• absolute structure;
  • Flack method;
  • Parsons' Q method;
  • Hooft method;
  • anomalous dispersion

The absolute structures of R-(−)-2-methylpiperazine (rmpip), S-(+)-2-methylpiperazine (smpip), R-(−)-2-methylpiperazinediium dibromide (rmpipBr) and S-(+)-2-methylpiperazinediium dibromide (smpipBr) have been determined by anomalous dispersion employing the Parsons' Q and Hooft methods. The studies were undertaken to determine the limitations of the absolute structure determination of light element structures (C, H, N) employing routine single-crystal X-ray diffraction laboratory conditions. The structures of the neutral methylpiperazines were known from a priori non-crystallographic methods and were confirmed by the absolute structure determination of their dibromide salts. By employing the full data collection of Bijvoet pairs and minimizing systematic errors, the absolute structure parameters 0.09 (8) (Hooft) for R-(−)-2-methylpiperazine and 0.05 (8) (Hooft) for S-(+)-2-methylpiperazine were determined.