An NMR spin lattice relaxation study of several N 2H 62+ salts and 14N 1H dipole-dipole contributions to relaxation

Abstract

The NMR spin-lattice relaxation times ( T 1) at 31 MHz of N 2H 6Cl 2, N 2H 6Br 2, N 2H 6Br 2·2H 2O, and N 2H 6SO 4 have been investigated over appropriate temperature ranges. Minima of T 1 for the halide salts have been identified as due to reorientation of both − NH 3 + groups at equal, or nearly equal, rates with activation energies 9.85, 7.83, and 8.59 kcal mol −1 for chloride, bromide, and hydrated bromide, respectively. A T 1 minimum for the orthorhombic phase of N 2H 6SO 4 has been assigned to reorientation of one − NH 3 + group, with an activation energy of 5.85 kcal mol −1, while the other group reorients very much faster. A low-temperature phase transition in N 2H 6SO 4 has been observed. The low-temperature phase is metastable above the temperature of the phase transition and gives a T 1 minimum which has been assigned to reorientation of both − NH 3 + groups at very similar rates, with an average activation energy of 6.16 kcal mo −1. The activation energies have been compared with the results of a previous inelastic neutron scattering study. A consideration of 14N 1H dipole-dipole interactions in − NH 3 + has indicated that these may make a contribution of about 14% to the maximum relaxation rate.