Measurement of hetero-nuclear distances using a symmetry-based pulse sequence in solid-state NMR

A Symmetry-based Resonance-Echo DOuble-Resonance (S-REDOR) method is proposed for measuring hetero-nuclear dipolar couplings between two different spin-1/2 nuclei, under fast magic-angle spinning. The hetero-nuclear dipolar couplings are restored by employing the SR4 2 1 sequence, which requires the rf-field strength to be only twice the spinning frequency. The S-REDOR experiment is extended to S-RESPDOR (Symmetry-based Resonance-Echo Saturation-Pulse DOuble-Resonance) for determining dipolar coupling between a spin-1/2 nucleus (e.g.¹³C) and ¹⁴N. It is demonstrated that S-REDOR and S-RESPDOR methods suppress efficiently the homo-nuclear dipolar interaction of the irradiated nucleus and benefit from high robustness to the rf-field inhomogeneity, chemical shielding and dipolar truncation. Therefore, these methods allow the measurement of ¹³C/^14,15N distances, with ¹³C observation, in uniformly ¹³C-labeled samples. Furthermore, we provide analytical solutions for the S-REDOR and S-RESPDOR dephasing curves. These solutions facilitate the measurement of hetero-nuclear distances from experimental data.