A Comparative Quantum Mechanical Charge Field Study of Uranyl Mono- and Dicarbonate Species in Aqueous Solution

A theoretical study of the structure and dynamics of the uranyl mono- and dicarbonate species in aqueous solution employing the quantum mechanical charge field-molecular dynamics (QMCF-MD) method is presented. The obtained structural and dynamical data were found to be in good agreement with several experimental data and theoretical investigations available in the literature. The fivefold coordination pattern observed for the equatorially bounded ligands of the uranyl ion was found to deviate from the results of a number of previous studies based on quantum chemical cluster calculations and classical molecular dynamics studies, however. The reason for the different description of the system can be seen on the one hand in the capability of QM/MM-type simulations to take charge transfer, polarization, and many-body effects into account, while the presence of a large number of MM solvent molecules ensures that the simulation system mimics the environment in the bulk of a liquid. In addition to pair, three-body and angular distributions, the use of spatial density data enabled a detailed characterization of the three-dimensional arrangement of ligands in the vicinity of the complex. Further analysis of dynamical data such as hydrogen-bond correlation functions and mean lifetime analysis enabled a detailed characterization of the properties of the complexes in aqueous solution. It could be shown that the bulk-oriented oxygen atoms of the carbonate ions form strong hydrogen bonds with bulk molecules, while the tendency of the oxygen atoms of the uranyl(VI) show decreasing tendency to form hydrogen bonds upon complexation.