Investigation of Multiple-Quantum NMR Coherence Growth and Intensity Profile in Silsesquioxanes

Multiple-quantum (MQ) nuclear magnetic resonance (NMR) was used to characterize the proton distribution in methyltrimethoxysilane gel (MS gel, [CH₃SiO_3/2]_n) and octavinylsilsesquioxane (OVS, [CH₂=CHSiO_3/2]₈). The Gaussian model of distribution of MQ coherence intensities was used to determine the effective ¹H spin-cluster size (N). The growth rate was described by the exponents α in the scaling law of the form N ~ τ^α. The obtained values of α were 1.65 for MS gel and 1.78 for OVS. The magnitudes of the growth exponents were compared to the previously obtained values for hybrid organic–inorganic gels. The influence of the dipolar coupling strength and the arrangement of spins in these systems on the growth of MQ NMR coherences is discussed. The shape of MQ NMR coherence profiles was investigated. It was found that the intensity profiles of MQ NMR coherences at different excitation times in the investigated hybrid silica gels are similar and may be well approximated by a stretched exponential function.