On the origin of spinning sidebands in MQMAS NMR experiments

We have recently demonstrated that a combination of multiple-quantum NMR and magic-angle spinning can provide high resolution spectra from half-integer quadrupolar nuclei. An interesting feature of these spectra is the appearance of extensive spinning sideband patterns along the indirectly detected dimension, whose origin is here discussed. It is shown that these sidebands arise even in the absence of explicit time modulations of any spin interactions, due to the orientation- and time-dependence characterizing the multiple- to single-quantum conversion process. An analytical formalism is developed to describe these effects, and corroborated by numerical calculations and NMR experiments.