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The sodium anion (Na − ) was once thought to behave like a ‘genuine’ anion, with both the [Ne] core and the 3s valence shell interacting very weakly with their environments. In the present work, following a recent study of the surprisingly small quadrupolar line widths of Na − , NMR shielding calculations were carried out for the Na − /Na + [2.2.2]cryptand system solvated in methylamine, based on ab initio molecular dynamics simulations, followed by detailed analyses of the shielding constants. The results confirm that Na − does not act like a quasi-free ion that interacts only weakly with its surroundings. Rather, the filled 3s shell of Na − interacts strongly with its chemical environment, but only weakly with the ion's own core and the nucleus, and it isolates the core from the chemical environment. As a consequence, the Na − ion appears in NMR experiments like a free ion. 

NMR relaxation rates for protons in liquid water and neat acetonitrile were computed based on ab initio molecular dynamics (aiMD) with forces from Kohn–Sham (KS) theory as well as force-field (FF) based classical dynamics. Intra- and intermolecular dipole–dipole contributions were separated, and nearly quantitative agreement with experiment was obtained for water. Spin-rotation (SR) contributions to the intramolecular relaxation rate in acetonitrile were computed using nuclear SR coupling tensors obtained from KS theory. Their inclusion improved the total computed intramolecular rate to within a factor of two of experiment. Insufficient sampling of rare short-time collision events between neighboring acetonitrile molecules in the simulations is hypothesized as a major source of error in the intermolecular contributions.