Flack crystallographic analysis of absolute configuration

• The secrets^* of a CIF file:
• The CIF _chemical_absolute_configuration 'code':
  • syn: Enantiomer assigned by reference to an:
    unchanging chiral centre in the synthetic procedures
  • rm: co-crystallisation with molecule of known absolute configuration
  • ad: Absolute configuration by anomalous dispersion/Flack analysis
  • rmad: rm + ad
  • co: the enantiomer assigned using chiro-optical procedures
  • ts: the enantiomer assigned using transition state simulation
  • unk: an arbitrary choice of enantiomer has been made.
• ~27% (of 19,455) such CIFs declare value for 'code', of which:
  2400=ad, 768=rm, 1007=syn.
• Assignment by syn:
  • is potentially error-prone,
  • May require a lot of literature detective work
  • • ... in the supporting information
• Need a completely independent check on the validity of the syn chain!