Study of the Motion of Water Molecules in NaBr·2H2O by the NMR Method

Abstract

The temperature variation of the second moment of the NMR spectrum of a powdered sample of NaBr·2H2O has been studied experimentally and explained on a theoretical basis. Below 126°K the second moment is constant at about 27.4 G2 (the rigid-lattice value); above this temperature it decreases at first slowly to 25.4 G2 at 174°K and then more rapidly until it reaches a value of 11.5 G2 at about 240°K, where it remains practically constant up to the room temperature (295°K). The slow diminution between 126 and 174°K is attributed to the flipping motion of the water molecules. Theoretical formulas are newly developed for the powder spectrum based on the single crystal formulas given by Pederson to explain quantitatively this slow fall. The agreement between the theoretical and experimental value is satisfactory. The large drop in the second moment beyond 174°K is explained partly on the basis of an onset of free rotation of one of the water molecules about one of its O–H axes and partly attributed to the increased rocking and twisting vibrations of the other water molecule which is bound more strongly in the lattice. The estimated value of the potential barrier hindering the rotation of this water molecule is found to be of the expected order of magnitude. The NMR line has no fine structure below 240°K but starts exhibiting it above this temperature. It is characteristic of a three-spin system. This feature is discussed in the light of the rotating water molecule model suggested above. The above results on the second moment confirm the hydrogen-bonding scheme suggested for NaBr·2H2O from an NMR study of the line splittings in single crystals.