5-Iodouracil: structure of a reflection twin.

The crystal structure of 5-iodouracil, C4H3IN2O2, has been determined in the noncentrosymmetric space group P21 on a nonmerohedrally twinned crystal. Both twin components are enantiomorphically pure, but the twin element is a mirror plane perpendicular to c*. The molecular structure is discussed and stacking faults in the two-dimensional packing are proposed as a reason for the twinning.