Nuclear magnetic relaxation rate dispersion in supercooled heavy water under high pressure

Abstract

Spin‐lattice (T1) and spin–spin (T2) relaxation times of the deuterons in supercooled D2O at 225 MPa, measured at two frequencies: 55.54 and 39.14 MHz down to 188 K are reported. The results show that T1 and T2 become frequency dependent in supercooled liquid water under high hydrostatic pressure at temperatures below ∼220 K. Theoretical expressions for the relaxation rates are deduced under the assumption that the orientational fluctuations of the water molecules are composed of fast librational oscillations and slower diffusional motions. The effect of the librations is to reduce the size of the deuterium quadrupole coupling constant. The diffusional motions are nearly isotropic and dominate the T dependence of the relaxation times. The autocorrelation function of the slow orientational fluctuations was assumed to be exponential at long times with a single time constant, the orientational correlation time τ2. The T dependence of the latter is well described by the VTF equation. The parameters obtained by least squares fitting the experimental spin‐lattice relaxation times to an isotropic motional model correctly predict the temperature and frequency dependence of the spin–spin relaxation times.