Rotational States of NH+4in Dilute Solution in Alkali Halide Lattices. II. An NMR Study of KBr/NH4Br

Nuclear magnetic relaxation of the protons of NH + 4 in dilute solution (0.5 mole %) in KBr is measured between 2 and 259 K. The approach to equilibrium at 4.2 K is slow, the time constant being 1.5±0.2 hr, owing to conversion between nuclear spin symmetry species. There is a minimum in the spin-lattice relaxation time of 3.8±0.2 s at 5.3 K. From the slope of T 1 vs T -1 on the high temperature side, the activation energy is found to be 105±10 J mole -1 , which approximately corresponds to some average of separations between low-lying rotational levels. The spin-spin relaxation time, T 2 , is also long (1.0±0.1 ms) and independent of temperature. The equilibrium value of < I ( I +1)>, the ensemble average of the resultant nuclear spin, is slightly less than 4.3 at 4.2 K. Deviations from Curie's Law are observed below about 10 K. These results are compatible with a model of the NH + 4 ion as a three-dimensional quantum rotor placed in a potential field of octahedral symmetry.