Structural Characteristics, Population Analysis, and Binding Energies of [An(NO₃)]²⁺(with An = Ac to Lr)

Abstract

Efficient predictive capabilities are essential for the actinide series since regulatory constraints for radioactive work, associated costs needed for specialized facilities, and the short half-lives of many actinides present great challenges in laboratory settings. Improved predictive accuracy is advantageous for numerous applications including the optimization and design of separation agents for nuclear fuel and waste. One limitation of calculations in support of these applications is that the large variations observed from predictions obtained with currently available methods can make comparisons across studies uncertain. Benchmarking currently available computational methodologies is essential to establish reliable practices across the community to guarantee an accurate physical description of the systems studied. To understand the performance of a variety of common theoretical methods, a systematic analysis of differences observed in the prediction of structural characteristics, electron withdrawing effects, and binding energies of [An(NO 3 )] 2+ (with An = Ac to Lr) in gas and aqueous phases is reported. Population analysis obtained with Mulliken and Löwdin reflect a large dependence on the level of theory of choice, whereas those obtained with natural bond orbital show larger consistency across methodologies. Predicted stability across the actinide series calculated with coupled cluster with perturbative doubles and triples at the triple ζ level is equivalent to the one obtained when extrapolated to the complete basis set limit. The ground state of [Fm(NO 3 )] 2+ and [Md(NO 3 )] 2+ is predicted to have an electronic structure corresponding to An III state in gas and An IV in aqueous phase, whereas the ground state of [An(NO 3 )] 2+ (with An = Ac to Es, Lr) presents an electronic structure corresponding to An IV in the gas and aqueous phase. The compounds studied with No in gas and aqueous phase present a preferred No III state, and the Lr compounds did not follow trends predicted for the rest of the actinide series, as previously observed in studies regarding its unusual electronic structure relative to its position in the periodic table.

Structural Analysis of the Complexation of Uranyl, Neptunyl, Plutonyl, and Americyl with Cyclic Imide Dioximes

“…40−50 A previous study of [An(NO 3 )] 2+ structures established that utilizing the 6-31G*, cc-pVDZ, 6-311++G**, cc-pVTZ, and cc-pVQZ basis set for non-An atoms, the Stuttgart RSC 1997 ECP and associated basis set for the actinides include all of the basis functions in the basis set, while changing the functional (LDA, TPSS, B3LYP, PBE0, and B972) predicted structures with an An–O distance of less than 0.03 Å and an O–An–O angle difference of less than 1°. 36 In the current study, the strongly constrained and appropriately normed semilocal (SCAN) 51 density functional is also included as it had not yet been tested for actinides. The SCAN functional is utilized to analyze structural characteristics, population analysis, and differences in predicted Gibbs free energies of reaction of the uranyl compounds with relative to those predicted with B3LYP.…”

Establishing Cost-Effective Computational Models for the Prediction of Lanthanoid Binding in [Ln(NO3)]2+ (with Ln = La to Lu)

“…A recent study on [An(NO 3 )] 2+ compounds where An = Ac–Lr has also shown a ground state of an An III configuration instead of a An IV configuration for Fm, Md, and No atoms in the gas phase (but Fm IV and Md IV in the aqueous phase). 6…”

“…This approach was recently utilized in a similar study of [An(NO 3 )] 2+ compounds, for which the thermochemical corrections for the Gibbs free energies of reaction are obtained at each level of theory utilized for the geometry and vibrational frequency calculations. 6 …”

“…DFT can describe electron correlation at a reasonable computational cost; however, the predicted thermochemical characteristics can be largely dependent on the choice of level of theory as it was recently shown in a systematic study of 5f elements in an actinoid nitrate study. 6 Furthermore, ab initio 7 and composite methods 8 have been used for thermochemical predictions, but these methodologies vary in computational cost and accuracy. Utilizing ab initio methods for lanthanoid-containing compounds can be challenging because of the number of electrons and basis functions needed to describe these systems, which increase the computational cost.…”

“…Among electronic structure methods in computational modeling, density functional theory (DFT) methods have been widely used to describe the thermochemical properties of lanthanoid-containing compounds. DFT can describe electron correlation at a reasonable computational cost; however, the predicted thermochemical characteristics can be largely dependent on the choice of level of theory as it was recently shown in a systematic study of 5f elements in an actinoid nitrate study . Furthermore, ab initio and composite methods have been used for thermochemical predictions, but these methodologies vary in computational cost and accuracy.…”