Recoupling in MAS NMR by rotorsynchronized pulse sequences,
A. Brinkmann,
M. Edén, M. Carravetta,
J. Schmedt auf der Günne,
X. Zhao, and M. H. Levitt,
15th European Experimental NMR Conference, Leipzig, Germany
(Jun. 2000),
talk.
We design rotorsynchronized 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 C14_{4}^{5}, doublequantum
excitation can be obtained at high spinning
frequencies (20 kHz). Recently we presented a new class of rotorsynchronized
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 xaxis. 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
R14_{2}^{6} can be used
to excite doublequantum coherence. A sequence with the symmetry
R4_{4}^{1} can be
used for zeroquantum 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 doublequantum coherence and to obtain heteronuclear
twodimensional correlation spectra.
