High Pressure O‐17 Longitudinal Relaxation Time Studies in Supercooled H₂O and D₂O

The spin‐lattice relaxation times T 1 of the oxygen‐17 isotope in light and heavy water have been measured at 13.56 MHz in the temperature range 457 K to 238 K and up to pressures of 250 MPa. Below 300 K all isotherms exhibit maxima of T 1 which become most pronounced at the lowest temperatures measured. A marked isotope effect is seen in going from light to heavy water. The ratio [ T 1 (H 2 17 O)/ T 1 (D 2 17 O)] T,p is temperature dependent and increases with decreasing temperature. Furthermore the isotherms in D 2 17 O exhibit a stronger pressure dependence than the isotherms in H 2 17 O. The correlation times τ θ derived from 2 H‐ T 1 and 17 O‐ T 1 in heavy water are identical at all pressures and temperatures and demonstrate the isotropic nature of the orientational fluctuations of the molecules in liquid water. The temperature dependence of τ θ can at low pressures ( p < 150 MPa) be described by a fractional power law with a singular temperature T s , whereas at high pressure ( p > 150 MPa) the isobars can only be fitted by the VTF‐equation with the ideal glass transition temperature T 0 . T s as well as T 0 are found to be higher in heavy water compared to light water.