Pulse sequence symmetries in the nuclear magnetic resonance of spinning solids: Application to heteronuclear decoupling

We develop the average Hamiltonian theory of a class of symmetrical radio-f requency pulse sequences in the NMR of rotating solids. Theorems are presen ted which allow one to predict the elimination of many average Hamiltonian terms, without detailed calculation. These results are applied to the probl em of heteronuclear decoupling in the presence of rapid magic angle spinnin g. We present sequences which minimize the number of heteronuclear terms at the same time as recoupling the homonuclear interactions of the irradiated spins. The performance of the new sequences is tested on C-13 labeled calc ium formate. Experimental measurements of double-quantum H-1 excitation ind icate a relationship between good heteronuclear decoupling of the observed spin species and efficient recoupling of the irradiated spin species. The h eteronuclear decoupling performance of the new sequences is significantly b etter than that obtained with an unmodulated radio-frequency field. The dec oupling performance is improved further by breaking the pulse sequence symm etry in a controlled fashion. (C) 1999 American Institute of Physics. [S002 1-9606(99)00228-7].