Campus Units

Chemistry, Ames Laboratory

Document Type

Article

Publication Version

Accepted Manuscript

Publication Date

2017

Journal or Book Title

Physical Chemistry Chemical Physics

Volume

19

Issue

41

First Page

28153

Last Page

28162

DOI

10.1039/C7CP04223J

Abstract

We demonstrate that natural isotopic abundance 2D heteronuclear correlation (HETCOR) solid-state NMR spectra can be used to significantly reduce or eliminate the broadening of 1H and 13C solid-state NMR spectra of organic solids due to anisotropic bulk magnetic susceptibility (ABMS). ABMS often manifests in solids with aromatic groups, such as active pharmaceutical ingredients (APIs), and inhomogeneously broadens the NMR peaks of all nuclei in the sample. Inhomogeneous peaks with full widths at half maximum (FWHM) of ∼1 ppm typically result from ABMS broadening and the low spectral resolution impedes the analysis of solid-state NMR spectra. ABMS broadening of solid-state NMR spectra has previously been eliminated using 2D multiple-quantum correlation experiments, or by performing NMR experiments on diluted materials or single crystals. However, these experiments are often infeasible due to their poor sensitivity and/or provide limited gains in resolution. 2D 1H–13C HETCOR experiments have previously been applied to reduce susceptibility broadening in paramagnetic solids and we show that this strategy can significantly reduce ABMS broadening in diamagnetic organic solids. Comparisons of 1D solid-state NMR spectra and 1H and 13C solid-state NMR spectra obtained from 2D 1H–13C HETCOR NMR spectra show that the HETCOR spectrum directly increases resolution by a factor of 1.5 to 8. The direct gain in resolution is determined by the ratio of the inhomogeneous 13C/1H linewidth to the homogeneous 1H linewidth, with the former depending on the magnitude of the ABMS broadening and the strength of the applied field and the latter on the efficiency of homonuclear decoupling. The direct gains in resolution obtained using the 2D HETCOR experiments are better than that obtained by dilution. For solids with long proton longitudinal relaxation times, dynamic nuclear polarization (DNP) was applied to enhance sensitivity and enable the acquisition of 2D 1H–13C HETCOR NMR spectra. 2D 1H–13C HETCOR experiments were applied to resolve and partially assign the NMR signals of the form I and form II polymorphs of aspirin in a sample containing both forms. These findings have important implications for ultra-high field NMR experiments, optimization of decoupling schemes and assessment of the fundamental limits on the resolution of solid-state NMR spectra.

Comments

This is a manuscript of an article published as Hanrahan, Mike Patrick, Amrit Venkatesh, Scott L. Carnahan, Julie L. Calahan, Joseph W. Lubach, Eric J. Munson, and Aaron J. Rossini. "Enhancing the Resolution of 1H and 13C Solid-State NMR Spectra by Reduction of Anisotropic Bulk Magnetic Susceptibility Broadening." Physical Chemistry Chemical Physics (2017). 10.1039/C7CP04223J . Posted with permission.

Copyright Owner

The Owner Societies

Language

en

File Format

application/pdf

Available for download on Tuesday, September 25, 2018

Published Version

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