Friday, January 25, 2008
Spinning Sideband Suppression in Solid State MAS NMR
In the MAS or CPMAS NMR spectra of solids, it may be difficult to distinguish between an isotropic peak and a spinning sideband or the spinning sidebands from one resonance may overlap with the isotropic peak of others. This tends to complicate the analysis of the data. One simple approach to alleviate these problems is to collect two spectra at different spinning speeds. The sidebands will occur at different frequencies whereas, the isotropic peaks will remain at the same frequency. An alternative to this approach is to use a pulse sequence to suppress the spinning sidebands leaving only the isotropic peaks. The most common of these sequences is the TOSS ( TOtal Suppression of Spinning sidebands) sequence (Dixon et. al., J. Magn. Reson. 49, 341 (1982)). In this approach four properly timed 180 degree pulses are applied before the acquisition of the FID. These pulses have the effect of randomizing the phases of the spinning sidebands while preserving the phase of the isotropic resonances. The figure below illustrates the use of this sequence. The lower trace is the 50 MHz 13C CPMAS spectrum of the ethanol inclusion of Dianin's compound with a spinning speed of 2500 Hz. The upper trace is the comparable 13C CPMAS-TOSS spectrum.