Redetermination of 5-iodouracil

The title compound (systematic name: 2,4-dihydroxy-5-iodopyrimidine), C4H3IN2O2, which was first reported by Sternglanz, Freeman & Bugg [Acta Cryst. (1975 ▶), B31, 1393–1395], has been redetermined, providing a significant increase in the precision of the derived geometric parameters. The asymmetric unit comprises a non-planar molecule in a slightly distorted B25 boat conformation. The molecules are associated in the crystal structure to form ribbons stabilized by N—H⋯O hydrogen bonds which involve NH groups and two carbonyl O atoms.

The title compound (systematic name: 2,4-dihydroxy-5-iodopyrimidine), C 4 H 3 IN 2 O 2 , which was first reported by Sternglanz, Freeman & Bugg [Acta Cryst. (1975), B31, 1393-1395], has been redetermined, providing a significant increase in the precision of the derived geometric parameters. The asymmetric unit comprises a non-planar molecule in a slightly distorted B 25 boat conformation. The molecules are associated in the crystal structure to form ribbons stabilized by N-HÁ Á ÁO hydrogen bonds which involve NH groups and two carbonyl O atoms.
The author thanks MIUR (Rome) for financial support in 2006 of the project 'X-ray diffractometry and spectrometry'.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: TK2237).

Redetermination of 5-iodouracil G. Portalone
Comment 5-iodouracil, 5IUrac, was determined some 30 years ago (Sternglanz et al., 1975). In this study, 591 unique reflections were collected at ambient temperature on an automatic diffractometer, and the heavy-atom method was employed to solve the crystal structure. Only non-H atoms were localized and refined. The final refinement, carried out on a fairly small data set, led to R = 0.044, a data-to-parameter ratio of 7.2, S = 2.52 and standard deviations of 0.018Å in C-C bond lengths and 0.9° in bond angles, As a part of a more general study of multiple-hydrogen-bonding DNA/RNA nucleobases as potential supramolecular reagents (Brunetti et al., 2000(Brunetti et al., , 2002Portalone et al., 1999;Portalone et al., 2002;Portalone & Colapietro, 2007), this paper reports a redetermination of the crystal structure of the title compound, (I), with greater precision and accuracy. The asymmetric unit of (I) comprises a non-planar independent molecule (Fig. 1) in a slightly distorted B 25 boat conformation (Cremer & Pople, 1975). Analysis of the crystal packing of (I), (Fig. 2), shows that the structure is stabilized by two intermolecular N-H···O interactions of descriptor C 1 1 (3) (Etter et al., 1990;Bernstein et al., 1995;Motherwell et al., 1999) (Table 1) between NH moieties and two carbonyl O atoms (O2 i and O1 ii ) [symmetry code: (i) x + 1, y − 1, z; (ii) −x + 1, y + 1/2, −z + 2] which link the molecules into ribbons.

Experimental
The title compound (0.1 mmol, Sigma Aldrich at 98% purity) was dissolved in water (6 ml) and heated under reflux for 1 h. After cooling the solution to ambient temperature, crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of the solvent.

Refinement
The H atoms were included in the riding model approximation with C-H = 0.96 Å and N-H = 0.86-0.88 Å, and with refined isotropic displacement parameters.
Figures Fig. 1. The molecular structure of (I), showing the atom-labelling scheme. Displacements ellipsoids are at the 50% probability level.