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

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Abstract
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.