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(OC2H5)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 (TSiH) and an intrinsic 1H spin-lattice relaxation time in the rotating frame (T1ρH); the latter was different from the apparent T1ρH value estimated by assuming single TSiH. The distribution ranges of TSiH 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.