Proton spin T1 relaxation dispersion in liquid H2O by slow proton-exchange

We have measured the longitudinal proton spin relaxation time T1 in pure water as a function of temperature (10, 37, 80°C) and Larmor frequency (50 Hz⩽ν0⩽50 kHz) by means of field-cycling techniques. T1 becomes frequency dependent below ν0≈5 kHz due to the slow proton-exchange between different oxygen environments (16O, 17O, 18O), which modulates the magnetic 1H–17O interaction. The proton-exchange time τe was found to be Arrhenius-like with activation energy 13.4 kJ/mol and pre-exponential 1.5×10−6 s (37 °C:τe=2.6×10−4 s). This is in fairly good agreement with results obtained previously in the literature by more indirect NMR methods. The new access to proton-exchange reduces the experiemtnal error limits and allows measurements at extremely small frequencies, where the familiar T1ρ technique no longer works.