High-pressure δ-Al(OH)₃ and δ-AlOOH phases and isostructural hydroxides/oxyhydroxides: New structural insights from high-resolution¹H and²⁷Al NMR

In order to shed light on the proton distributions and order/disorder in high-pressure δ-Al(OH)₃ and δ-AlOOH phases, two-dimensional, high-resolution ¹H CRAMPS (FSLG)-MAS NMR and ²⁷Al 3QMAS NMR spectra have been obtained. For δ-Al(OH) ₃, the ¹H CRAMPS-MAS NMR revealed two peaks with an intensity ratio close to 2:1. The ²⁷Al MAS and 3QMAS NMR suggest a single Al site with a well-defined local structure. For δ-AlOOH, the ¹H and ²⁷Al NMR indicate the presence of a single H and Al site each. These results are consistent with crystal structures refined from X-ray diffraction. For comparison, ¹H MAS and CRAMPS-MAS NMR spectra were also obtained for several other hydroxides/oxyhydroxides, including In(OH)₃ and InOOH that have similar structures to δ-Al(OH) ₃ and δ-AlOOH, respectively. These data not only provide additional insights into the proton distributions in these important crystal structure classes but also together provide a better defined quantitative correlation between ¹H chemical shift and hydrogen-bonding O⋯O distance.