Probing the 1H spin distribution in hybrid organic-inorganic gels by multiple-quantum NMR spectroscopy

Multiple-quantum (MQ) NMR was used to characterize the organic phase distribution in a series of hybrid organic-inorganic gels namely methyltrimethoxysilane aerogel and QM resins (trimethylsiloxysilicate polymers) with different composition. The Gaussian model of distribution of MQ coherence intensities was used to determine the effective 1H spin cluster size (N). The growth of the effective cluster size with the time of excitation of MQ NMR coherences τ indicates the uniform distribution of organic moieties in the studied gels, but the growth rate is significantly slower than that of adamantane which was used for comparison as an example of the systems with uniform distribution of 1H spins. The growth rate was described by the exponents α in the scaling law of the form N~τα. The obtained values of α were 3.07 for adamantane, 1.27 for aerogel and 1.8 for QM resins. This difference was attributed to the surface distribution of the 1H spins in the studied gels. The growth rate of spin clusters in aerogel is slower in comparison with QM resins with different content of 1H spins which show similar growth rate among each other. This result was explained by the higher local concentration of 1H spins on the surfaces of silica regions in QM resins.