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

Abstract

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 ( R 3) irradiation, or on two continuous rotor-synchronized modulations (SFAM 1 and SFAM 2), or on four symmetry-based sequences ( R 2 1 1 , SR 4 1 2 , R 12 3 5 , R 20 5 9 ) , 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 R 3 methods behave poorly due to the narrowness of their rf-matching curves, and that the best methods are SR 4 1 2 and SFAM (SFAM 1 or SFAM 2 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, SFAM 1 and SFAM 2 will be the most efficient methods.