Non-covalent interactions of uranyl complexes: a theoretical study

Abstract

We report a set of theoretical calculations designed to examine the potential of model uranyl complexes to participate in hydrogen- and halogen-bonding. Potential energy scans for the interaction of [UO₂Cl₂(H₂O)₃] and [UO₂(NCSe)₂(H₂O)₃] with a single water molecule demonstrate that uranyl is a weak hydrogen bond acceptor, but that equatorially coordinated water is a strong hydrogen bond donor. These predictions are supported by a survey of contacts reported in the Cambridge Structural Database. At the minima of each scan, we show that the interaction energy is only weakly dependent on the choice of the theoretical method, with standard density functional theory methods comparing well with coupled-cluster, MP2 and double-hybrid DFT predictions. Geometry optimisation of a 1 : 1 uranyl : water complex results in a cyclic structure, in which vibrational frequencies, atoms-in-molecules and natural bond orbital analysis support the weakness of U–O_yl as an acceptor. The origin of this behaviour is traced to the electronic structure of uranyl, and in particular covalency in the U–O_yl bonds resulting from donation into formally empty 5f and 6d orbitals on U.