The Pressure Dependence of Self Diffusion in Supercooled Light and Heavy Water

Measurements of self‐diffusion coefficients D in H 2 O and D 2 O at pressures up to 400 MPa and temperatures down to 200 K are reported. Upon cooling molecular motions are strongly retarded leading to a pronounced non‐Arrhenius temperature dependence of D . In contrast, initial compression of cold water facilitates translational and rotational motions. Unexpectedly, the rates of translational diffusion increase less dramatically than those of rotational diffusion. These results are discussed in relation to the peculiar structure of the hydrogen bond network of cold water. Further all dynamic isotope effects can be removed by shifting all temperature scales by ∼ 7 K in going from H 2 O to D 2 O.