Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems

We have recently shown that the sensitivity of single- and multiple-quantum NMR experiments of half-integer (N/2) quadrupolar nuclei can be increased significantly by introducing so-called double frequency sweeps (DFS) in var ious pulse schemes. These sweeps consist of two sidebands generated by an a mplitude modulation of the RF carrier. Using a time-dependent amplitude mod ulation the sidebands can be swept through a certain frequency range. Inspi red by the work of Vega and Naor (J. Chem. Phys. 75, 75 (1981)), this is us ed to manipulate +/- (m - 1) <-> +/- m (3/2 less than or equal to m less th an or equal to N/2) satellite transitions in half-integer spin systems simu ltaneously. For Na-23 (I = 3/2) and Al-27 (I = 5/2) spins in single crystal s it proved possible to transfer the populations of the outer +/- m spin le vels to the inner +/- 1/2 spin levels. A detailed analysis shows that the e fficiency of this process is a function of the adiabaticity with which the various spin transitions are passed during the sweep. In powders these swee p parameters have to be optimized to satisfy the appropriate conditions for a maximum of spins in the powder distribution. The effects of sweep rate, sweep range, and RF field strength are investigated both numerically and ex perimentally. Using a DFS as a preparation period leads to significantly en hanced central transition powder spectra under both static and MAS conditio ns, compared to single pulse excitation. DFSs prove to be very efficient to ols not only for population transfer, but also for coherence transfer. This can be exploited for the multiple- to single-quantum transfer in MQMAS exp eriments. It is demonstrated, theoretically and experimentally, that DFSs a re capable of transferring both quintuple-quantum and triple-quantum cohere nce into single-quantum coherence in I = 5/2 spin systems. This leads to a significant enhancement in signal-to-noise ratio and strongly reduces the R F power requirement compared to pulsed MQMAS experiments, thus extending th eir applicability. This is demonstrated by Al-27 3QMAS experiments on 9Al(2 )O(3) . 2B(2)O(3) and the mineral andalusite. In the latter compound, Al ex periences a quadrupolar-coupling constant of 15.3 MHz in one of the sites. Finally a 5QMAS spectrum on 9Al(2)O(3) . 2B(2)O(3) demonstrates the sensiti vity enhancement of this experiment using a double frequency sweep. (C) 200 0 Academic Press.