Study of the Structure of OH Groups on MgO by 1D and 2D 1H MAS NMR Combined with DFT Cluster Calculations

Abstract

Complex 1H MAS NMR spectra of hydroxylated MgO powders have been assigned by combining DFT embedded cluster calculations and experiments using single pulse, Hahn-echo, and 2D NOESY like sequences. Chemical shifts calculations suggest the qualitative classification of protons into three main categories, characterized by different chemical shifts ranges. The highest chemical shifts (δH > −0.7 ppm) are proposed to be characteristic of hydrogen-bond donor OH groups (threefold O3C−H, fourfold O4C−H, and fivefold O5C−H localized on corners, edges, and in valleys respectively). The lowest chemical shifts (δH < −0.7 ppm) are associated to isolated and hydrogen-bond acceptor twofold O2C−H and onefold O1C−H, whereas the central signal at δH = −0.7 ppm would correspond to isolated O3C−H and O4C−H on kinks and divacancies. These assignments can be refined by considering dipolar interactions between vicinal protons observed thanks to the NOESY like sequence. It is thus shown that some hydrogen bond donor OH groups are characterized by a lower chemical shift than expected from calculations and also contribute to the central signal. Calculated thermal stabilities and chemical shifts suggest that these protons correspond to O4C−H on monatomic steps. The final assignment is fully consistent with previous experimental results on CD3OH adsorption and quantitative analysis of the evolution of spectra with temperature. This study illustrates the synergism between experiments and theory, by comparison with the results obtained by either one.