Studies of Successive Phase Transitions and Molecular Motions

The successive phase transitions of [Mg(H?0)6][SiF6] were studied by measuring 2 H NMR spectra. The quadrupole coupling constant eQq/h and asymmetry parameter 77 changed drastically at each transition temperature. 1 2 H and 1 F NMR T\ were measured for this compound to study the relation between the molecular motions and the successive phase transitions. The activation energy Ea and the pre-exponential factor TO for the reorientation of [SiFö]~ were estimated as 28 kJmol" 1 and 6 .0x 10~ s, and those of the 180° flip of H 2 0 as 33 k J m o l 1 and 4 . 0 x 10" 4 s. These two motions occur rapidly even in phase V. For the reorientation of [Mg(H20)6] + , E a = 62 k J m o l 1 and TO = 1.1 x 1 0 1 6 s were obtained f rom the simulation of : H NMR spectra. The jump rate of this motion is of the order of 1010 s _ I in phase II. These results suggest that the successive phase transitions are closely related to the motion of [Mg(H 2 0) 6 ] 2 + .