Nuclear relaxation and spin exchange in ammonium hexafluorophosphate (NH4PF6)

The three phases of solid ammonium hexafluorophosphate (NH₄PF₆) have been studied by cw and pulsed NMR methods between 298° and 77°K. The proton and fluorine second moments of 1.8 and 1.9 G², respectively, at 298°K are consistent with rapid, randomized reorientations of NH₄⁺ and ${\mathrm{PF}}_6^{‐}$ ions, while the larger values at 77°K require that there be little or no motional narrowing by reorientations of the PF₆⁻ ions even though the narrowing by NH₄⁺ reorientations is still present. For both nuclear species the spin‐lattice relaxation time (T₁) studies of the two low‐temperature phases reveal three distinct regions: (1) Between 109° and 110°K their relaxation is exponential with time. (2) Between 105° and 87°K the relaxation is nonexponential, but may be expressed as the sum of two exponential terms. At each temperature the same pair of time constants applies to both nuclear species. (3) At 77°K the relaxation is again exponential, with different T₁'s for protons and fluorine. The nonexponential behavior in region (2) is attributed to spin exchange between protons and fluorine nuclei. It results from modulation of the H–F dipole‐dipole interactions by relatively slow reorientations of the PF₆⁻ ions. This interpretation was confirmed by a study of the fluorine T₁ of ND₄PF₆, for which only exponential decays were observed. Activation parameters for the ${\mathrm{PF}}_6^{‐}$ reorientation were determined from the fluorine T₁ in ND₄PF₆. The parameters depend somewhat upon temperature, being 4.1 ± 0.2 kcal/mole and (3.7 ± 0.5) × 10⁻¹⁶ sec above 130°K and 4.4 ± 0.3 and (5.2 ± 1.5) × 10⁻¹⁷ below. A discontinuity occurs in the T₁'s at the 192°K phase transition, and in the high temperature NaCl phase the T₁'s are almost independent of temperature and probably are governed by spin‐rotation interactions. DTA experiments indicate that no additional phase transitions occur in NH₄PF₆ between 300° and 473°K.