Efficient and sideband-free 1H-detected 14N magic-angle spinning NMR.

Indirect detection via sensitive spin-1/2 nuclei like protons under magic-angle spinning (MAS) has been developed to overcome the low spectral sensitivity and resolution of 14N NMR. The 14N quadrupolar couplings cause inefficient encoding of the 14N frequency due to large frequency offsets and make the rotor-synchronization of the evolution time necessary. It is shown that 14N rf pulses longer than the rotor period can efficiently encode 14N frequencies and generate spinning sideband free spectra along the indirect dimension. Average Hamiltonian and Floquet theories in the quadrupolar jolting frame (QJF) are used to treat the spin dynamics of the spin-1 quadrupolar nucleus under long 14N rf pulses and MAS. The results show that the rf action can be described by a scaled and phase-shifted effective rf field. The large quadrupolar frequency offset becomes absent in the QJF and therefore leads to sideband-free spectra along the indirect dimension. More importantly, when a pair of long 14N rf pulses are used, the distribution of the phase shift of the effective rf field does not affect the 14N encoding for powder samples; thus, high efficiencies can be obtained. The efficient and sideband-free features are demonstrated for three 1H/14N indirectly detected experiments using long 14N pulses under fast MAS.