A NOVEL-APPROACH FOR EVALUATING THE NMR-SPECTRA OF SPINNING SOLIDS - APPLICATION TO THE CASE OF CHEMICAL-EXCHANGE

An approach is introduced that allows the evaluation of the effects of magic‐angle spinning (MAS) in the NMR spectra of inhomogeneously broadened spin‐1/2 nuclei. The method, which consists in replacing the time‐dependent MAS Hamiltonian by a set of three time‐independent Hamiltonians, is able to predict one of the main characteristics of the MAS experiment, namely the appearance of rotor echoes. Spectra were calculated using this approach and compared with 13C NMR spectra of model compounds, showing good agreement in the range of spinning rates normally used. A simple extension of the method also makes it useful for simulating some cases of variable‐angle spinning NMR. Although the approach is suitable for the evaluation of normal MAS NMR spectra, its main applications may be found in cases where the spin Hamiltonian is no longer self‐commuting. As an example, the method was employed for analysing the case of an exchange process between two equally populated chemically equivalent sites. Calculated spectra obtained in this way allowed the correct reproduction of the changes observed in the MAS NMR spectra of two compounds which are known to undergo reorientations in the solid state. Further possible applications of the approach are discussed. Copyright © 1990 John Wiley & Sons, Ltd.