High-resolution H-1 solid-state NMR spectroscopy using windowed LG4 homonuclear dipolar decoupling

International audienceWe 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 (