Dynamical structure of paramagnetic [M(H₂O)₆][SiF₆] (M = Fe²⁺,Ni²⁺) crystal studied by means of²H nuclear magnetic resonance

Abstract

The temperature dependences of the 2 H nuclear magnetic resonance (NMR) spectra and the spin-lattice relaxation time T 1 were measured forThe motional modes for both compounds were discussed on the basis of the spectral simulation. The temperature variations of the 2 H NMR spectra at high temperatures could be explained by threehowever, six-site jumps of [Fe(H 2 O) 6 ] 2+ about the C 3 axis were found to be most probable form of motion at high temperatures. At low temperatures, the 2 H NMR spectra of both compounds could be explained by 180 • flips of the water molecule. The 2 H NMR T 1 was dominated by the fluctuations of the electric field gradient caused by the molecular motion and of the magnetic interaction between the 2 H 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 energies, the jumping rates at infinite temperature for each form of motion and the quadrupole interaction parameters (e 2 Qq/ h, η) were obtained from the 2 H NMR spectra and T 1 . The conclusions from the spectral simulation are in good agreement with the results for T 1 . These results suggest that [Fe(H 2 O) 6 ][SiF 6 ] possesses dynamic disorder structure in the high-temperature phase.

Molecular Dynamics in Paramagnetic Materials as Studied by Magic-Angle Spinning 2H NMR Spectra

“…The activation energies of the three-site jump of [M(H 2 O) 6 ] 2+ (Ni 2+ = 89 kJ mol -1 , Zn 2+ = 83 kJ mol -1 , Co 2+ = 74 kJ mol -1 , and Mn 2+ = 42 kJ mol -1 ) were obtained by the analysis of 2 H NMR spectra. We have pointed out that the structure of [M(H 2 O) 6 ] 2+ elongated along the C 3 axis makes the activation energy small. , The activation energies of the three-site jump of [M(H 2 O) 6 ] 2+ obtained by this study reflected the degree of the elongation of [M(H 2 O) 6 ] 2+ .…”

“…For the 180° flip of water molecules, a constant k H 2 O value (= 1.0 × 10 9 s -1 ) was used. The linebroadening due to anisotropic spin−spin relaxation which is caused by the dipolar interaction between the 2 H nuclei and the Ni 2+ was estimated using eq 9 and τ e obtained from the observed T 1 value at each temperature . The paramagnetic shift ν D due to the electron−deuteron dipolar coupling for the nearest Ni 2+ and the next nearest Ni 2+ were estimated from eq 5 as 38 and 9 kHz for D1 and 45 and 6 kHz for D2 at 296 K and 23 and 5 kHz for D1 and 27 and 4 kHz for D2 at 493 K. Figure shows the temperature dependence of the rate k re for the three-site jump of [Ni(H 2 O) 6 ] 2+ about its C 3 axis obtained by the 2 H NMR MAS and broadline spectra.…”

“…The paramagnetic shift ν D due to the electron−deuteron dipolar coupling for the nearest Ni 2+ and the next nearest Ni 2+ were estimated from eq 5 as 38 and 9 kHz for D1 and 45 and 6 kHz for D2 at 296 K and 23 and 5 kHz for D1 and 27 and 4 kHz for D2 at 493 K. Figure shows the temperature dependence of the rate k re for the three-site jump of [Ni(H 2 O) 6 ] 2+ about its C 3 axis obtained by the 2 H NMR MAS and broadline spectra. k re in the range of 10 3 −10 6 s -1 were obtained by the 2 H NMR broadline spectra . On the contrary, the 2 H MAS NMR spectra of [Ni(H 2 O) 6 ][SiF 6 ] provided information of molecular motion in the dynamic range of 10 5 s -1 ≤ k re ≤ 10 7 s -1 and 10 2 s -1 ≤ k re ≤ 10 3 s -1 .…”

“…The 2 H NMR frequency at site i (ω i ( t )) for a sample under MAS is written as where ω CS i is the time-independent contribution of Fermi's contact shift. ω Qi ( t ) and ω P ij ( t ) are the time-dependent contributions of the quadrupole interaction and the dipolar interaction between 2 H nucleus and the j th paramagnetic ion, which are written by the second-order Wigner rotation matrix (Ω) as ,,,,, where θ m ( = cos -1 1/ ) is the magic angle and ω r is the sample spinning speed. (α qi , β qi , γ qi ), (α pij , β pij , γ pij ), (α gj , β gj , γ gj ), and (α, β, γ) are the Euler angles for the transformation from the molecular axes to the principal axes of the electric field gradient (EFG) tensor, from the molecular axes to the principal axes of the dipolar interaction between 2 H nucleus and the j th paramagnetic ion, from the molecular axes to the principal axes of the g-tensor of the j th paramagnetic ion and from the rotor axes to the molecular axes, respectively.…”

“…11,12 The simulation method for the 2 H NMR broadline spectrum of paramagnetic compounds obtained by these pulse sequences has been developed for the study of molecular dynamics in paramagnetic compounds. 2,[13][14][15][16][17][18][19] 2 H magic-angle spinning (MAS) NMR spectra with spinning sidebands have also been shown to be sensitive to molecular motions with the rates, 10 2 s -1 e k e 10 3 s -1 and 10 7 s -1 e k e 10 9 s -1 , which are almost undetectable in the 2 H NMR broadline spectra. [25][26][27] Therefore, information of molecular motions in a wide dynamic range can be obtaind by the combined use of 2 H NMR MAS and broadline spectra.…”