Proton Magnetic Resonance Study of Hydrated Potassium Trioxalatoaluminate: Free Rotation of Proton Pairs

A proton magnetic resonance study of hydrated potassium trioxalatoaluminate revealed that the lines are extremely narrow and intense at room temperature and much broader at 90°K, indicating the presence of free rotation of the proton pairs. Detailed investigations were carried out on both single-crystal and powder samples, and it was found that the p–p vectors rotate about the c-axis generating a cone having a semi-vertical angle of about 33°. The temperature variation of the second moment showed a transition around 155°K. This is in agreement with Saha's susceptibility measurements on the isomorphous chromium salt. Similar rotational narrowing was observed in the chromium and ferric salts, also. The Pake method was used to determine the length and orientation of the p–p vectors in the rigid lattice (at 90°K). The hydrogen-bond schemes suggested by the x-ray workers did not satisfy the criterion of parallelism between the p–p line and the line joining the acceptor oxygen atoms. Alternate schemes could not be found. An order-of-magnitude calculation gave a very low value for the barrier hindering the rotation (≈ 2kcal/mole). These results suggest that hydrogen bonds are either absent or, if present, extremely weak and are consistent with the presence of free rotation. A preliminary infrared study gave an insignificant shift of the O–H stretching frequency thus supporting our conclusions.