Suppressing t₁-noise in solid-state NMR D-HMQC using spin-echo and dipolar-echo based sequences

The D-HMQC (dipolar-based heteronuclear multiple-quantum correlation) method has been extensively used in solid-state NMR for probing internuclear through-space correlations in materials, employing a dipolar recoupling sequence under MAS (magic angle spinning) conditions. However, the D-HMQC spectra often suffer from concomitant inevitable t₁-noise, which typically arises from the interferences of residual undesired interactions, e.g. CSA (chemical shift anisotropy) and MAS frequency fluctuations. Mitigating these interferences is essential for broader applications, especially at high magnetic fields. Herein, we propose two new methods to decrease these interferences: SE_CW-HMQC and DE-HMQC. The former incorporates two additional spin-echoes with the echo delays filled by CW decoupling, while the latter employs two dipolar-echoes. Through comprehensive simulations and experiments at slow-to-moderate MAS rates (≤ 25 kHz), we benchmark the performances of these two new methods against the recently reported TONE-HMQC and conventional D-HMQC sequences, where the wR4 (windowed R4) recoupling sequence is applied for achieving ¹H-{²⁷Al/⁷¹Ga}, ¹³C-{¹⁵N} and ¹⁹F-{¹³C} through-space correlations. These results show that the SE_CW-HMQC and TONE-HMQC sequences perform similarly in suppressing the CSA interference, while maintaining a high robustness to homonuclear dipolar couplings. However, among all these methods, DE-HMQC exhibits the highest tolerance to CSA and MAS instability, yielding the best-quality 2D correlation spectra with the lowest t₁-noise. For optimal performance under slow-to-moderate MAS rates, particularly in systems with strong homonuclear dipolar couplings, a wR4 recoupling sequence with a large window is highly recommended.