6-Methyl-uracil: a redetermination of polymorph (II)

摘要

6-Methyluracil, C 5 H 6 N 2 O 2 , exists in two crystalline phases: form (I), monoclinic, space group P 2 1 / c [Reck et al. (1988 ). Acta Cryst. A 44 , 417–421] and form (II), monoclinic, space group C 2/ c [Leonidov et al. (1993 ). Russ. J. Phys. Chem. 67 , 2220–2223]. The structure of polymorph (II) has been redetermined providing a significant increase in the precision of the derived geometric parameters. In the crystal, mol-ecules form ribbons approximately running parallel to the c -axis direction through N—H?O hydrogen bonds. The radical differences observed between the crystal packing of the two polymorphs may be responsible in form (II) for an increase in the contribution of the polar canonical forms C—(O ? )=N—H + relative to the neutral canonical form C(=O)—N—H induced by hydrogen-bonding inter-actions.