Motion of Water Molecules and Hydrogen Bonds in Zinc Hexachlorostannate (IV) Hexahydrate as Studied by 1H NMR and 35Cl NQR

Abstract

Abstract The 1H NMR spin-lattice relaxation time T1, 35Cl NQR frequency νQ, and 35Cl NQR spin-lattice relaxation time T1Q of [Zn(H2O)6] [SnCl6] (zinc hexachlorostannate(IV) hexahydrate) have been measured at temperatures between 77 and 350 K. The NQR spin echo signal with νQ = 15.689 MHz at 77 K showed a positive temperature coefficient attributable to O-H-Cl type H-bonds in the crystal. νQ at 77 K is strongly correlated with the electronegativity χM of the metal M in the series of stannates [M(H2O)6] [SnCl6] (M = Mg, Ca, Mn, Co, Ni, Zn). A T1 minimum observed for the Zn salt is ascribed to 180° flips of water molecules with an activation energy of 20 kJ mol-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. T1Q proved to be mainly governed by lattice vibrations, weakly modulated by the fluctuating electric field gradient caused by the 180° flip motions.