Ab initio molecular dynamics study of sodium NMR chemical shifts in the methylamine solution of [Na+ [2.2.2]cryptand Na−]

Abstract

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.