High‐Resolution ¹H Solid‐State NMR Spectroscopy Using Windowed LG4 Homonuclear Dipolar Decoupling

Abstract

We investigate the optimal implementation of windowed LG4 decoupling for the direct acquisition of high‐resolution 1H solid‐state NMR spectra in the moderate magic‐angle spinning regime (ωr<2π × 35 kHz). We determine how the optimal value of the azimuth of the LG rotation axis, α, is affected by the choice of windowing parameters. We find that for both the windowless and windowed implementation of LG4, the optimal value of α is that for which the chemical‐shift scaling factor is approximately 0.43. We then provide an analytical expression for estimating the chemical‐shift scaling factor as a function of both α and the windowed decoupling parameters, which allows the optimal value of α to be easily determined for any implementation of LG4. We also introduce a simple and general method for improving the performance of any phase‐modulated homonuclear dipolar decoupling sequence by compensating for pulse imperfection errors that accumulate in windowed spectra due to the repeated switching of the RF irradiation. This method involves the introduction of short (<0.5 µs) compensation pulses at the start and end of each decoupling pulse, the duration and phase of which are chosen to cancel the effects of the amplitude and phase transients that arise as a result of the RF switching. We demonstrate how this method can be implemented for homonuclear dipolar decoupling sequences such as wPMLG and wLG4 to reduce the chemical‐shift scaling factor distortions generated by phase transients such that high‐quality, high‐resolution 1H NMR spectra can be acquired at arbitrary RF offsets.

Compensating Pulse Imperfections in Solid‐State NMR Spectroscopy: A Key to Better Reproducibility and Performance

“…[11] Such an optimization can include the basic characteristics of the resonance circuit in the form of the transfer function [12,13] but this time-consuming approach needs to be repeated, ideally,f or each sample and temperature as changing the experimental conditions changes the transfer function. [14,15] A related approach which has specifically been used to suppress effects of phase transients is the experimental optimization of pulse sequence parameters that can partially counteract them [5,16] using ag rid search. [14,15] A related approach which has specifically been used to suppress effects of phase transients is the experimental optimization of pulse sequence parameters that can partially counteract them [5,16] using ag rid search.…”

“…Another approach is the direct optimization of pulse sequences on the spectrometer which also takes the characteristic of the resonance circuit into account. [14,15] A related approach which has specifically been used to suppress effects of phase transients is the experimental optimization of pulse sequence parameters that can partially counteract them [5,16] using ag rid search. In all these approaches the obtained optimized pulse sequences will be specific to the transfer function of the experimental setup and must be reoptimized if the experimental setup or the tuning and matching changes.I nt he latter two cases,t he optimization should take place on the sample to be studied which is challenging for biomolecules with limited signal-to-noise ratio.A sa na pproximation, at est sample with sufficient signal-to-noise could be used.…”

Origin of the residual line width under frequency-switched Lee–Goldburg decoupling in MAS solid-state NMR

“…5a and 6). Based on numerical simulations, we believe this to be due to the better compensation of the effective nutation over a full FSLG cycle and the additional removal of fictitious fields (second-order one-spin terms) by applying transient compensation (Ernst et al, 2005;Hellwagner et al, 2017). The effect of changing the effective-field angle from the magic angle to 60 • is very small and is hard to judge from the spectra.…”

Origin of the Residual Linewidth Under FSLG-Based Homonuclear Decoupling in MAS Solid-State NMR

“…The most commonly used super cycles or alterations include an inversion of the phase ramp (PMLGxx !) (Leskes et al, 2007;Paul et al, 2009) or a relative phase shift between two PMLG cycles with an inversion of the second cycle (LG-4) (Halse et al, 2014;Halse and Emsley, 2013). These super cycles have the disadvantage of reducing the scaling factors of the chemical shifts leading to reduced separation of the lines assuming similar decoupling efficiency.…”