Reliable Potential Energy Surfaces for the Reactions of H₂O with ThO₂, PaO₂⁺, UO₂²⁺, and UO₂⁺

Abstract

The potential energy surfaces for the reactions of H2O with ThO2, PaO2(+), UO2(2+), and UO2(+) have been calculated at the coupled cluster CCSD(T) level extrapolated to the complete basis set limit with additional corrections including scalar relativistic and spin-orbit. The reactions proceed by the formation of an initial Lewis acid-base adduct (H2O)AnO2(0/+/2+) followed by a proton transfer to generate the dihydroxide AnO(OH)2(0/+/2+). The results are in excellent agreement with mass spectrometry experiments and prior calculations of hydrolysis reactions of the group 4 transition metal dioxides MO2. The differences in the energies of the stationary points on the potential energy surface are explained in terms of the charges on the system and the populations on the metal center. The use of an improved starting point for the coupled cluster CCSD(T) calculations based on density functional theory with the PW91 exchange-correlation functional or Brueckner orbitals is described. The importance of including second-order spin-orbit corrections for closed-shell molecules is also described. These improvements in the calculations are correlated with the 5f populations on the actinide.

Gas Phase Hydrolysis and Oxo‐Exchange of Actinide Dioxide Cations: Elucidating Intrinsic Chemistry from Protactinium to Einsteinium

“…extended the experiments to PaO 2 + . We used very high levels of theory (coupled cluster theory with single and double excitations and an approximate triples correction (CCSD(T)) extrapolated to the complete basis set limit plus additional corrections to predict potential energy surfaces for the oxo‐exchange by hydrolysis of AnO 2 0/+/2+ for An=Th IV O 2 0 , Pa V O 2 + , U V O 2 + and U VI O 2 2+ , with results in agreement with experiment. Subsequently, the energetics of bare AnO 2 + and AnO + were obtained at the CCSD(T) and complete active space (CAS) levels of theory; predictions included that AnO 2 + for An=Pa to Lr exhibit a minimum bond dissociation energy ( D [(OAn + )‐O]) at CmO 2 + .…”

“…[a] A =CCSD(T)/awT‐DK3; B =CCSD(T)/awT‐DK3(PW91); C = CV‐CBS(wCV n Z)‐DK; D =CCSD(T)/CBS(awCV n Z)‐PP(PW91); E =CCSD(T)/CBS‐PP‐DK(PW91)+corrections [b] NB=No Barrier (barrier could not be located for EsO 2 + ). [c] For TS1 we used the corrections from B3LYP/aug‐cc‐pvdz(O,H)/cc‐pvdz‐PP(Pa,U) optimized geometries.…”

Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO4–

“…Although the net bond dissociation energy, to An + and two O atoms, of PaO 2 + (∼1580 kJ/mol) is somewhat greater than that of UO 2 + (∼1520 kJ/mol), it has been established that molecular Pa−O bonds are less covalent than U−O bonds due to energy-degeneracy driven covalency. 31,49,50 A QTAIM analysis of two of the bond critical point (BCP) metrics along the An-O bond, namely the electron (Pa = 0.237 au, U = 0.275 au) and energy (Pa = −0.168 au, U = −0.229 au), show an increase in value that is indicative of an increased covalency going from Pa to U. 50 The greater deviation from linearity for protactinyl can be attributed to this lesser covalency, which relaxes the necessity for symmetry correspondence between the Pa 5f and O 2p orbitals.…”

“…This disparity in chemistry may be ascribed to lesser covalency of molecular Pa−O bonds, as discussed previously. 31,49,50 Addition of water to the AnO 2 (C 2 O 4 ) − complexes, whether the product is a bis-hydroxide or a hydrate, results in no change in oxidation state with the result that the energetics for the Pa and U complexes are similar. This is in contrast to reactions for which there is a change in oxidation state, specifically from U .…”

Remarkably High Stability of Late Actinide Dioxide Cations: Extending Chemistry to Pentavalent Berkelium and Californium

“…[39,40] The aug-ccpVDZ basis set [41,42] was used for Oa nd the aug-cc-pVDZ-PP basis sets [43][44][45][46] with effective core potentials were used for An = Ua nd Pa, and the Stuttgart large core effective core potential [47] and associated basis sets [48,49] for the remaining An. The DFT calculations provided starting geometries for subsequent CCSD(T) calculations.…”