Uranyl Carbonate Complexes in Aqueous Solution and Their Ligand NMR Chemical Shifts and 17O Quadrupolar Relaxation Studied by ab Initio Molecular Dynamics.

Abstract

Dynamic structural effects, NMR ligand chemical shifts, and 17O NMR quadrupolar relaxation rates are investigated in the series of complexes UO22+, UO2(CO3)34-, and (UO2)3(CO3)66-. Car-Parrinello molecular dynamics (CPMD) is used to simulate the dynamics of the complexes in water. NMR properties are computed on clusters extracted from the CPMD trajectories. In the UO22+ complex, coordination at the uranium center by water molecules causes a decrease of around 300 ppm for the uranyl 17O chemical shift. The final value of this chemical shift is within 40 ppm of the experimental range. The UO2(CO3)34- and (UO2)3(CO3)66- complexes show a solvent dependence of the terminal carbonate 17O and 13C chemical shifts that is less pronounced than that for the uranyl oxygen atom. Corrections to the chemical shift from hybrid functionals and spin-orbit coupling improve the accuracy of chemical shifts if the sensitivity of the uranyl chemical shift to the uranyl bond length (estimated at 140 ppm per 0.1 Å from trajectory data) is taken into consideration. The experimentally reported trend in the two unique 13C chemical shifts is correctly reproduced for (UO2)3(CO3)66-. NMR relaxation rate data support large 17O peak widths, but remain below those noted in the experimental literature. Comparison of relaxation data for solvent-including versus solvent-free models suggest that carbonate ligand motion overshadows explicit solvent effects.