_yl Stretching Vibrations as a Quantitative Measure of the Equatorial Bond Covalency in Uranyl Complexes: A Quantum-Chemical Investigation

The molecular structures of a series of uranyl (UO₂²⁺) complexes in which the uranium center is equatorially coordinated by a first-row species are calculated at the density functional theory level and binding energies deduced. The resulting electronic structures are investigated using a variety of density-based analysis techniques in order to quantify the degree of covalency in the equatorial bonds. It is shown that a consideration of the properties of both the one-electron and electron-pair densities is required to understand and rationalize the variation in axial bonding effected by equatorial complexation. Strong correlations are found between density-based measures of the covalency and equatorial binding energies, implying a stabilizing effect due to covalent interaction, and it is proposed that uranyl U–O_yl stretching vibrational frequencies can serve as an experimental probe of equatorial covalency

Using molecular dynamics to simulate realistic structures of nitrocellulose of different nitration levels

Nitrocellulose is a reactive derivative of cellulose, one of the most commonly occurring natural materials. Nitration of cellulose decreases the stability of the structure, meaning less is understood about its structure and reactions. Although cellulose is often found in fully crystalline forms, nitrocellulose is more commonly paracrystalline, or amorphous. We present a protocol based on molecular dynamics simulations for creating realistic structures of nitrocellulose, particularly focusing on the crystallinity of the systems being created. We will also provide a detailed analysis of the geometric and dynamical parameters used to quantify the degree of crystallinity for the structures created here, with nitration levels varying from 0-14.14 wt% nitrogen content. Paracrystalline cellulose was not created using the protocol designed here, although it was found that the more nitrated a nitrocellulose system, the more the structure tends to paracrystallinity. This is due to a decrease in the number of hydrogen bonds present, and an increase in the size of the functional groups pushing the chains apart and weakening the interactions between the chains of the structure. The structures created are representative of realistic systems, which in the future will be able to be used to build further understanding of long-term storage of nitrocellulose.</p

CCDC 1032119: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 1032118: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures