NMR line shape analysis for two-site exchange in rotating solids

The theory for the analysis of magic angle spinning (MASS) NMR spectral line shapes of spin systems in the solid state undergoing two-site chemical exchange is presented. The theoretical approach which is based on the Floquet formalism is valid for the whole range of exchange rates—from slow to fast, as well as for any sample spinning rate. The effect of the exchange on the widths and positions of the center and sidebands is discussed. For fast and slow exchange rates expressions for these parameters are derived using perturbation theory. Spectral line fitting between calculated and experimental spectra can provide information on the rate constants and the activation energies for the exchange processes together with relative structural parameters of the sites involved, thus extending the line shape analysis methods to include MASS.