Nuclear magnetic relaxation in water revisited

In this study, we revisited nuclear magnetic relaxation of (1)H in water at very low Larmor frequencies that has been studied intensively in earlier years. We make use of the recently developed superconducting quantum interference device based ultra-low field NMR technique, which enables much easier access to the longitudinal spin-lattice relaxation time T(1) and the transversal spin-spin relaxation time T(2) below several kHz than traditional field cycling methods. Our data reproduce and complement the earlier results, in that they corroborate the finding of an exchange process with a correlation time of about 0.34 ms at room temperature which can be attributed to the migration of hydronium and hydroxyl ions in neutral water via hydrogen bridges. The corresponding relaxation process is driven by the interaction of the protons with (17)O and contributes to the T(1) and the T(2) relaxation rate by about 0.12 s(-1). In addition, we found evidence of a very slow exchange process at about 100 Hz that has hitherto not been reported.