Double-quantum NMR spectroscopy of ³¹P species submitted to very large CSAs

We introduce an original pulse sequence, BR 2₂¹ (τ π τ), which is a block super-cycled R 2₂¹ sequence employing as basic element a π pulse sandwiched by 'window' intervals. This homonuclear dipolar recoupling method allows the efficient excitation of double-quantum coherences between spin-1/2 nuclei submitted to very large chemical shift anisotropy. We demonstrate that this technique can be employed in double-quantum ↔ single-quantum ³¹P homonuclear correlation experiment at high magnetic field (B₀ ≥ 14 T) and high MAS frequencies (ν_R ≥ 30 kHz). The performances of BR 2₂¹ (τ π τ) are compared to those of the double-quantum recoupling methods, such as BABA and bracketed fp-RFDR, which were already employed at fast MAS rates. The BR 2₂¹ (τ π τ) sequence displays a higher robustness to CSA and offset than the other existing techniques.