Abstract
Radio-frequency field inhomogeneity is one of the most common imperfections in NMR experiments. They can lead to imperfect flip angles of applied radio-frequency (rf) pulses or to a mismatch of resonance conditions, resulting in artefacts or degraded performance of experiments. In solid-state NMR under magic angle spinning (MAS), the radial component becomes time-dependent because the rf irradiation amplitude and phase is modulated with integer multiples of the spinning frequency. We analyse the influence of such time-dependent MAS-modulated rf fields on the performance of some commonly used building blocks of solid-state NMR experiments. This analysis is based on analytical Floquet calculations and numerical simulations, taking into account the time dependence of the rf field. We find that, compared to the static part of the rf field inhomogeneity, such time-dependent modulations play a very minor role in the performance degradation of the investigated typical solid-state NMR experiments.
Highlights
Radio-frequency field inhomogeneity describes the spatial inhomogeneity of the rf field inside the coil or sample volume and is one of the major experimental imperfections that leads to artefacts or reduced efficiency in NMR experiments
The gap between the rotor and the coil is minimized in order to optimize the filling factor. This design choice typically leads to large rf field inhomogeneity that can manifest itself in reduced efficiency in experiments such as cross-polarization (Hartmann and Hahn, 1962; Stejskal et al, 1977), homonuclear decoupling (Bielecki et al, 1989, 1990; Mote et al, 2016), heteronuclear decoupling (Purusottam et al, 2015; Frantsuzov et al, 2017), symmetry-based recoupling sequences (Levitt, 2007), or even pulsed recoupling experiments like rotational-echo double-resonance (REDOR; Nishimura et al, 2001)
Magic angle spinning in combination with inhomogeneous radial rf fields leads to a time-dependent modulation of the rf field amplitude and phase
Summary
Radio-frequency (rf) field inhomogeneity describes the spatial inhomogeneity of the rf field inside the coil or sample volume and is one of the major experimental imperfections that leads to artefacts or reduced efficiency in NMR experiments. The gap between the rotor and the coil is minimized in order to optimize the filling factor This design choice typically leads to large rf field inhomogeneity that can manifest itself in reduced efficiency in experiments such as cross-polarization (Hartmann and Hahn, 1962; Stejskal et al, 1977), homonuclear decoupling (Bielecki et al, 1989, 1990; Mote et al, 2016), heteronuclear decoupling (Purusottam et al, 2015; Frantsuzov et al, 2017), symmetry-based recoupling sequences (Levitt, 2007), or even pulsed recoupling experiments like rotational-echo double-resonance (REDOR; Nishimura et al, 2001)
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