The BIRD Filter

Many modern NMR experiments exploit coupling interactions between protons and heteronuclei (eg. ¹³C). In such sequences the goal is to selectively observe the protons bound to ¹³C and suppress those bound to ¹²C. Since ¹³C is only 1 % naturally abundant, this means that 99% of the signal must be suppressed. One particularly simple scheme to accomplish this is the BIRD (BIlinear Rotation Decoupling) filter. The BIRD filter uses a heteroneuclear spin echo with delays equal to 1/(2¹J_CH) to align the ¹H(¹²C) and ¹H(¹³C) spin vectors along the -y and y axes of the rotating frame of reference, respectively. The 180 degree phase difference between the ¹H(¹²C) and ¹H(¹³C) spin vectors allows a 90 degree pulse to align the these vectors on the -z and z axes, respectively. At this point the ¹H(¹²C) spins are allowed to relax according to their T₁ to the null point. A final 90 degree read pulse puts the ¹H(¹³C) spins in the transverse plane for observation. The first of the two figures below demonstrates the use of the BIRD filter on the lineshape sample. The second figure shows a vector diagram explaining the sequence.