Absolute configuration determination by anomalous scattering of light atoms. Have the limits been reached?

The absolute conformation determination of the achiral olefin 4,4'-dimethylchalcone (DMC), C₁₇H₁₆O, crystallizing in a chiral space group (P2₁2₁2₁), was accomplished from the anomalous scattering of oxygen (Cu Kα) despite the small O:C ratio (0.059). The symmetry-equivalent intensities of seven enantiomer-sensitive reflections of two crystal specimens were measured with graphite-monochromated Cu Kα radiation on a Picker diffractometer. In addition, an extensive data set of one of the specimens was collected on a CAD4 diffractometer with Ni-filtered Cu Kα radiation. Bijvoet-difference analysis established that the two specimens were enantiomers. The probability that the absolute conformation has been wrongly determined is <0.1% in all three measurements. The results of this study established, for the first time, the correlation between the absolute conformation, in the crystal, of an achiral optically inactive molecule and the absolute configuration and optical rotation of one of the two possible enantiomers of its dibromide adduct obtained by gas/solid bromination reaction. Statistical analysis of the CAD4 data shows that the use of Friedel pairs tends to minimize the effects of systematic errors and yields a higher statistical confidence in the results.