Efficient symmetry-based γ-encoded DQ recoupling sequences for suppression of t₁-noise in solid-state NMR spectroscopy at fast MAS

Solid-state NMR spectroscopy has played a significant role in elucidating the structure and dynamics of materials and biological solids at a molecular level for decades. In particular, the ¹H double-quantum/single-quantum (DQ/SQ) chemical shift correlation experiment is widely used for probing the proximity of protons, rendering it a powerful tool for elucidating the hydrogen-bonding interactions and molecular packing of various complex molecular systems. Two factors, namely, the DQ filtering efficiency and t₁-noise, dictate the quality of the 2D ¹H DQ/SQ spectra. Experimentally different recoupling sequences show varied DQ filtering efficiencies and t₁-noise. Herein, after a systematic search of symmetry-based DQ recoupling sequences, we report that the symmetry-based γ-encoded RN_n^ν sequences show superior performance to other DQ recoupling sequences, which not only have a higher DQ recoupling efficiency but can also significantly reduce t₁-noise. The origin of t₁-noise is further discussed in detail via extensive numerical simulations. We envisage that such γ-encoded RN_n^ν sequences are superior candidates for DQ recoupling in proton-based solid-state NMR spectroscopy due to its capability of efficiently exciting DQ coherences and suppressing t₁-noise.