Redetermination of 5,5-dihydroxybarbituric acid trihydrate (alloxan tetrahydrate)

The crystal structure of 5,5-dihydroxybarbituric acid trihydrate, (I) (originally misnamed alloxan tetrahydrate) was determined by Mootz & Jeffrey (1965). In that original roomtemperature study, only three of the six H atoms in the asymmetric unit could be located by means of difference Fourier syntheses, and the structure re®ned to a ®nal R value of 0.097. We have redetermined this crystal structure at 150 K, with a ®nal R value of 0.028, to gain more accurate data for our theoretical modelling studies. The low-temperature redetermination located all the H atoms, which were re®ned isotropically. The precision of the unit-cell dimensions was improved by an order of magnitude. The unit-cell volume decreased by ca. 27 AÊ , consistent with the determination at low temperature. In general, the molecular geometric parameters are not signi®cantly different, the exception being the C6ÐO6 bond length, which is shorter in the lowtemperature structure, while C2ÐO2 is actually longer in the low-temperature structure, both by ca. 0.1 AÊ .


Comment
The crystal structure of 5,5-dihydroxybarbituric acid trihydrate, (I) (originally misnamed alloxan tetrahydrate) was determined by Mootz & Jeffrey (1965). In that original roomtemperature study, only three of the six H atoms in the asymmetric unit could be located by means of difference Fourier syntheses, and the structure re®ned to a ®nal R value of 0.097. We have redetermined this crystal structure at 150 K, with a ®nal R value of 0.028, to gain more accurate data for our theoretical modelling studies. The low-temperature redetermination located all the H atoms, which were re®ned isotropically. The precision of the unit-cell dimensions was improved by an order of magnitude. The unit-cell volume decreased by ca. 27 A Ê 3 , consistent with the determination at low temperature. In general, the molecular geometric parameters are not signi®cantly different, the exception being the C6ÐO6 bond length, which is shorter in the lowtemperature structure, while C2ÐO2 is actually longer in the low-temperature structure, both by ca. 0.1 A Ê .
Compound (I) crystallizes in the monoclinic space group C2/m, with the organic molecule on a mirror plane plus one water molecule in a general position and a second on a mirror plane (Fig. 1). The crystallographic plane is normal to the pyrimidine ring, passing through atoms O2, C2, C5, O7 and O8. The hydroxyl H atoms attached to the ring are disordered across the mirror plane. The water molecule O2W, in a general position, is disordered with one of the H atoms re®ned over two positions. The third water of solvation lies on the mirror plane at (0, y, 1), with the mirror bisecting the HÐOÐH angle.
The CÐN bond lengths in the ring range from 1.3666 (11) to 1.3752 (9) A Ê , and the C5ÐC6 bond length is 1.5272 (10) A Ê . The packing (Fig. 2) consists of centrosymmetric dimers hydrogen-bonded to form a chain, with the water molecules lying between these chains, forming a buckled sheet structure. The water molecules in general positions form OÐHÁ Á ÁO bonds to the organic hydroxyl groups, whilst the water molecules on the mirror plane bond to the unique hydrogen-bond carbonyl acceptor on the organic molecules (Fig. 3). The water molecules on the mirror planes and in general positions also hydrogen-bond to each other in the sheet. The DÁ Á ÁA distance within the bonded chains of molecules is 2.8580 (6) A Ê , whilst the OÐHÁ Á ÁO hydrogen bonds range from 2.7217 (11) to 2.9343 (9) A Ê . All potential donors and acceptors are used in the hydrogen bonding.

Experimental
To complement the results from an experimental polymorph search on alloxan, 5,5-dihydroxybarbituric acid trihydrate was obtained from Aldrich as colourless plate-like crystals of alloxan tetrahydrate (Fig. 4). These crystals were very sensitive and decompose rapidly in air. The crystal packing of 5,5-dihydroxybarbituric acid trihydrate, showing the NÐHÁ Á ÁO and OÐHÁ Á ÁO hydrogen-bonding interactions as dashed lines.

Figure 1
View of the 5,5-dihydroxybarbituric acid molecule and three water molecules (twice the asymmetric unit), showing the atom labelling scheme. Displacement ellipsoids are drawn at the 50% probability level.
The non-H atoms were re®ned freely with anisotropic displacement parameters, with the H atoms re®ned independently with an isotropic model. Data collection: SMART (Bruker, 2000); cell re®nement: SAINT (Bruker, 2000); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to re®ne structure: SHELXL97 (Sheldrick, 1997); molecular graphics: MERCURY (Bruno et al., 2002) and SHELXTL (Bruker, 2000); software used to prepare material for publication: SHELXL97. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.