Exploring structures, bonding, energetics and interactions of aqua complexes of actinide ions at various oxidation states using density functional theory

Abstract

A systematic study of interactions of U, Np and Pu ions with water in their all possible oxidation states (OSs) has been performed using DFT with triple zeta basis sets, hybrid functional and D3 dispersion correction. In gaseous phase, the maximum coordination of 9 water molecules is observed for U in both +3 and + 4 OSs and Np in +4 OS, whereas the same of 10 water molecules is obtained in +3 OS of Np and both +3 and + 4 OSs of Pu. However, in +5 and + 6 OSs, the actinide oxy ions are solvated by maximum 5 water molecules for all the three actinides, whereas in +7 state of both Np and Pu, there are 4 water molecules. Application of COSMO solvation model, although changes binding energy significantly, but does not change the maximum coordination number of all the complexes except for Np⁴⁺, in which the maximum coordination number changes from 9 to 10. However, the most favourable coordination number calculated using step-wise hydration energy in cases of complexes in gaseous phase and COSMO bulk phase is quite different. The calculated negative binding energies with the non-negative vibrational frequencies of the optimized complexes signify true minimum energy structures. The stretching frequencies of the actinide-oxygen (Ac=O) double bonds decrease whereas those of water O-H bonds (of water ligand) increase as the number of water molecules in the first solvation shell of the ions increases. One of the major highlights of the present investigation is the use of non-covalent interaction (NCI) and reduced density gradient (RDG) methods to reveal the importance of small but significant weak interactions like van der Waals, dipole-dipole and steric repulsion in stabilizing these complexes. Present study provides a comprehensive systematic analysis of structure, bonding, energetics and interaction of aqua complexes of three actinides in their all available oxidation states by treating all these complexes at the same level of theory.