High-pressure δ-AI(OH)3 and δ-A1OOH phases and isostructural hydroxides/oxyhydroxides:: New structural insights from high-resolution 1H and 27Al NMR

In order to shed light on the proton distributions and order/disorder in high-pressure delta-Al(OH)(3) and delta-AlOOH phases, two-dimensional, high-resolution H-1 CRAMPS (FSLG)-MAS NMR and Al-27 3QMAS NMR spectra have been obtained. For delta-Al(OH)(3), the H-1 CRAMPS-MAS NMR revealed two peaks with an intensity ratio close to 2:1. The Al-27 MAS and 3QMAS NMR suggest a single Al site with a well-defined local structure. For delta-AlOOH, the H-1 and Al-27 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 delta-Al(OH)(3) and delta-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 center dot center dot center dot O distance.