Periodic Density Functional Theory Calculations of Uranyl Tetrachloride Compounds Engaged in Uranyl–Cation and Uranyl–Hydrogen Interactions: Electronic Structure, Vibrational, and Thermodynamic Analyses

Abstract

Solid-state uranyl hybrid structures are often formed through unique intermolecular interactions occurring between a molecular uranyl anion and a charge-balancing cation. In this work, solid-state structures of the uranyl tetrachloride anion engaged in uranyl–cation and uranyl–hydrogen interactions were studied using density functional theory (DFT). As most first-principles methods used for systems of this type focus primarily on the molecular structure, we present an extensive benchmarking study to understand the methods needed to accurately model the geometric properties of these systems. From there, the electronic and vibrational structures of the compounds were investigated through projected density of states and phonon analysis and compared to the experiment. Lastly, we present a DFT + thermodynamics approach to calculate the formation enthalpies (ΔH f) of these systems to directly relate to experimental values. Through this methodology, we were able to accurately capture trends observed in experimental results and saw good quantitative agreement in predicted ΔH f compared to the value calculated through referencing each structure to its standard state. Overall, results from this work will be used for future combined experimental and computational studies on both uranyl and neptunyl hybrid structures to delineate how varying intermolecular interaction strengths relates to the overall values of ΔH f.

Synthesis, Characterization, and Density Functional Theory Investigation of the Solid-State [UO2Cl4(H2O)]2– Complex

“…To further probe changes in the primary coordination sphere, we have plotted the Projected Density of States (PDOS) for the materials containing the [UO 2 Cl 4 (H 2 O)] 2– or [UO 2 Cl 4 ] 2– species (Figure ). The electronic structure of the gas-phase uranyl cation is well-described experimentally and computationally. ,− Sigma bonding in the uranyl cation is a result of U 6d σ mixing with O 2p σ and U 5f σ mixing with O 2p σ to form σ g and σ u molecular orbitals, respectively. The π g orbitals are a result of U 6d π and 6p π blending with O 2p π and the overlap of U 6p π and U 5f π with O 2p π results in the formation of the π u orbitals (Figure a). , Turning to the PDOS of the [UO 2 Cl 4 (H 2 O)] 2– system, we noticed that the energy of U 5f, 6d, and O axial 2p states had shifted −0.5 eV compared to the [UO 2 Cl 4 ] 2– system.…”

“…When predicting the thermochemical, vibrational, electronic, and secondary interactions of uranyl halide compounds, DFT calculations have proven to be highly effective, requiring significantly less computation time than wavefunction-based methods. ,,, Thus, periodic DFT was chosen as our computational method of choice. All DFT calculations were done using the Vienna Ab initio Simulation Package (VASP). − The generalized gradient approximation of Perdew–Burker–Enrzerhof (GGA-PBE) was used to model exchange-correlation energy and projected augmented wave (PAW) pseudopotentials , were used to represent the atoms.…”

“…1,4 For example, the commonality of the [UO 2 Cl 4 ] 2− anion enabled a systematic evaluation of noncovalent interactions in solid-state materials that led to insights regarding the crystallization of U(VI) coordination compounds. [1][2][3][4]8,9 In addition, the [UO 2 Cl 4 ] 2− complex has been crystallized with a range of alkali and organic cations and used to determine the optical, 1 vibrational, 3,5,6 and thermodynamic 2,9,10 properties of these phases to provide insights into the nature of bonding within the 5f system.…”

“…Solid-state uranyl (U­(VI)­O 2 2+ ) chloride compounds have been widely used to evaluate fundamental f -block chemistry and intermolecular interactions within actinide coordination compounds. − Most uranyl chloride solid phases are synthesized through evaporation of U­(VI) in an acidic chloride media, which typically leads to the formation of the [UO 2 Cl 4 ] 2– coordination compounds. , Currently, there are 112 structures containing the [UO 2 Cl 4 ] 2– anions in the CCDC database . The vast structural catalog of this system provides a basis to explore rational design principles in hybrid actinide compounds. , For example, the commonality of the [UO 2 Cl 4 ] 2– anion enabled a systematic evaluation of noncovalent interactions in solid-state materials that led to insights regarding the crystallization of U­(VI) coordination compounds. − ,, In addition, the [UO 2 Cl 4 ] 2– complex has been crystallized with a range of alkali and organic cations and used to determine the optical, vibrational, ,, and thermodynamic ,, properties of these phases to provide insights into the nature of bonding within the 5 f system.…”

(NH4)2[UO2Cl4]·2H2O, a new uranyl tetrachloride with ammonium charge-balancing cations

“…An = U VI , Np VI and Pu VI ) have been reported for organic-based counter-cations (Schnaars & Wilson, 2013;Surbella III et al, 2017;Schnaars & Wilson, 2018). Quite recently, focus has been placed on the cationic influence on supramolecular assembly as well as actinyl bondstrength changes (Schnaars & Wilson, 2013;Surbella III et al, 2016;Carter et al, 2018;Pyrch et al, 2020;Augustine et al, 2023). Despite these numerous studies with actinyl tetrahalide species, we report a new inorganic uranyl tetrachloride not charge-balanced by an alkali cation, with formula (NH 4 ) 2 (UO 2 Cl 4 )Á2H 2 O (compound 1).…”