Figure 1.

REDOR pulse sequence used to measure ¹⁹F–³¹P dipolar couplings. Initial phosphorus magnetization is produced by cross polarization from ¹H to ³¹P. Application of a series of alternating rotor-synchronized fluorine and phosphorus 180° pulses dephases ³¹P transverse magnetization, resulting in a reduced ³¹P signal. The extent of dephasing observed is determined by the ¹⁹F–³¹P dipolar couplings. Comparison of this reduced signal with a reference signal obtained in the absence of the fluorine dephasing pulses allows the determination of the dipolar coupling, and thereby, direct extraction of the internuclear separation between ¹⁹F and ³¹P labeled sites.