Chemical exchange effects in the NMR spectra of rotating solids

We present a theoretical analysis based on Floquet theory describing the effect of intermediate rate exchange on line shapes in magic angle sample spinning (MASS) spectra. As a test case, ¹³C spectra were obtained of dimethyl sulfone (DMS) from 25-55 °C. DMS exhibits an axially symmetric powder pattern at 25 °C which becomes asymmetric due to 180° flips about the twofold axis bisecting the CH₃-S-CH₃ angle. As the temperature is increased, the principal effect on the MASS spectrum is a pronounced broadening of both the center and sidebands. Calculated spectra fit the experimental spectra well and provide information on the rate constants and the activation energy for the exchange process. These results extend the line shape methods successfully used to study molecular motion in static samples to include high resolution MASS.