The role of 1H solid-state NMR in structure elucidation of solids is becoming more predominant, especially as faster magic-angle spinning rates (MAS) become available which enable 1H detected assignment strategies.
However, in comparison to other nuclei, current 1H spectral resolution is poor, with linewidths of typically a few hundred Hz, even at the fastest MAS rates available today. Here we describe and measure the factors limiting proton linewidths in MAS experiments.
The sources of broadening contributing to the total proton linewidth are either of a homogeneous nature, such as the dipolar couplings, which would be completely removed at infinite MAS rate, or of an inhomogeneous nature. The latter consists mainly of broadening due to anisotropic bulk magnetic susceptibility (ABMS) and due to distributions of chemical shifts due to structural disorder.
In order to understand these different contributions, a set of experiments was used, including sample dilution to identify ABMS, two-dimensional experiments to identify any correlated inhomogeneous broadening, and extensive deuteration to identify 1H-1H dipolar contributions.
By studying several organic solids, the results show that homogeneous contributions can be up to 40 % of the 1H linewidth at 100 kHz MAS. These contributions can be successfully removed with pure isotropic approaches. [1,2]. The remaining linewidth is dominated by inhomogeneous broadening. We find that ABMS broadening is highly sample dependent. Two-dimensional 1H-1H correlation experiments allow to obtain high-resolution even in the presence of ABMS.
These results light the path to new methodology to attain high resolution in 1H NMR in solids.
[1] Moutzouri P.; Simões de Almeida B.; Torodii D.; Emsley L., J. Am. Chem. Soc. 2021, 143, 9834
[2] Cordova M.; Moutzouri P.; Simões de Almeida B.; Torodii D.; Emsley L., Angew. Chem. Int. Ed. 2023