Evaluating saturation pulses in solid-state NMR using two-dimensional exchange spectroscopy

We report a new solid-state NMR spectroscopy approach for investigating the transition pathways that result from applying radiofrequency (RF) pulses. This approach relies on observable J splittings in the NMR spectrum of a spin-1/2 nucleus coupled to a quadrupolar nucleus. Because the J-separated resonances identify the eigenstates of the quadrupolar nucleus, we can follow their evolution using a two-dimensional exchange spectroscopy (EXSY) experiment for which the mixing period contains RF irradiation of the quadrupole. We performed experiments using K₂NbF₇ which displays a resolved¹J(¹⁹F,⁹³Nb) multiplet. The resolution in the ¹⁹F EXSY spectra were improved using a Hahn echo assisted deconvolution procedure. EXSY experiments were used to compare the effectiveness of square, wideband uniform rate smooth truncation (WURST), and phase-modulated (PM) saturation pulses in rotational-echo saturation-pulse double-resonance (RESPDOR) experiments. We observed that the PM-pulses required the lowest RF amplitude to reach uniform ⁹³Nb saturation. Moreover, we observed that, in all cases, the central eigenstates were saturated at a lower RF amplitude. We envision that this approach could be extended to investigate transition pathways of relevance to other NMR pulse sequences such as satellite-transition MAS (STMAS) and multiple-quantum MAS (MQMAS) experiments.