Nuclear spin-lattice relaxation in the sodium anion, Na−

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Abstract
We report direct measurements of nuclear spin‐lattice relaxation times (T1n) for the sodium anion, Na−, in solutions containing both sodium and a heavier alkali metal in 1,4,7,10‐tetraoxacyclododecane (12‐crown‐4, 12C4). Nuclear spin‐lattice relaxation in Na− is found to be essentially independent of the alkali counterion and to depend only weakly upon the concentration of sodide ion in solution. The temperature dependence of T1n for Na− was used to determine an activation energy for the processes responsible for spin relaxation. The results are consistent with a dominant, but very inefficient, quadrupolar relaxation mechanism which involves the modulation of the electric field gradient at Na− via the reorientation and/or translational motion of surrounding 12C4 molecules in the liquid. Furthermore, we find that solvation of Na− in 12C4 as well described by a model in which there is neither preferential orientation of 12C4 molecules nor a clearly identifiable first solvation shell around the sodide ion. Consistent with this description, the activation energies for the processes responsible for spin‐lattice relaxation in Na− are close to those observed for the processes causing 1H and 13C relaxation in the neat liquid crown. We outline two methods, which take into account electron–electron correlation effects, for determining the Sternheimer antishielding factor β2 for the sodium anion. The most realistic estimate for β2 (−14.98) is used to calculate the NMR linewidth for Na− when it is associated with an uncomplexed (unsolvated) sodium cation and also in the contact and solvent‐separated Na+–Na− ion pairs. A rms value of 0.03 MHz for the quadrupole coupling constant (e2qQ/ℏ) of Na− in 12C4 solutions is deduced. We conclude that Na− in these solutions suffers only minor perturbations on its gas‐phase electronic structure; neither does it exist as part of a contact or solvent‐separated ion pair. It appears that the sodide ion represents the closest realization of a gas‐like ionic moiety in solution.

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