Here is an example how to choose appropriate symmetry numbers. We focus on 1H chemical shift anisotropy (CSA) recoupling at ultrafast MAS. The sequence should be satisfy the following conditions:
- (1) 1H CSA is recoupled at 1st order.
- (2) 1H-1H dipolar and 1H isotropic chemical shift are decoupled.
- (3) RF field amplitude is kept below 200 kHz at a MAS rate of 70 kHz.
- (4) a large scaling factor is desired.
- (5) the effect of RF field inhomogeneity is minimized.