Imidazolidin-2-one: pseudosymmetry and twinning.

The title compound, C3H6N2O, crystallizes with imposed twofold symmetry in the space group I4(1)/a. The five-membered ring displays a half-chair conformation. N-H···O hydrogen bonds connect the molecules to form R2(2)(8) rings and thence ribbons parallel to the a and b axes. These intersect via O2H2 rings involving longer H···O contacts. The crystal was merohedrally twinned. Preliminary indications of the higher symmetry space group I4(1)/amd, which would require the ring to be planar, proved to be incorrect. A previous brief report of the structure in Fdd2 is also probably incorrect.