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.