Charge densities in actinide compounds: strategies for data reduction and model building

Abstract

The data quality requirements for charge density studies on actinide compounds are extreme. Important steps in data collection and reduction required to obtain such data are summarized and evaluated. The steps involved in building an augmented Hansen–Coppens multipole model for an actinide pseudo-atom are provided. The number and choice of radial functions, in particular the definition of the core, valence and pseudo-valence terms are discussed. The conclusions in this paper are based on a re-examination and improvement of a previously reported study on [PPh4][UF6]. Topological analysis of the total electron density shows remarkable agreement between experiment and theory; however, there are significant differences in the Laplacian distribution close to the uranium atoms which may be due to the effective core potential employed for the theoretical calculations.

Charge Density Analysis of Actinide Compounds from the Quantum Theory of Atoms in Molecules and Crystals

“…In particular, crystal field effects are such to increase the charge of the two apical F atoms, which is consistent with the experimental evidence of a larger deformation density on the apical atoms. 31 Inspection of Figure 2 thus suggests a higher ionicity ( i.e. , lower covalency) in the two apical U–F a bonds than in the four equatorial U–F e bonds.…”

“… 31 − 33 Their improved protocol allowed for the reconstruction of the charge density (and its topological analysis via the QTAIMAC) of the tetraphenyl phosphate uranium hexafluoride cocrystal [PPh 4 + ][UF 6 – ]. 31 The accuracy of such an experimental procedure can be evaluated from a comparison with the outcomes of quantum-mechanical simulations.…”

“… 34 − 41 In particular, Gianopoulos et al . 31 , 32 computed the charge density of the UF 6 – molecular fragment extracted from the [PPh 4 + ][UF 6 – ] crystal, performed a QTAIMAC study, and compared their theoretical results with those from the experiment on the crystals. While an overall agreement between the molecular calculations and the experiments on the crystal was observed for some features of the chemical bonding, some significant quantitative, and even qualitative, discrepancies remained, which require further analysis.…”

“…Table 2 reports several bond descriptors evaluated at the bond critical points from the QTAIMAC. Computed values for a-UF 6 – and cry-UF 6 – are reported and compared with experimental values obtained from two different models in ref ( 31 ) (referred to as models 1b and 1c). The overall agreement between computed and experimental values is remarkable, both clearly confirming the mixed ionic/covalent nature of the U–F bonds based on the various descriptors with ∇ 2 ρ > 0, H < 0, 1 < | V |/ G < 2, and small and negative H /ρ at the bond critical points.…”

“…The difference of each quantity between the two bonds Δ = U–F a – U–F e is also reported. Computed values at the B3LYP/BSA level (this study) for a-UF 6 – and cry-UF 6 – are reported and compared with experimental values of the crystal as obtained from two different models from ref ( 31 ). Results from calculations performed on the experimental geometry of the crystal are also reported (cry-eg-UF 6 – ).…”