Quadrupolar NMR of Inorganic Materials: The Multiple-Quantum Magic Angle Spinning Experiment

Over two-thirds of all NMR-accessible nuclides are quadrupolar with half-integer spin quantum numbers (I = 3/2, 5/2, 7/2, etc.). Far from being a minority interest, therefore, NMR of nuclei such as ⁷Li, ¹¹B, ²³Na, ³⁹K, ⁷¹Ga, ⁸⁷Rb (I = 3/2), ¹⁷O, ²⁵Mg, ²⁷Al (I = 5/2), ⁴⁵Sc, ⁵¹V, ⁵⁹Co (I = 7/2), and ⁹³Nb (I = 9/2) is central to the characterization and study of a wide range of minerals, glasses, novel oxide materials, and microand mesoporous solids, including zeolites. Although their quadrupolar coupling constants can be large (typically in the range 0–20 MHz), NMR of these nuclei is facilitated by the presence of a |m _I = +1/2〉 ↔ | - 1/2〉 “central transition” (CT) which, to a first-order approximation, is not broadened by the quadrupolar interaction. Figure 1 a shows, however, that the ⁸⁷Rb central-transition NMR spectrum of polycrystalline RbNO₃, although relatively narrow, is still essentially a broad, almost featureless, “hump.” The success of magic angle spinning (MAS) as a line-narrowing technique for spin I = 1/2 nuclei has encouraged its application to spectra of this type and yet, as the example in Figure 1b shows, despite achieving a significant narrowing of the ⁸⁷Rb lineshape, MAS still fails to resolve the three crystallographically distinct sites in RbNO₃.