Motion of the H2O Molecule in Potassium Oxalate Monohydrate

The proton magnetic resonance doublet splitting was measured for potassium oxalate monohydrate in the range from 74° to 395°K. The equilibrium position of the proton—proton vector was always perpendicular to the magnetic field. The measurements were made for orientations of the crystal such that the H–O–H bisector was parallel or perpendicular to the field. The doublet splitting is virtually independent of the temperature up to 395°K. However, the H2O molecules is in zero-point vibration. The angular amplitudes of the torsional and rocking motions are about 11° and 10°, respectively. The orientational potential energy of the H2O molecule appears to be made up of a hydrogen-bond component and an electrostatic component due to nearest potassium ion neighbors. A reasonable, nominal H-bond energy and a reasonable, simple charge model for the H2O molecule yielded computed torque constants of 28 and 36 kcal/mole for the torsional and rocking motions, respectively. The proton—proton separation is found to be 1.54 Å when the raw value is corrected for the effects of molecular vibration. The results favor the crystal structure given recently by Pedersen.