Sonically induced narrowing of the NMR spectra of solids and other novel NMR techniques

Abstract

A typical liquid state NMR spectrum is composed of a number of discrete absorptions which can be readily interpreted to yield detailed information about the chemical environment of the nuclei found within the sample. The same cannot be said about the spectra of solid samples. For these the absorptions are typically broad, featureless and yield little information directly. This situation may be further exacerbated by the characteristically long T1 values of nuclei bound within a solid lattice which, consequently, require long inter-sequence delays that necessitate lengthy experiments. This work attempts to address both of these inherent problems. Classically, the resolution of the broad-line spectra of solids into discrete resonances has been achieved by imparting to the sample coherent rotation about specific axes in relation to the polarising magnetic field, as implemented in the magic-angle spinning (MAS) [1], dynamic angle spinning (DAS) [2] and double rotation (DOR) [3] NMR experiments. Recently, an alternative method, sonically induced narrowing of the NMR spectra of solids (SINNMR) [4], has been reported which yields the same well resolved solid-state spectra as the classic solid-state NMR experiments, but which achieves the resolution of the broad-line spectra through the promotion of incoherent motion in a suspension of solid particles. The first part of this work examines SINNMR and, in particular, concentrates on ultrasonically induced evaluation, a phenomenon which is thought to be essential to the incoherent averaging mechanism. The second part of this work extends the principle of incoherent motion, implicit in SINNMR, to a new genre of particulate systems, air fluidized beds, and examines the feasibility of such systems to provide well resolved solid state NMR spectra. Samples of trisodium phosphate dodecahydrate and of aluminium granules are examined using the new method with partially resolved spectra being reported in the case of the latter.