Homonuclear, broadbanded dipolar mixing sequences have recently been developed which can drive polarization transfer among spin-pairs in solids over a wide range of chemical shifts. One expressed use of these techniques is their application to multiply-labeled samples, where short mixing time experiments would yield correlations between near-neighbors and hence facilitate spectral assignment, and longer mixing time experiments would yield, in addition, correlations between more separated nuclei in a distance-dependent manner, from which information about molecular structure might be extracted. Numerous experiments have demonstrated the validity of the first postulate: multidimensional correlation spectra of uniformly 13C and/or 15N model compounds have been obtained using these sequences (with short mixing times), the presence of crosspeaks between near-neighbor nuclei has been confirmed, and the pattern of these crosspeaks in the correlation spectrum has been used for assignment. The second postulate has run into difficulty, however: in addition to the obscuring effects of successive polarization transfers among a series of directly bonded nuclei that can mimic the transfer of polarization through weaker coupling between more distant nuclei, calculations have indicated that the strong interactions between directly-bonded nuclei can often block transfer between the weaker couplings. Both of these effects make it difficult to extract the structural information contained in the magnitudes of the weak couplings from longer mixing-time correlation spectra, obtained using broadbanded dipolar mixing sequences. We have investigated the nature of the latter effect using a model 3-spin system. Experiments using several different broadbanded recoupling techniques (RFDR, MELODRAMA) confirm the truncating effect of strong couplings over weak. Experiments were performed using several different pulse sequences which attempt to get around this problem.
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