Effects of paramagnetic cations on the nonexponential spin-lattice relaxation of rare spin nuclei in solids

Solid‐state NMR spectra of paramagnetic silica‐based materials: observation of 29Si and 27Al nuclei in the first coordination spheres of manganese ions

“…13 The pronounced dipolar electron-nucleus interactions are independently supported by the strongly reduced 29 Si T 1 times: samples 2C390 and 3C540 show the 29 Si T 1 values of ¾35 ms and 13 ms, respectively, (versus) 20-100 s as measured for diamagnetic silica-based materials. 3 These T 1 times are reasonably shorter than those measured in silica xerogels, doped by small concentrations of Mn 2C ( 29 SiT 1 ¾ 1s) 21 or in aryl-sulfonate silica gels 22 with a fullsurface coverage by Cu 2C ions. At the same time, the T 1 values do not contradict the fact that the separate lines in the 29 Si MAS NMR spectra of the paramagnetic samples remain relatively narrow.…”

“…In terms of the distance dependence, 17 this difference is very small due to the presence of the spin-diffusion mechanism. 22 Second, electron-nucleus couplings for the 29 Si nuclei located in the first coordination sphere can be significantly larger than the spinning rate. This effect is often observed in diamagnetic solids with strong homo-nuclear dipolar couplings.…”

“…4, the wide component is not symmetric. It stretches from +1400 (taken as υ 11 ) to 400 ppm (υ 33 ) and shows a maximum ¾at C100 ppm (υ 22 ). Such a shape could roughly correspond to an isotropic chemical shift of C300 ppm.…”

Solid-state NMR investigations of Si-29 and N-15 enriched silicon nitride

“…3(c) shows the spectrum obtained for t ¼ 10.5 h. A plot of the integrated 29 Si signal intensities (I) versus t is displayed in Fig. 4, together with a best-fit of the experimental data to a stretched exponential function [23][24][25], according to…”

Structure of a paramagnetic supermicroporous silica‐based material via a multinuclear solid‐state NMR monitoring

“…were performed in the dried gel and calcined materials 1C504-1C650 spinning at 10 kHz. We have found that in all the cases the 29 Si inversion-recovery curves are strongly nonexponential 9,20 (Fig. 5) and the isotropic lines and their sidebands relax identically.…”