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

Abstract

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 artifacts or degraded performance of experiments. In solid-state NMR under magic-angle spinning, the radial component becomes time-dependent because the rf-irradiation amplitude and phase is modulated with integer multiples of the spinning frequency. We analyze 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 as well as 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.

Maximizing efficiency of dipolar recoupling in solid-state NMR using optimal control sequences

“…In this case, the stronger interaction allows larger deviations from the recoupling condition, and the transfer can occur in a larger volume (see the Supplementary Materials). Numerical simulations also show that temporal rf modulations have negligible effects on ramp-CP and adiabatic-CP methods ( 16 ), while it is essential to include these in the design of the optimal control sequences ( 15 ).…”

“…These variations depend on the position along the coil axis and on the radial distance from the axis. The effect of the temporal rf inhomogeneities was first described in relation to rotary resonance recoupling ( 13 , 14 ) and was studied in detail recently ( 12 , 15 , 16 ).…”

Echo modulations under homonuclear decoupling

“…Thus, a total effective field orthogonal to the refocusing pulse results and no echo modulation will be observed. However, experimental imperfections in the radio-frequency irradiation, i.e., pulse transients [17][18][19] or static or MAS-modulated rf-field inhomogeneity [26,27] can lead to additional effective fields (x eff in Eq. ( 14)) with arbitrary directions.…”