Proton Magnetic Resonance Studies in Sodium—Ammonia Solutions

Abstract

High-resolution techniques have been applied to the measurements of the proton magnetic resonance shifts in sodium-ammonia solutions. Concentrations ranging from R = 1.6×10—4 to 1.7×10—1 were studied, and the temperature was varied from below —70° to above +25°C. The miscibility gap was noted in the relatively concentrated solutions. In dilute solutions, a triplet, arising from the spin—spin interaction of the nitrogen nucleus on the proton nuclei within an ammonia molecule, was sometimes observed. In order to interpret the data taken on samples having a mole ratio R≤1.15×10—2, a modified form of the Knight shift expression applicable to this system is derived: (ΔHH0)I=−8π3L0ρ(I)χe,theo,where χe, theo = L0μe2/kT. Using this relation, the data taken at —33.2°C are found to offer strong support for the existence of some kind of monomers down to R≤3.41×10—4. The value deduced for the total negative spin density of the electron on the monomer protons is —8.1×1023 cm—3 using the equilibrium constants of Becker, Lindquist, and Alder. Application of this same formalism to published Knight shift data yields corresponding values for the total electron density associated with the monomer of 9.0×1024 cm—3 on the nitrogen nuclei and of approximately 3.3×1022 cm—3 on the sodium-ion core. The possibility that the electron relaxation occurs on the monomer rather than on the ammonia molecules bordering a singly occupied cavity is also discussed.