Determination of absolute configurations of light-atom molecules by means of direct detection of Bijvoet differences

The absolute configurations of two light-atom molecules were determined sufficiently well by direct detection of Bijvoet differences. The compounds examined were (I) beta-cytidine C9H13N3O5 and (II) (S)-3-[(R)-4,4,4-Trifluoro-3-[4-methoxyphenyl]butanoyl]-4-(phenylmethyl)oxazolidin-2-one, C21H20NO4F3. Both compounds crystallize in orthorhombic system with the space group P2(1)2(1)2(1) and Z = 4. The crystal structures were carefully refined by the technique of conventional structure analysis. All possible reflections were measured on a laboratory diffractometer with Cu Kalpha radiation. The multiple-diffraction effect was often observed especially as remarkable intensity enhancement in weak reflections. After such unreliable reflections were eliminated by comparisons among the equivalent mates, data were averaged to a set of Bijvoet pairs. Afterwards, additional measurements by the psi-scan technique were tested. Since psi-scan data showed a slight systematic error probably owing to some shape-effect, an artificial absorption correction DIFABS was adopted to remove the error. Small but significant intensity differences could be detected for many Bijvoet pairs, and the absolute configurations were correctly determined without ambiguity in all cases. The R and wR values for separate refinements of enantiomorphs supported the results with slight differences. Flack parameters indicated no contradictions as well.