2 research outputs found
Uranyl Carbonate Complexes in Aqueous Solution and Their Ligand NMR Chemical Shifts and <sup>17</sup>O Quadrupolar Relaxation Studied by ab Initio Molecular Dynamics
Dynamic structural
effects, NMR ligand chemical shifts, and <sup>17</sup>O NMR quadrupolar
relaxation rates are investigated in the series of complexes UO<sub>2</sub><sup>2+</sup>, UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4–</sup>, and (UO<sub>2</sub>)<sub>3</sub>(CO<sub>3</sub>)<sub>6</sub><sup>6–</sup>. 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 UO<sub>2</sub><sup>2+</sup> complex, coordination at the uranium
center by water molecules causes a decrease of around 300 ppm for
the uranyl <sup>17</sup>O chemical shift. The final value of this
chemical shift is within 40 ppm of the experimental range. The UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4–</sup> and (UO<sub>2</sub>)<sub>3</sub>(CO<sub>3</sub>)<sub>6</sub><sup>6–</sup> complexes show a solvent dependence of the terminal carbonate <sup>17</sup>O and <sup>13</sup>C 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 Ă…
from trajectory data) is taken into consideration. The experimentally
reported trend in the two unique <sup>13</sup>C chemical shifts is
correctly reproduced for (UO<sub>2</sub>)<sub>3</sub>(CO<sub>3</sub>)<sub>6</sub><sup>6–</sup>. NMR relaxation rate data support
large <sup>17</sup>O 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
Quadrupolar NMR Relaxation from <i>ab Initio</i> Molecular Dynamics: Improved Sampling and Cluster Models versus Periodic Calculations
Quadrupolar
NMR relaxation rates are computed for <sup>17</sup>O and <sup>2</sup>H nuclei of liquid water, and of <sup>23</sup>Na<sup>+</sup>, and <sup>35</sup>Cl<sup>–</sup> in aqueous solution
via Kohn–Sham (KS) density functional theory <i>ab initio</i> molecular dynamics (aiMD) and subsequent KS electric field gradient
(EFG) calculations along the trajectories. The calculated relaxation
rates are within about a factor of 2 of experimental results and improved
over previous aiMD simulations. The relaxation rates are assessed
with regard to the lengths of the simulations as well as configurational
sampling. The latter is found to be the more limiting factor in obtaining
good statistical sampling and is improved by averaging over many equivalent
nuclei of a system or over several independent trajectories. Further,
full periodic plane-wave basis calculations of the EFGs are compared
with molecular-cluster atomic-orbital basis calculations. The two
methods deliver comparable results with nonhybrid functionals. With
the molecular-cluster approach, a larger variety of electronic structure
methods is available. For chloride, the EFG computations benefit from
using a hybrid KS functional