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

The use of double frequency sweeps (DFSs) consisting of two sidebands generated by a time-dependent amplitude modulation of the rf-carrier frequency is demonstrated in NMR experiments of spin I=3/2 nuclei. This can be used to manipulate the satellite transitions of these spin systems simultaneously. An adiabatic passage of the satellites to invert their spin populations prior to a selective excitation of the central transition can be used for signal enhancement in both static and magic-angle spinning spectra, as will be shown for a number of model compounds. Furthermore, DFSs prove to be very efficient for the conversion of triple- to single-quantum coherence in MQMAS spectroscopy. This not only means an enhancement in signal to noise ratio, but also relieves the rf-power requirements. As a result undistorted MQMAS line shapes are obtained as is demonstrated for the four 23Na resonances in Na4P2O7.