Efficiency of purging sequences in the long-range heteronuclear shift correlation experiment

The efficiencies of a number of pulse sequences designed to remove directly bonded C-H correlations from long-range C-H shift correlation maps are evaluated. A two-step J filter sequence is shown to give good suppression in 1 D experiments. Its incorporation into the long-range C-H shift correlation experiment with a BIRD sequence at the center of the refocusing period gives the BIRDTRAP sequence, which is shown to yield 2D maps with a few very weak direct correlations and no artifacts. BIRDTRAP has a sensitivity higher than that of FLOCK.