Rotational Resonance (R2) allows selective polarization transfer between homonuclear spin pairs under MAS, specifically when the sample spinning speed is set equal to (or a submultiple of) the isotropic chemical shift difference of the pair. Viewed in the appropriate interaction frame, the effect is due to an interference between the MAS-induced modulation of the dipolar coupling, and the chemical-shift induced modulation of the coupling. By applying a weak rf-field to the system, we can increase the effective precession frequency of the nuclear spins, and so modify the spinning speed at which rotational resonance occurs. This is useful for several reasons. First, pairs of inhomogeneously broadened resonances (such as occurs in the 13C spectra of amyloid peptides) give rise to broadened rotational resonance conditions that preclude precise matching of the condition for all spin pairs in the sample. This significantly reduces recoupling efficiency when the magnitude of the broadening approaches or exceeds the size of the recoupled dipolar interaction. By performing experiments in which the sample spinning speed is set somewhat higher than the standard R2 condition, and then applying an rf field of increasing magnitude that sweeps the resonance condition across the range covered by the inhomogeneously broadened chemical shift difference of the relevant spin pair, one can more effectively drive polarization transfer among the two. As an extension of this effect, in samples with more than one spin pair, one can ramp the applied rf field such that a series of resonance conditions, for different spin pairs in the sample, are sequentially matched. This extends the selectivity of the R2 experiment, useful in extracting the effects of weak couplings from a strongly-coupled spin matrix (such as a uniformly labeled sample), to a more broadbanded regime, without significantly sacrificing selectivity. Thus the technique is useful as a dipolar mixing sequence in multidimensional correlation experiments, where recoupling selectivity is desired.
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