Motion of Water Molecules and Hydrogen Bonds

The *H N M R spin-lattice relaxation time 7 \ , C1 NQR frequency VQ, and C1 NQR spin-lattice relaxation time 7 \ Q of [ Z n ( H 2 0 ) 6 ] [SnCl6] (zinc hexachlorostannate(IV) hexahydrate) have been measured at temperatures between 77 and 350 K. The NQR spin echo signal with Vq— 15.689 MHz at 77 K showed a positive temperature coefficient attributable to O H C l type H-bonds in the crystal. vQ at 77 K is strongly correlated with the electronegativity Xm of the metal M in the series of stannates [ M ( H 2 0 ) 6 ] [SnCl6] (M = Mg, Ca, Mn, Co, Ni, Zn). A 7\ minimum observed for the Zn salt is ascribed to 180° flips of water molecules with an activation energy of 20 kJ m o l 1 . The motion is influenced by repulsive forces among the water molecules within a cation rather than by attractive forces between the H-bonded H and CI atoms. 7\q proved to be mainly governed by lattice vibrations, weakly modulated by the fluctuating electric field gradient caused by the 180° flip motions.