J. Am. Chem. Soc., ASAP Article 10.1021/ja055570j S0002-7863(05)05570-8
Web Release Date: January 12, 2006
Characterizing Challenging Microcrystalline Solids with Solid-State NMR Shift Tensor and Synchrotron X-ray Powder Diffraction Data: Structural Analysis of Ambuic Acid
James K. Harper, David M. Grant,* Yuegang Zhang, Peter L. Lee, and Robert Von Dreele
Contribution from the Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112, and Argonne National Laboratory, Advanced Photon Source, Argonne, Illinois 60439
Synchrotron X-ray powder diffraction and solid-state 13C 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 P21 space group with unit cell parameters a = 15.5047(7), b = 4.3904(2), and c = 14.1933(4) Å and = 110.3134(3). This combined analysis yields structural improvements at two dihedral angles over prior NMR predictions with differences of 103 and 37 found. Only minor differences of ±5.5, on average, are observed at all remaining dihedral angles. Predicted hydroxyl hydrogen-bonding orientations also fit NMR predictions within ±6.9. This refinement corrects chemical shift assignments at two carbons and reduces the NMR error by ~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.