Comparison of several hetero-nuclear dipolar recoupling NMR methods to be used in MAS HMQC/HSQC

We compare several hetero-nuclear dipolar recoupling sequences available for HMQC or HSQC experiments applied to spin-1/2 and quadrupolar nuclei. These sequences, which are applied to a single channel, are based either on the rotary resonance recoupling (R³) irradiation, or on two continuous rotor-synchronized modulations (SFAM₁ and SFAM₂), or on four symmetry-based sequences (R 2₁¹, SR 4₁², R 12₃⁵, R 20₅⁹), or on the REDOR scheme. We analyze systems exhibiting purely hetero-nuclear dipolar interactions as well as systems where homo-nuclear dipolar interactions need to be canceled. A special attention is given to the behavior of these sequences at very fast MAS. It is shown that R³ methods behave poorly due to the narrowness of their rf-matching curves, and that the best methods are SR 4₁² and SFAM (SFAM₁ or SFAM₂ if homo-nuclear interactions are not negligible). REDOR can also recouple efficiently hetero-nuclear dipolar interactions, provided the sequence is sent on the non-observed channel and homo-nuclear dipolar interactions are negligible. We anticipate that at ultra-fast spinning speed, SFAM₁ and SFAM₂ will be the most efficient methods.