Hindered Rotation of Water Molecules in Potassium Oxalate Monohydrate: A Pulsed Deuteron Magnetic Resonance Study

The spin–lattice relaxation time $T_1$ of the deuterons in potassium oxalate monohydrate, K₂C₂O₄·D₂O, has been measured in the temperature range − 19 to 75°C. $T_1$ decreases with temperature from about 70 sec at − 15°C to 0.02 sec at 75°C. The magnitude of $T_1$ and the strong temperature dependence show that the spin–lattice relaxation is caused by fluctuations of the nuclear quadrupolar interaction resulting from the 180° flip motion of the water molecules in the lattice. The average waiting time between flips, $\tau$ , is determined directly at −15°C to be 1.0 ± 0.1 msec. The ratio $T_1\mathrm{\hspace{0.17em}/\hspace{0.17em}\tau }$ is found to be (3.8 ± 0.3) × 10⁴, in good agreement with the value calculated from the proposed relaxation mechanism, (4.00 ± 0.05) × 10⁴. From the temperature dependence of $T_1$ the potential barrier hindering the flip motion is found to be 16.3 ± 0.1 kcal/mol. This value is in good agreement with the value calculated earlier from a point‐charge model by Pedersen.