Selective excitation of multiple quantum coherence

Non-selective excitation excites all possible MQCs, however most of the spectral intensity remains in the single quantum and lower-order MQ spectra.18 There have been a number of methods employed to excite one order of coherence in a spin system or to selectively excite only a few orders of coherence. In principle, this more effectively focuses the available intensity into one order or a set of selected orders and should lead to improved quality of higher-order MQ spectra. 

Order selective excitation has been achieved by the use of phase-cycled sequences of Rf pulses19-24 and composite pulse trains.25 Warren and Pines19-24 established that it was possible to construct TV/c-quantum selective excitation sequences (k = 0, +1, +2.) by systematically repeating an appropriate train of pulses with a phase increment 2n/N between repetitions (Fig. 5). 

Pulse sequences, which are selective for desired orders of coherence in spin systems of given topology and symmetry have also been developed.26,27 

DANTE-type frequency-selective pulses have also been used to selectively excite specific nuclei in the spin system and this has been employed to simplify MQ spectra. This approach is limited to relatively simple spin systems where the resonances of some nuclei are well separated from the normally overlapped signals in the spectrum.28,29 

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K. Doray, A. Kumar, Cascate selective pulses on connected single quantum transitions leading to selective excitation of multiple quantum coherences, J. Magn. Reson. Ser. A 114 (1995) 155-162. 

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