Proton Magnetic Relaxation Study of Oxalic Acid Dihydrate

Proton magnetic relaxation times T1, T1ρ, and T1D were measured on a single crystal of α-form oxalic acid dihydrate in two modes: temperature dependence at one or two specific crystal orientations, and angular dependence at several temperatures. Results were compared with the calculation based on an isolated three proton system for the assumed modes of motions. The temperature dependence of T1 and that of T1ρ and T1D in the low temperature range are reasonably accounted for by the 180°-flip of the water molecule. The activation energy for the motion of 43.6 kJ mol−1 is obtained. T1ρ and T1D above about 35 °C decreased as the temperature was raised. The comparison of the observed results in this temperature range with the calculated ones suggests that there is exchange motion among the three spins. An activation energy for this motion of 80.8 kj mol−1 is obtained. This three spin exchange motion may be considered as a combination of a jump of carboxyl proton to a higher potential trough corresponding to th...