Spin-lattice relaxation in the alkaline earth hexaammines

The proton spin-lattice relaxation times T¹ of Ca(NH₃)₆, Sr(NH₃)₆, and Ba(NH₃)₆ have been measured at 8 MHz in the range 30-200 K by using standard pulsed NMR techniques. Dispersed samples were prepared from ingots of the compounds and were mixed with alumina powder to ensure electrical isolation of the metal grains. The samples exhibited T₁ behavior differing significantly from that of ammonia. Three different mechanisms for nuclear relaxation appear to operate in this temperature range: (1) rotational diffusion of ammonia at high temperatures (above ∼100 K), (2) activated rotation of ammonia about the pseudo C₃ axis in the intermediate temperature range (∼50-100 K), and (3) rotational tunneling which results in temperature independent T₁ values in the low temperature range (below ∼50 K). Activation barriers to ammonia diffusion and rotation were determined. Nonexponential relaxation was observed in the vicinity of the T₁ minima. These samples exhibited asymmetrical line shapes expected of metal powders with an average particle size somewhat larger than their skin depths. Knight shifts in the narrow line region were determined to be less than 10 ppm, in agreement with recent measurements by cw techniques. The line widths also agreed, to within experimental error, with previously published cw data.