Dynamical structure of paramagnetic [M(H2O)(6)][SiF6] (M = Fe2+, Ni2+) crystal studied by means of H-2 nuclear magnetic resonance

The temperature dependences of the H-2 nuclear magnetic resonance (NMR) spe ctra and the spin-lattice relaxation time T-1 were measured for [Ni(H2O)(6) ][SiF6] and [Fe(H2O)(6)][SiF6]. The motional modes for both compounds were discussed on the basis of the spectral simulation. The temperature variatio ns of the H-2 NMR spectra at high temperatures could be explained by three- site jumps of [Ni(H2O)(6)](2+) about the C-3 axis for [Ni(H2O)(6)][SiF6]. F or [Fe(H2O)(6)][SiF6], however, six-site jumps of [Fe(H2O)(6)](2+) about th e C-3 axis Were found to be most probable form of motion at high temperatur es. At low temperatures, the H-2 NMR spectra of both compounds could be exp lained by 180 degrees hips of the water molecule. The H-2 NMR T-1 was domin ated by the fluctuations of the electric field gradient caused by the molec ular motion and of the magnetic interaction between the H-2 nucleus and the unpaired electron spin in the metal ion. T-1 was analysed in terms of the motional modes predicted from the spectral simulation. The activation energ ies, the jumping rates at infinite temperature for each form of motion and the quadrupole interaction parameters (e(2) Qq/h, eta) were obtained from t he H-2 NMR spectra and T-1. The conclusions from the spectral simulation ar e in good agreement with the results for T-1. These results suggest that [F e(H2O)(6)][SiF6] possesses dynamic disorder structure in the high-temperatu re phase.