The dynamic isotope effects (IE) on the translational and rotational motion in liquid ammonia and water are reassessed by NMR measurements. For H2O/D2O the translational and rotational IE are clearly distinct. At 298 K, 23% and 30% are obtained, respectively. Both effects as well as the slopes of the temperature dependencies increase with decreasing temperature. For NH3/ND3 a rotational IE of 37% was observed at 298 K. A small increase to 40% at 222 K could be ascertained. The translational IE is about 15% at room temperature and exhibits a stronger temperature dependence. It is suggested that the observed deviations of the IE’s from the square root of mass and square root of moments of inertia laws are caused by translation-rotation coupling as well as quantum effects. The experimental data obtained in the present paper are also of importance for the correct interpretation of all kinds of experiments on water and ammonia, where isotopic substitutions are involved.

Precise temperature dependent H, D,By a new method, the acetonitrile deuterium quadrupole coupling constant was determined to 162(6) kHz.An isotope effect of about 5% on the acetonitrile correlation time of the reorientation of the symmetry axis is observed. It decreases if the water content is increased. For the effective correlation time of the C-H/D vector the isotope effect is about 25%. This effect was separated into a contribution of 3% for only one molecule which is deuterated and 21% for the deuteration of the remaining molecules.The anisotropy of the reorientation of acetonitrile was calculated in the limit of rotational diffusion. In neat deuterated ammonia the reorientation about the symmetry axis is about ten times faster than the reorientation of the symmetry axis itself. Due to different activation energies the anisotropy increases with decreasing temperature. The isotope effect is also anisotropic and the rotational anisotropy in protonated acetonitrile is about fourteen. The anisotropy is higher in the binary mixtures. At the critical composition it is fourteen for the deuterated and twenty for the protonated sample.In the mixture of critical composition simultaneous relaxation measurements on both phases were possible. This allows a determination of the coexistence curve. No anomaly of the rates was observed on approaching the coexistence curve.