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

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 (ω_r<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.