STUDIES OF SUCCESSIVE PHASE-TRANSITIONS AND MOLECULAR MOTIONS IN [MG(H2O)(6)][SIF6] BY H-1,H-2 AND F-19 NMR

The successive phase transitions of [Mg(H2O)(6)][SiF6] were studied by measuring H-2 NMR spectra. The quadrupole coupling constant e(2)Qq/h and asymmetry parameter eta changed drastically at each transition tem perature. H-1,H-2 and F-19 NMR T-1 were measured for this compound to study the relation between the molecular motions and the successive ph ase transitions. The activation energy E-a and the pre-exponential fac tor tau(0) for the reorientation of [SiF6](2-) were estimated as 28 kJ mol(-1) and 6.0 x 10(-14) s, and those of the 180 degrees flip of H2O as 33 kJ mol(-1) and 4.0 x 10(-14) s. These two motions occur rapidly even in phase V. For the reorientation of [Mg(H2O)(6)](2+), E-a = 62 kJ mol(-1) and tau(0) = 1.1 x 10(-16) s were obtained from the simulat ion of H-2 NMR spectra. The jump rate of this motion is of the order o f 10(4) - 10(6) s(-1) in phase II. These results suggest that the succ essive phase transitions are closely related to the motion of [Mg(H2O) (6)](2+).