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

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• 作者：Cé ; line Chizallet ; Guylè ; ne Costentin ; Hé ; lè ; ne Lauron-Pernot ; Michel Che ; Christian Bonhomme ; Jocelyne Maquet ; Franç ; oise Delbecq ; Philippe Sautet
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：December 13, 2007
• 年：2007
• 卷：111
• 期：49
• 页码：18279 - 18287
• 全文大小：232K
• 年卷期：v.111,no.49(December 13, 2007)
• ISSN：1932-7455

文摘

Complex ¹H MAS NMR spectra of hydroxylated MgO powders have been assigned by combining DFTembedded 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 (_H > -0.7 ppm) are proposedto be characteristic of hydrogen-bond donor OH groups (threefold O_3C-H, fourfold O_4C-H, and fivefoldO_5C-H localized on corners, edges, and in valleys respectively). The lowest chemical shifts (_H < -0.7ppm) are associated to isolated and hydrogen-bond acceptor twofold O_2C-H and onefold O_1C-H, whereasthe central signal at _H = -0.7 ppm would correspond to isolated O_3C-H and O_4C-H on kinks and divacancies.These assignments can be refined by considering dipolar interactions between vicinal protons observed thanksto the NOESY like sequence. It is thus shown that some hydrogen bond donor OH groups are characterizedby a lower chemical shift than expected from calculations and also contribute to the central signal. Calculatedthermal stabilities and chemical shifts suggest that these protons correspond to O_4C-H on monatomic steps.The final assignment is fully consistent with previous experimental results on CD₃OH adsorption andquantitative analysis of the evolution of spectra with temperature. This study illustrates the synergism betweenexperiments and theory, by comparison with the results obtained by either one.