Molecular dynamics and phase transitions in paramagnetic [Mn(H2O)6][SiF6] investigated by 2H NMR

The temperature dependences of 2H NMR spectra and spin-lattice relaxation time T1 have been measured for paramagnetic [Mn(H2O)6][SiF6]. The obtained 2H NMR spectra were simulated by considering the quadrupole interaction and paramagnetic shift. The variation of the spectra measured in phase III was explained by the 180° flip of water molecules. The activation energy Ea and the jumping rate at infinite temperature k0 for the 180° flip of H2O were obtained as 35 kJ mol−1 and 4×1014 s−1, respectively. The spectral change in phases I and II was ascribed to the reorientation of [Mn(H2O)6]2+ around the C3 axis where the Ea and k0 values were estimated as 45 kJ mol−1 and 1×1013 s−1, respectively. From the almost temperature independent and short T1 value, the correlation time for electron-spin flip-flops, τe, and the exchange coupling constant J were obtained as 3.0×10−10 s and 2.9×10−3 cm−1, respectively. The II-III phase transition can be caused by the onset of the jumping motion of [Mn(H2O)6]2+ around the C3 axis.