17 O NMR as a measure of basicity of alkaline-earth oxide surfaces: A theoretical study.

The surface basicity of the alkaline-earth metal oxides has been investigated by studying the properties of ¹⁷ O nuclear magnetic resonance (NMR). To this end, we performed density functional theory calculations and determined the ¹⁷ O chemical shift and the quadrupolar coupling constants of the regular and stepped surfaces of MO (M = Mg, Ca, Sr, and Ba) oxides. The computed average chemical shift (δ _iso ^av ) for ¹⁷ O NMR of bulk MgO, CaO, SrO, and BaO is 46, 301, 394, and 636 ppm, respectively, in excellent agreement with the experiment. The ¹⁷ O NMR chemical shifts correlate linearly with the Madelung potential in the four oxides. Next, we considered the changes in the ¹⁷ O chemical shift due to the adsorption of BR ₃ (R = F and OCH ₃ ) and pyrrole as probe molecules. We found that the ¹⁷ O NMR signal of the O ion directly bound to the probe molecule shifts considerably compared to the clean surface. This is due to a change in the polarization of the O charge distribution due to the molecular adsorption. This change is the largest for BaO, with the strongest bond and the shortest surface-adsorbate distance, and the smallest for MgO, thus showing a direct correlation between ¹⁷ O NMR and surface basicity. The ¹⁷ O chemical shift of the basic site correlates linearly also with several properties of the adsorbed molecules, providing a direct measure of the surface basicity.