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

Abstract The temperature dependence of the 1 H NMR spin-lattice relaxation time T 1 and the second moment M 2 have been measured for [Mg(H 2 O) 6 ]SnCl 6 (magnesium hexachlorostannate(IV) hexahydrate) and [Mg(H 2 O) 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 observed at 195 K for the Sn(IV) complex and at 153 K for the Te(IV) complex. The T 1 minimum has been explained by the 180° flips of water 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, activation 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 activation energy between the two complexes have been interpreted in terms of the hydrogen bonds in the crystals.