Pressure and temperature dependence of the longitudinal proton and deuteron relaxation rates in NH 3 and ND 3

The molecular mobility of liquid ammonia is derived from the determination of the longitudinal relaxation times of the protons and deuterons. The experiments were performed in the temperature interval between the melting pressure curve and 467 K for NH 3 and to 351 K for ND 3 at pressures up to 250 MPa. At temperatures below ∼ 350 K the molecular mobility can be described by the isotropic small‐step diffusion model. The activation energies at constant pressure are derived for the rotatoric diffusion to 7.0 ± 0.5 kJ · mol −1 and for the translatoric diffusion to 6 ± 1 kJ · mol −1 from the temperature dependence of the relaxation rates. In addition the activation energy at constant volume for the rotatoric diffusion has been determined to 5.7 ± 0.5 kJ · mol −1 . The isotherms for all relaxation rates are linear in a log(1/ T 1 ) versus 1/ T plot, yielding Δ V ≠ intra = 2.5 ± 0.6 cm 3 · mol −1 and Δ V ≠ inter = 5 ± 1 cm 3 · mol −1 .