Isotopic Fingerprints of Proton-Mediated Dielectric Relaxation in Solid and Liquid Water

We report cross-validated measurements of the isotope effect on dielectric relaxation for four isotopologues of ice and water, including the 1–105 Hz region, in which only sporadic and inconsistent measurements were previously available. In ice, the relaxation rates exhibit an activated temperature dependence with an isotope-independent activation energy. Across 248–273 K, the H2O-to-D2O relaxation rate ratio remains constant at 2.0±0.1. This scaling agrees with Kramers’ theory in the high-friction limit if the moving mass is the proton or deuteron, indicating that dielectric relaxation is governed by a classic proton transfer over an energy barrier rather than molecular reorientation.