Overcoming Prohibitively Large Radiofrequency Demands for the Measurement of Internuclear Distances with Solid-State NMR under Fast Magic-Angle Spinning.

Solid-state NMR is a powerful tool to measure distances and motional order parameters which are vital tools in characterizing the structure and dynamics of molecules. Magic-angle spinning (MAS), widely employed in solid-state NMR, averages out dipole-dipole couplings that carry such information. Hence, rotor-synchronized radiofrequency (RF) pulses, that interfere with MAS averaging, are commonly employed to measure such couplings. However, most of the methods that achieve this, rotational echo double resonance (REDOR) being a classic example, require RF amplitudes that are greater than or equal to the MAS frequency. While feasible at MAS frequencies <40 kHz, these requirements become prohibitively large for higher MAS frequencies (40-110 kHz), which are now commercially available. Here, we redesign the REDOR experiment so that RF amplitudes as low as 0.5-0.7 times the spinning frequency can be used. This sequence, name deferred rotational echo double resonance (DEDOR), thus extends the utility of this method to the fastest MAS frequencies currently commercially available (111 kHz). The generality of this strategy is shown by extending it to other methods that utilize the same principle as REDOR. They will be useful in obtaining structural parameters for a wide range of molecules using solid-state NMR under fast MAS with the additional advantage of higher spectral resolution under these conditions.