Characterizing challenging microcrystalline solids with solid-state NMR shift tensor and synchrotron X-ray powder diffraction data: Structural analysis of ambuic acid

Synchrotron X-ray powder diffraction and solid-state C-13 NMR shift tensor data are combined to provide a unique path to structure in microcrystalline organic solids. Analysis is demonstrated on ambuic acid powder, a widely occurring natural product, to provide the complete crystal structure. The NMR data verify phase purity, specify one molecule per asymmetric unit, and provide an initial structural model including relative stereochemistry and molecular conformation. A refinement of X-ray data from the initial model establishes that ambuic acid crystallizes in the P2(1) space group with unit cell parameters a = 15.5047(7), b = 4.3904(2), and c = 14.1933(4) angstrom and beta = 110.3134(3)degrees. This combined analysis yields structural improvements at two dihedral angles over prior NMR predictions with differences of 103 degrees and 37 degrees found. Only minor differences of +/- 5.5 degrees, on average, are observed at all remaining dihedral angles. Predicted hydroxyl hydrogen-bonding orientations also fit NMR predictions within +/- 6.9 degrees. This refinement corrects chemical shift assignments at two carbons and reduces the NMR error by similar to 16%. This work demonstrates that the combination of long-range order information from synchrotron powder diffraction data together with the accurate shorter range structure given by solid-state NMR measurements is a powerful tool for studying challenging organic solids.