Abstract
Recent developments in magic angle spinning (MAS) technology permit spinning frequencies of ≥100 kHz. We examine the effect of such fast MAS rates upon nuclear magnetic resonance proton line widths in the multi-spin system of β-Asp-Ala crystal. We perform powder pattern simulations employing Fokker-Plank approach with periodic boundary conditions and 1H-chemical shift tensors calculated using the bond polarization theory. The theoretical predictions mirror well the experimental results. Both approaches demonstrate that homogeneous broadening has a linear-quadratic dependency on the inverse of the MAS spinning frequency and that, at the faster end of the spinning frequencies, the residual spectral line broadening becomes dominated by chemical shift distributions and susceptibility effects even for crystalline systems.
Highlights
The superior sensitivity of 1H detected experiments and the wealth of information on molecular structure and dynamics that they provide has always rendered observation of protons (1Hs) as very important for the NMR spectroscopy
To combat the broadening effects of 1H–1H dipolar couplings and improve spectral resolution in 1H observed spectra, studies usually employ either dilution of the dense networks of protons with deuterium atoms [8,9,10], combined rotation and multiple pulse spectroscopy (CRAMPS) techniques to decouple the homonuclear dipolar couplings [11,12,13] or combination of these two solutions [14]
Simulated and experimental 1D 1H solid-state NMR spectra of b-Asp-Ala at 100 kHz MAS frequency. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Summary
The superior sensitivity of 1H detected experiments and the wealth of information on molecular structure and dynamics that they provide has always rendered observation of protons (1Hs) as very important for the NMR spectroscopy. To combat the broadening effects of 1H–1H dipolar couplings and improve spectral resolution in 1H observed spectra, studies usually employ either dilution of the dense networks of protons with deuterium atoms [8,9,10], combined rotation and multiple pulse spectroscopy (CRAMPS) techniques to decouple the homonuclear dipolar couplings [11,12,13] or combination of these two solutions [14] The first of these approaches is most commonly used for proteins where incorporation of 2H is achieved by biosynthetic methods [15]. These investigations aid in guiding the relatively straightforward and time-efficient ‘‘fast MAS only” method that is appropriate for the characterization of small organic molecules at natural abundance (i.e. without any isotopic enrichment) in the solid state
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