Quadrupole Nuclei in Inorganic Materials

Abstract

The solid-state NMR of quadrupolar nuclei is becoming more important to date. The anisotropy of quadrupolar coupling cannot be eliminated by magic-angle spinning. Although large quadrupolar couplings often prevent us to observe NMR signals of quadrupolar nuclei as sufficiently separated signals, the recent development of the equipment and methods including DNP, are spreading our opportunities of NMR observation to a wider range of materials and in deeper levels of information. The quadrupolar parameters, which can be taken from NMR spectra of quadrupolar nuclei, have a potential usefulness to distinguish atoms depending on their local electrostatic environment. MQMAS or STMAS enable us not only to observe quadrupolar nuclei in separated signals but also to get quadrupolar parameters and isotopic chemical shifts of each the separated signal component in a 2D spectrum. J- or D-HMQC, CP-HETCOR and related techniques provide us information about inter-atomic chemical bonding or geometrical correlations. When the sensitivity is insufficient, besides isotopes enrichment, for quadrupolar nuclei specifically, enhancement through intra-spin population transfer methods, RAPT, FAM, DFS, WURST and HS can be applied. WURST, when combined with QCPMG method, can provide us a method for obtaining NMR spectra for those nuclei having huge quadrupolar coupling that we cannot observe the whole signal shape by simple methods. This article is dealing with the NMR methods being standard at the present, or becoming standard in the near future, especially for the practical samples, for example, mixed samples or amorphous samples. A number of experimental hints and a couple of experimental examples of 27Al and 17O are described.