Proton-Specific Characterization to Explore the Nature of Hydroxyl Hydrogen in Australian Opals

Abstract

Solid-state proton (1H) nuclear magnetic resonance (NMR), neutron diffraction (ND) and inelastic neutron scattering (INS) spectra are shown to be distinct for Australian opal-AG, opal-AN and opal-CT samples. 1H NMR spectra of opal-AG and opal-AN samples are strikingly different while both show a trend of increasing proton content associated with a peak due to molecular water. Inversion–Recovery and Dephasing plots reveal additional peaks which may be characterized by semiquantifiable peak positions, fwhm and relaxation times. While peak assignment is incomplete, results are consistent with the presence of several examples of molecular water and silanol, including H-bonding. ND and INS spectra do not support the presence of water in sufficient local quantities to form ice. INS spectra for opal-AG and opal-CT show peaks consistent with multiple translational and librational vibrations indicating “mobile” or “displaceable” protons. Opal-AN samples show less INS activity with little definition in the spectra. While insights from this work support indicative 2-dimensional models showing interactions between proton-bearing sites, more information is needed for proposal of 3-dimensional structural models comprising Q4, Q3 and water components.