Low-Temperature Crystal Structure of S-camphor Solved from Powder Synchrotron X-ray Diffraction Data by Simulated Annealing

The ordered, low-temperature crystal structure of the pure enantiomer of camphor (C10H16O) has been solved from highresolution powder synchrotron X-ray diffraction data. The structure is orthorhombic, space group P212121, Z=8, with a= 8.9277(2) ?, b=27.0359(5) ?, and c=7.3814(1) ? at 100 K. The structure was solved by autoindexing of the pattern, space group determination, and then optimization of the positions and orientations of the two independent molecules in the unit cell by simulated annealing. The molecular structure obtained from the restrained Rietveld refinement shows reasonable agreement with that optimized from ab initio molecular orbital calculations. In the crystal structure, the molecules are aligned antiferroelectrically and weak C-H...O hydrogen bonds link together the independent molecules.