Investigation of the water exchange mechanism of the Plutonyl(VI) and Uranyl(VI) ions with quantum chemical methods

The water exchange reactions of [PuO₂(OH₂)₅]²⁺ and [UO₂(OH₂)₅]²⁺ were investigated with density functional theory (DFT) and wave function theory (WFT). Geometries and vibrational frequencies were calculated with DFT and CPCM hydration. The electronic energies were evaluated with general multiconfiguration quasi-degenerate second-order perturbation theory (GMC-QDPT2). Spin-orbit (SO) effects, computed with SO configuration interaction (SO–CI), are negligible. Both Actinyl(VI) ions react via an associative exchange mechanism, most likely I_a. The Gibbs activation energies (ΔG^?) at 25 °C are 33–34 and 30–37 kJ mol^?1 for [PuO₂(OH₂)₅]²⁺ and [UO₂(OH₂)₅]²⁺, respectively. ΔG^? for dissociative mechanisms (D, I_d) is higher by more than 15 kJ mol^?1.