The 29Si T1 and T2 NMR relaxation in porous paramagnetic material SiO 2–MnO–Al 2O 3

Abstract

The 29Si spin–lattice relaxation in porous silica-based material 1, doped by ions Mn 2+ at a Si/Mn ratio of 3.5, is non-exponential, independent of magic-angle spinning (MAS) rates and governed by direct dipolar coupling between electron and nucleus where an electron relaxation time is estimated to be about 10 −8 s. In the absence of mutual energy-conserving spin flips (spin diffusion) in 1, the 29Si T 2 time increases linearly with spinning rates. None was observed in diamagnetic porous system 2. The unexpected 29Si T 2 dependence has been interpreted in terms of the large bulk magnetic susceptibility (BMS) effects. It has been shown that editing the 29Si Hahn-echo MAS NMR spectra eliminates wide lines, belonging to 29Si nuclei in the proximity of paramagnetic centers, and reduces the BMS broadenings in sideband patterns for nuclei remote from these centers.