Silicon-29 cross-polarization/magic-angle-spinning NMR study of inorganic-organic hybrids: Homogeneity of sol-gel derived hybrid gels

The structures of sol-gel derived hybrid gels prepared by co-hydrolysis of tetraethoxysilane (TEOS)-organotrialkoxysilane (RTES, RSi(OC₂H ₅)₃, R = CH₃, C₅H₁₁, C₈H₁₇ and C₆H₅) mixtures (TEOS:RTES:CH₃CH₂OH:H₂O:HNO₃ = 0.5:0.5:10:x:0.3; x = 2, 5, 10 and 20) were characterized based on signal intensities of ²⁹Si CP/MAS NMR as a function of the contact time. The contact time dependencies of the signals for Q⁴ (Si(OSi) ₄) units were successfully simulated by assuming distributed cross relaxation times (T_SiH) and an intrinsic ¹H spin-lattice relaxation time in the rotating frame (T_1σ^H); the latter was different from the apparent T_1σ^H value estimated by assuming single T_SiH. The distribution ranges of T _SiH for the TEOS-RTES gels broadened as the water content increased, suggesting that the Q⁴ units tended to be separated from the T units and that the local concentration of ¹H spins around the Q⁴ unit tended to decrease.