The FLOCK Experiment

The FLOCK experiment (Section 6-2) is an X-nucleus-detected experiment which, despite its lower sensitivity compared with that of HMBC, is still very useful in certain circumstances. Another longer range H-X chemical-shift correlation experiment is COLOC, and the X nucleus for both experiments is C. 

The principal disadvantage of the COLOC sequence derives from the fact that it is a jce /-evolution-time experiment (i.e., t is incorporated into the delay time Aj). The major limitation with fixed-/] pulse sequences is that C-H correlations are considerably diminished, or completely absent, when the two- and three-bond coupling con- 

The second BIRD pulse is selective for vectors during A ] and acts as a simple C 180° pulse for Jch vectors, which tend to refocus. Nonetheless, there may be some one-bond polarization transfer. The Jqw vectors, however, move to antiphase for maximum polarization transfer. In addition, the BIRD pulse refocuses magnetic-field inhomogeneities for protons that are indirectly bonded to and refocuses the individual vector components (which themselves are defocusing) prior to polarization transfer. 

The third BIRD pulse, during A2, also is selective for Vch vectors, which are focused from an antiphase orientation. nuclei that are directly bonded to protons behave in either of two ways. On the one hand, the 7ch vectors of the majority of nuclei, which have not undergone any polarization transfer, remain focused, but are eliminated by phase-cycling subtraction. On the other hand, the minority of nuclei that have undergone some polarization transfer remain at antiphase and are not observed. 

In addition to a variable /], FLOCK has four fixed delay times. The first delay time (for relaxation) is a function of the H-, not the X-nucleus, T S because, like HETCOR, FLOCK is a polarization transfer experiment. The second (t, the delay in the BIRD pulses) is a 

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The two best longer range, heteronuclear chemical-shift correlation experiments are HMBC and FLOCK. Not surprisingly, HMBC ( H detection) is more commonly used than the FLOCK experiment (X-nucleus detection) because of its much greater sensitivity. Since the X nucleus in these experiments is overwhelmingly carbon, X refers to " C in the sections that follow. 

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