A Comparative ¹H -²⁹Si Cross-Polarization Solid-State Nuclear Magnetic Resonance Study of Opal-A and Opal-CT

We report an extensive ¹H to ²⁹Si cross-polarization (CP) nuclear magnetic resonance (NMR) investigation of a wide range of opal-AG, opal-AN and opal-CT samples, including both spectra and contact time dependent kinetics. After an extensive study of Hartmann-Hahn optimization, spin rates and power levels we are forced to conclude that the kinetics of the system is only amenable to comparative analysis rather than determination of absolute values. Q₃ peaks showed both signal growth (T_IS) and decay (T_1ρ^I) while Q₄ centers only showed the T_IS component for all opals studied, consistent with isolated proton sources in the latter. Q₂ centers are only a minor factor in most cases. Initial ¹H-²⁹Si 2D-HETCOR spectral evidence suggests that multiple Q₃ and Q₄ sites, with differing chemical shifts, are involved in the CP process. Active silicate centers and water sites may differ for single pulse (SP) and CP modes. Both SP and CP techniques are best used for comparative studies within each and between opal classes. Differing geometries are implied for all three types of opal.