Accuracy of molecular structures determined from high-resolution powder diffraction. The example of m-fluorobenzoic acid

The accuracy with which it is possible to determine atomic coordinates of light atoms at arbitrary positions using X-ray powder data alone is demonstrated. For this purpose use is made of synchrotron powder diffraction data obtained for an organic molecule, m-fluorobenzoic acid, consisting of ten non-hydrogen atoms (C₇H₅FO₂). The results are compared with those of a single-crystal study on the same compound, and in particular the differences in bond lengths and bond angles are studied. The structure solution produced by direct methods is compared with the final result after the Rietveld refinement. The effect of performing the data collection at room temperature and at 100 K is also investigated. The bond lengths extracted from the powder data were found to differ from the single-crystal values by an average of 0.025 Å when data collected at room temperature were employed, and by 0.021 Å when using 100 K data. For the bond angles the corresponding values were 3.5 and 1.5°, respectively. With the low-temperature data it was possible to refine all atoms, including the hydrogen atoms, without restraints, whereas for the data collected at room temperature, strong restraints had to be used for the hydrogen atoms.