The BIRD cluster is a group of pulses that permit discrimination on the basis of the size of proton-carbon (J_CH) coupling constants. Differentiation is based on the fact that the ¹J_CH values are in the range from 125-220 Hz while ⁿJ_CH values are generally less than 15 Hz.

A BIRD-recovery time cluster applied to a spin system with z-magnetization of I spins (I_z) inverts the non-S-bound (S=¹³C or ¹⁵N) I spins (I=¹H), whereas the S-bound protons are untouched. After an appropriate recovery period (the null period), the z-magnetization of the non-S-bound protons is zero and the pulse sequence is started with only the S-bound protons. The major application of this preparation period has been in phase-cycled HMQC-type experiments in which the large signal arising from unwanted ¹H-¹²C magnetization is efficiently removed.

The BIRD building block has also been incorporated in evolution periods to distinguish between direct and long-range components of the magnetization.

Easy implementation on AVANCE spectrometers.

The interpulse delay of the BIRD block must be set to 1/(2* ¹J_CH) and the recovery delay must be optimized for each sample. For further details about implementation of several 2D inverse experiment incorporating the BIRD-recovery module in AVANCE spectrometers see:

• Tutorial: 2D inverse experiments

The most standard way to implement BIRD-recovery cluster in a pulse program is:

... p1 ph1 d2 pl2:f2 (p2 ph2) (p4 ph4):f2 d2 p1 ph3 d7 ...

in which:

• p1 and p2 are the 90º and 180º ¹H pulses (in microseconds) applied at a power level pl1 from the f1 channel.
• p4 is the 180º ¹³C pulse (in microseconds) applied at a power level pl2 from the f2 channel.
• d2 is set to 1/(2*¹J_CH).
• d7 is the recovery-null delay.
• All phases must be specified at the end of pulse program.