Bilinear Rotation Decoupling

Introduction:

In many experiments one wishes to selectively observe protons that are directly attached to either 13C or 15N.  In order to do so, the strong signals originating from the protons attached to 12C or 14N need to be suppressed prior to the pulse sequence such that the receiver gain may be properly adjusted to observe the desired signals.  A popular method  in which this may be accomplished is to make a BIRD (BIlinear Rotation Decoupling) sandwich.  This BIRD sandwich consists of an imbedded pulse sequence which serves to rotate the magnetization of the protons attached to the 12C carbons into the -z-direction of the rotating frame, yet does not effect the magnetization of the 13C bound protons.  As such, the unwanted signals are suppressed.  Using a judicious choice to time following the BIRD pulse sequence, the signals originating from the 12C bound protons are at a null, hence not excited. 

References:
  • J. R. Garbow, D. P. Weitekamp and A. Pines, Chem. Phys. Lett. 1982, 93, 504-508.