Characterizing challenging microcrystalline solids with solid-state NMR shift tensor and synchrotron X-ray powder...

Area of Science:

• Crystallography
• Solid-state chemistry
• Organic chemistry

Background:

• Accurate crystal structure determination is crucial for understanding organic solids.
• Microcrystalline organic solids present challenges for traditional structure determination methods.
• Solid-state NMR and X-ray diffraction offer complementary structural information.

Purpose of the Study:

• To develop and demonstrate a combined approach using synchrotron X-ray powder diffraction and solid-state NMR for crystal structure determination.
• To elucidate the complete crystal structure of ambuic acid, a natural product.
• To improve the accuracy of structural parameters compared to NMR predictions alone.

Main Methods:

• Synchrotron X-ray powder diffraction (SXRPD) was used to obtain long-range order data.
• Solid-state Carbon-13 (13C) NMR shift tensor data provided short-range structural information, including molecular conformation and relative stereochemistry.
• An initial structural model from NMR data was used to refine SXRPD data.

Main Results:

• The complete crystal structure of ambuic acid was determined, crystallizing in the P2(1) space group.
• The combined method refined dihedral angles with improved accuracy compared to prior NMR predictions.
• Hydroxyl hydrogen-bonding orientations were accurately predicted, and chemical shift assignments were corrected, reducing NMR error by ~16%.

Conclusions:

• The integration of synchrotron X-ray powder diffraction and solid-state NMR is a powerful and accurate method for studying microcrystalline organic solids.
• This combined approach overcomes limitations of individual techniques, providing enhanced structural resolution.
• The refined structure of ambuic acid demonstrates the efficacy of this synergistic methodology.