Silica and Hybrid Silica Gels Revisited: New Insight by Solid State Nuclear Magnetic Resonance

Abstract

The “old” ¹H→²⁹Si CP MAS (Cross Polarization Magic Angle Spinning) experiment is revisited in the frame of silica hybrid gels and silsesquioxanes. It is proved that the analysis of the CP curves can lead to erroneous interpretation in terms of quantification. We show that this results from false assumptions concerning the dynamical CP parameters THSi (cross relaxation time constant) and T_1ρ^H (¹H relaxation time in the rotating frame). In other words, at least one parameter must be measured independently, in order to constrain the fits of the CP curves. Moreover, we demonstrate that the well-known (and universally used…) “spin bath” assumption is not always valid in the frame of ¹H→²⁹Si CP MAS NMR. This point is clearly demonstrated on model silsesquioxanes exhibiting short Si-H bonds. In this case, the transfer of magnetization (called coherent transfer) presents clearly oscillations, which can lead to the precise measurement of Si-H distances by solid state NMR! Curiously, the coherent transfer of magnetization is also demonstrated for weakly coupled spin systems, encountered in the silsesquioxane (SiO_1.5CH₃)₈ or T units gels. In this case, a numerical simulation of the CP curves gives a deep insight in the chemical environment of the ²⁹Si sites in terms of Si-H distances and local molecular reorientations. For weakly coupled systems, ¹H-¹H spin diffusion must be suppressed, in order to reveal the coherent character of the transfer: the quenching of spin diffusion is demonstrated by using a modified version of the CP MAS experiment. We introduce here the Lee-Goldburg CP MAS experiment (CPLG MAS) for that purpose. The “off resonance” ¹H irradiation (at the magic angle) ensures the strong suppression of the ¹H-¹H homonuclear dipolar interaction and therefore the efficient quenching of spin diffusion during the CP transfer.