Self-diffusion in CD4 and ND3: With notes on the dynamic isotope effect inliquids

Self-diffusion coefficients D of CD4 and ND3 were measured with the NMR-PGS E-technique over a wide range of temperature and pressure (10-200 MPa, 150- 450 K for CD4, 10-200 MPa, 200-450 K for ND3). When compared to the protona ted species, both substances show a dynamic isotope effect D-r = DX-H/DX-D that was found to rise to 1.3 (CD4) and 1.4 (ND3) at the lowest temperature s studied. This behavior is similar to a number of other simple liquids (HF , CH3OH, H2O). Classical theories for single particle motion in liquids sug gest a dependence of D-r on the square root of the inverse mass ratio, or t he square root of the inverse ratio of the moments of inertia, if translati on-rotation coupling is dominant. D-r should, however, be temperature-indep endent. The strong temperature-dependence of D-r and its high value at low temperatures found in many liquids can thus not be explained by single part icle properties, but rather has to be viewed as a collective phenomenon. It was suggested earlier that the stronger hydrogen bonds expected in the deu terated liquids are responsible for this behavior. However, the fact that m ethane shows a similar dynamic isotope effect is an indication that more co mplex mechanisms are responsible for the deviations from classical models o f liquid dynamics. Quantum mechanical calculations suggest that backscatter ing effects may describe this interesting phenomenon. (C) 1999 American Ins titute of Physics. [S0021-9606(99)50706-X].