Advantages of double frequency sweeps in static, MAS and MQMAS NMR of spinI=3/2 nuclei

The use of double frequency sweeps (DFSs) consisting of two sidebands gener ated by a time-dependent amplitude modulation of the rf-carrier frequency i s demonstrated in NMR experiments of spin I = 3/2 nuclei. This can be used to manipulate the satellite transitions of these spin systems simultaneousl y. An adiabatic passage of the satellites to invert their spin populations prior to a selective excitation of the central transition can be used for s ignal enhancement in both static and magic-angle spinning spectra, as will be shown for a number of model compounds. Furthermore, DFSs prove to be ver y efficient for the conversion of triple- to single-quantum coherence in MQ MAS spectroscopy. This not only means an enhancement in signal to noise rat io, but also relieves the rf-power requirements. As a result undistorted MQ MAS line shapes are obtained as is demonstrated for the four Na-23 resonanc es in Na4P2O7. (C) 1999 Elsevier Science B.V. All rights reserved.