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

The spin–lattice relaxation time T1 of the deuterons in potassium oxalate monohydrate, K2C2O4·D2O, has been measured in the temperature range − 19 to 75°C. T1 decreases with temperature from about 70 sec at − 15°C to 0.02 sec at 75°C. The magnitude of T1 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, τ, is determined directly at −15°C to be 1.0 ± 0.1 msec. The ratio T1 / τ 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 T1 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.