Abstract

We design rotor-synchronized pulse sequences in MAS NMR by exploiting symmetry theorems, which link the symmetry of the pulse sequences to selection rules for the average Hamiltonian. We have generalized the concept of C7 to sequences denoted CN_n^ν, consisting of N cycles, with phases incremented in steps of 2πν/N. The entire CN_n^ν sequence lasts exactly n rotational periods. Using the symmetry C14₄^-5, double-quantum excitation can be obtained at high spinning frequencies (20 kHz). Recently we presented a new class of rotor-synchronized pulse sequences, denoted RN_n^ν. The sequences consist of N elements each of which rotates the spins by an angle π about the rotating frame x-axis. The phases of the basic elements are alternated by ±πν/N. The sequence of N elements spans exactly n rotational periods. A sequence with the symmetry R14₂⁶ can be used to excite double-quantum coherence. A sequence with the symmetry R4₄¹ can be used for zero-quantum recoupling. The CN_n^ν and RN_n^ν sequences can be used to irradiate at the Larmor frequencies of several spin species at the same time. We present symmetry theorems for two simultanous RN_n^ν sequences which allow the construction of rf sequences for recoupling and/or decoupling of certain spin interactions in heteronuclear spin systems. We present solutions for decoupling homonuclear dipolar and CSA interactions while recoupling the heteronuclear dipolar couplings. We use these sequences to excite heteronuclear double-quantum coherence and to obtain heteronuclear two-dimensional correlation spectra.