Effects of radio-frequency field inhomogeneity on MAS solid-state NMR experiments

Abstract

NMR experiments rely on the manipulation of nuclear spins using resonant radio-frequency (rf) fields. Different coil designs have been developed to generate these fields. In solid-state NMR probes, solenoid coils wound around the rotor inside the magic-angle-spinning (MAS) stator are most commonly used. The field generated by such a coil is inhomogeneous and depends on the position within the sample space. This rf inhomogeneity is a prevalent problem in NMR which leads to non-ideal rotations of spins in parts of the sample. Radial contributions to the rf field cause time-dependent modulations of the rf amplitude and phase, as spin packets travel through areas with different rf fields during MAS. The influence of these rf modulations on the performance of hetero- and homonuclear recoupling (Rotational Echo Double Resonance and symmetry-based C7) and homonuclear frequency-switched Lee-Goldburg decoupling pulse sequences are studied using numerical simulations and Floquet theory. Significant effects are only observed for frequency-switched Lee-Goldburg decoupling, where time-dependent amplitude modulations lead to line broadening. Experimentally, these effects can be studied by a physical restriction of the sample volume to areas close to the coil windings where modulations are strongest. As an alternative to physical sample restriction, the implementation of band-selective pulses in the spin-lock frame for the application of B1-field selective inversions to spins experiencing selected parts of the rf-field distribution is presented. Any frequency band-selective pulse can be used for this purpose and the family of I-BURP pulses (H. Geen, R. Freeman, Band-Selective Radiofrequency Pulses, J. Magn. Reson. 93 (1991) 93141) chosen for the measurements demonstrated here. Significant improvements in homonuclear frequency-switched Lee-Goldburg decoupled proton spectra are shown, when parts of the rf-field distribution close to the nominal rf amplitude are selected. However, residual observed linewidths are not decoupling limited and therefore broadening due to amplitude modulations cannot be studied.