1. Purpose

In many experiments one wants to selectively observe protons that are attached to ¹³C or ¹⁵N. The strong signals of those protons attached to ¹²C or ¹⁴N need to be suppressed prior to the actual pulse sequence, in order to be able to adjust the receiver gain for the desired signals only. One method of discriminating between these two kinds of signals is to insert the BIRD (BIlinear Rotation Decoupling) sandwich, which rotates the magnetization of the protons attached to ¹²C into the −z-direction of the rotating frame, whereas the magnetization of the ¹³C-bonded protons has returned into the +z-direction. If one waits a suitable relaxation time after the BIRD sandwich, the signals of the former are at the null point and therefore not excited during the following pulse sequence. In this educational experiment the use of the BIRD sandwich is shown for chloroform. g-BIRD sequences with additional pulsed field gradients are now in common use.

2. Literature

J. R. Garbow, D. P. Weitekamp, A. Pines, Chem. Phys. Lett. 1982, 93, 504–508.
D. Uhrin, T. Liptaj, K. E. Kövér, J. Magn. Reson. Ser. A 1993, 101, 41–46.
R. T. Williamson, J. R. Carney, W. H. Gerwick, J. Nat. Prod. 2000, 63, 876–878.

3. Pulse Scheme and Phase Cycle

BIRD filter pulse sequence and phase cycle

Note that the BIRD building-block consists of the pulses p1 to p4, whereas p5 is only used for detection.