H-1-NMR STUDIES ON THE MOTION OF WATER-MOLECULES IN MAGNESIUM HEXACHLOROSTANNATE(IV) AND HEXACHLOROTELLURATE(IV) HEXAHYDRATES

The temperature dependence of the H-1 NMR spin-lattice relaxation time T-1 and the second moment M(2) have been measured for [Mg(H2O)(6)]SnC l6 (magnesium hexachlorostannate(IV) hexahydrate) and [Mg(H2O)(6)]TeCl 6 (magnesium hexachlorotellurate(IV) hexahydrate). The same length of a T-1 minimum, with 215 ms at the Larmor frequency of 18 MHz, was obse rved at 195 K for the Sn(IV) complex and at 153 K for the Te(IV) compl ex. The T-1 minimum has been explained by the 180 degrees flips of wat er molecules about their C-2 axes. Activation energies associated with the motion were evaluated as 21 kJ mol(-1) for the Sn(IV) complex and 15 kJ mol(-1) for the Te(IV) complex. Steep drops of T-1 observed for both complexes above room temperatures were assigned to the uniaxial rotation of the hydrated cations. From the slope of the drops, activat ion energies of 83 kJ mol(-1) and 76 kJ mol(-1) were estimated for the Sn(IV) and Te(IV) complexes, respectively. The differences in the act ivation energy between the two complexes have been interpreted in term s of the hydrogen bonds in the crystals.