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Crystals of a second polymorph of violuric acid monohydrate [systematic name: pyrimidine-2,4,5,6(1H,3H)-tetrone monohydrate], C4H3N3O4·H2O, have higher density and a more extensive hydrogen-bonding arrangement than the previously reported polymorph. Violuric acid and water mol­ecules form essentially planar hydrogen-bonded sheets, which are stacked in an offset ...ABCABC... repeat pattern involving no ring-stacking inter­actions.

Supporting information


Crystallographic Information File (CIF)
Contains datablocks global, I


Structure factor file (CIF format)
Contains datablock I

CCDC reference: 652507

Comment top

For some time now, an area of our reseach has concentrated on the synthesis and crystallographic characterization of s-block metal complexes of anions of barbituric acid, violuric acid and other related ligands because of the pharmaceutical importance of their derivatives. Barbituric acid is also known for its ability to form polymorphs and has thus been used as a model compound for developing computational polymorph prediction techniques (Lewis et al., 2004). Violuric acid is a 5-substituted derivative of barbituric acid, with a CN—OH substituent allowing extra coordination and hydrogen-bonding possibilities compared with barbituric acid.

The structure of violuric acid monohydrate, (I), was first determined in 1964, by both X-ray diffraction (Craven & Mascarenhas, 1964) and neutron diffraction (Craven & Takei, 1964) at room temperature, in order to determine the tautomeric form, the detailed molecular structure and the nature of the hydrogen bonding. The neutron diffraction study permitted the precise determination of the D atom positions. For these two studies, the space group was suggested as orthorhombic Cmc21, with all the atoms lying on crystallographic mirror planes and an interlayer separation of 3.11 Å. Both papers reported unsatisfactory features of the structure refinement and suggested either unresolved static or dynamic disorder, together with uncertainty about the true space group.

A definitive redetermination at 150 K (Nichol & Clegg, 2005) confirmed that the correct space group was Cmc21 and that the structure, at this lower temperature, is fully ordered and gives entirely satisfactory results for the geometry and displacement parameters; there appeared to be no phase transition between room temperature and 150 K. In this first polymorph (referred to here as polymorph A), the violuric acid molecule is exactly planar. Violuric acid and water molecules form planar hydrogen-bonded sheets. The crystal packing consists of stacked sheets separated by 3.0377 (6) Å (half the a-axis length) and there is no ring stacking between adjacent sheets. All the O atoms of the carbonyl groups act as acceptor atoms for hydrogen bonds; this is uncommon for barbituric derivatives, where one carbonyl group is usually not involved in hydrogen bonding (Lewis et al., 2005). An R22(8) motif (Bernstein et al., 1995) links the violuric acid rings together, and the water molecule is hydrogen bonded to the third carbonyl group and to the O atom of the isonitroso group. One of the H atoms of the water molecule thus acts as a birfurcated donor.

We have now obtained a second polymorph (B) of violuric acid monohydrate. The asymmetric unit is shown in Fig. 1 and consists of one molecule of violuric acid and one water molecule, both lying in general positions with no imposed crystallographic symmetry. Nevertheless, the violuric acid molecule is essentially planar, with an r.m.s. deviation of 0.021 Å, and the O atom of the water molecule lies only 0.065 Å from this mean plane. The water H atoms also lie approximately in this plane, enabling the formation of a hydrogen-bonded sheet, which is parallel to (103); adjacent sheets are separated by an average of 3.08 Å (Fig.2), slightly more than in the orthorhombic polymorph (A). The stacking sequence of sheets here is an offset ABCABC···, whereas it is ABAB··· in polymorph A. In neither case are there any ring stacking, water molecules or NOH groups lying over the rings of adjacent layers in polymorph B. please clarify

Fig. 3 shows the hydrogen bonding within a sheet, and the details of the hydrogen bonds are given in Table 1. There are both similarities and significant differences between this and the arrangement in polymorph A. In both structures, the water molecule acts as a donor to a carbonyl group through one H atom, and as a bifurcated donor to both a carbonyl and the isonitroso O atom of a second violuric acid molecule. The water O atom accepts a hydrogen bond from an NH group in polymorph B, but from the NOH group in polymorph A; in B, the NOH group donates to two carbonyl O atoms in two different molecules. The second NH group in B serves as a bifurcated donor, to a carbonyl group and the N atom of the NOH group of a single violuric acid molecule, whereas both NH groups in A act as simple donors to carbonyl O atoms. In A, two carbonyl groups of each violuric acid molecule accept hydrogen bonds only from water molecules (one each), while the third accepts two hydrogen bonds (from two NH groups); in B, one carbonyl group is a single acceptor (from water), one accepts hydrogen bonds from water and from NOH, and the other accepts from NH and NOH. Thus the water molecule shows the same degree of hydrogen bonding in both polymorphs (three as donor and one as acceptor), but the violuric acid molecule forms more hydrogen bonds in B (five as donor and seven as acceptor) than in A (three as donor and five as acceptor); only in B does the N atom of the NOH group accept a hydrogen bond. Comparison of the two structures shows that some potential hydrogen-bonding interactions in polymorph A do not occur because the donor–acceptor distances are too great (Desiraju & Sharma, 1996; Taylor et al., 1984a,b). The calculated density of polymorph B (1.788 Mg m-3) is a little higher than that of polymorph A (1.773 Mg m-3), both of these values being rather high for an organic compound as a result of the extensive hydrogen bonding.

The specific individual ring motifs generated by the hydrogen bonding in polymorph B, with their binary graph-set notation (Bernstein et al., 1995), are as follows: (i) the bifurcated hydrogen bonding of N1/H1 to O1i and N3i generates an R21(5) motif involving two violuric acid molecules; (ii) one branch of this bifurcated hydrogen bond, N1—H1···N3i, together with O4i—H4i···O1 (symmetry-equivalent to O4—H4···O1iii in Table 1) generates an R22(7) motif between the same pair of molecules; the other branch, N1—H1···O1i, is symmetry-equivalent to N1iv—H1iv···O1iii, which generates an R32(6) motif with O4—H4···O1iii and O4—H4···O2iv, linking three violuric acid molecules; (iii) the bifurcated hydrogen bonding of O5/H5A to O3 and O4 generates an R21(6) motif involving one violuric acid and one water molecule; (iv) O5—H5A···O4, O5—H5B···O2iv and O4—H4···O2iv generate an R23(6) motif linking the water molecule to two violuric acid molecules; (v) O5—H5A···O3, N2—H2···O5ii and their inversion-related equivalents O5ii—H5Aii···O3ii and N2ii—H2ii···O5 generate a centrosymmetric R44(12) motif; (vi) finally, N2—H2···O5ii and O5v—H5B···O2 (equivalent to O5—H5B···O2iv), together with their inversion-related equivalents N2vi—H2vi···O5v and O5ii—H5Bii···O2vi [symmetry codes (v) x, 1 + y, z; (vi) 2 - x, 2 - y, 1 - z], generate another centrosymmetric R44(12) motif, with both of these larger ring motifs containing two water and two violuric acid molecules. Hydrogen bonds formed exclusively by violuric acid molecules link these molecules into ribbons along the b axis, and the larger ring motifs involving water molecules provide lateral connections between the ribbons, forming the overall sheet structure shown in Fig. 3.

Bond lengths and angles in the two polymorphs are essentially the same.

Related literature top

For related literature, see: Bernstein et al. (1995); Craven & Mascarenhas (1964); Craven & Takei (1964); Desiraju & Sharma (1996); Lewis et al. (2004, 2005); Nichol & Clegg (2005); Taylor et al. (1984a, 1984b).

Experimental top

Small colourless block crystals were obtained from an aqueous solution prepared by adding one equivalent of barbituric acid to three equivalents of violuric acid and one equivalent of magnesium sulfate in water, in an attempt to prepare a complex containing both ligands. The solution was stirred and boiled to reduce its volume to 40% of the original, and allowed to cool slowly to room temperature. Crystals appeared after approximately three weeks.

Refinement top

All H atoms were located in a difference Fourier map and were freely refined.

Computing details top

Data collection: COLLECT (Nonius, 1998); cell refinement: EVALCCD (Duisenberg et al., 2003); data reduction: EVALCCD; program(s) used to solve structure: SHELXTL (Sheldrick, 2001); program(s) used to refine structure: SHELXTL; molecular graphics: DIAMOND (Brandenburg & Putz, 2004); software used to prepare material for publication: SHELXTL and local programs.

Figures top
[Figure 1] Fig. 1. The asymmetric unit of (I), with 50% probability displacement ellipsoids.
[Figure 2] Fig. 2. A parallel projection, along the b axis, of the packing of (I).
[Figure 3] Fig. 3. The hydrogen bonding (dashed lines) in a single sheet.
pyrimidine-2,4,5,6(1H,3H)-tetrone monohydrate top
Crystal data top
C4H3N3O4·H2OF(000) = 360
Mr = 175.11Dx = 1.788 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
a = 7.862 (5) ÅCell parameters from 74 reflections
b = 8.869 (5) Åθ = 2.0–27.5°
c = 9.479 (5) ŵ = 0.17 mm1
β = 100.25 (3)°T = 150 K
V = 650.4 (6) Å3Block, colourless
Z = 40.30 × 0.20 × 0.10 mm
Data collection top
Nonius KappaCCD
1266 independent reflections
Radiation source: sealed tube864 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.060
ω scansθmax = 26.0°, θmin = 4.4°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2006)
h = 99
Tmin = 0.960, Tmax = 0.985k = 1010
7836 measured reflectionsl = 1111
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.045Hydrogen site location: difference Fourier map
wR(F2) = 0.118All H-atom parameters refined
S = 1.05 w = 1/[σ2(Fo2) + (0.0612P)2 + 0.0912P]
where P = (Fo2 + 2Fc2)/3
1266 reflections(Δ/σ)max < 0.001
129 parametersΔρmax = 0.21 e Å3
0 restraintsΔρmin = 0.27 e Å3
Crystal data top
C4H3N3O4·H2OV = 650.4 (6) Å3
Mr = 175.11Z = 4
Monoclinic, P21/nMo Kα radiation
a = 7.862 (5) ŵ = 0.17 mm1
b = 8.869 (5) ÅT = 150 K
c = 9.479 (5) Å0.30 × 0.20 × 0.10 mm
β = 100.25 (3)°
Data collection top
Nonius KappaCCD
1266 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2006)
864 reflections with I > 2σ(I)
Tmin = 0.960, Tmax = 0.985Rint = 0.060
7836 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0450 restraints
wR(F2) = 0.118All H-atom parameters refined
S = 1.05Δρmax = 0.21 e Å3
1266 reflectionsΔρmin = 0.27 e Å3
129 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
O10.22673 (19)0.64592 (17)0.2348 (2)0.0456 (5)
O20.6336 (2)1.00545 (16)0.38623 (19)0.0476 (5)
O30.8092 (2)0.52118 (18)0.4385 (2)0.0565 (6)
O40.5621 (2)0.31557 (16)0.34906 (19)0.0437 (5)
H40.490 (4)0.229 (4)0.324 (3)0.086 (10)*
O50.9314 (2)0.2017 (2)0.4654 (2)0.0518 (6)
H5B0.847 (5)0.126 (4)0.455 (3)0.089 (11)*
H5A0.887 (4)0.287 (4)0.466 (3)0.061 (10)*
N10.4277 (2)0.82721 (19)0.3123 (2)0.0341 (5)
H10.349 (3)0.899 (3)0.287 (2)0.041 (7)*
N20.7176 (3)0.7616 (2)0.4070 (2)0.0359 (5)
H20.819 (4)0.790 (3)0.438 (3)0.046 (7)*
N30.4498 (2)0.43043 (19)0.3098 (2)0.0357 (5)
C10.3758 (3)0.6813 (2)0.2880 (3)0.0337 (6)
C20.5970 (3)0.8733 (2)0.3699 (2)0.0341 (6)
C30.6921 (3)0.6092 (2)0.3965 (3)0.0347 (6)
C40.5113 (3)0.5643 (2)0.3320 (2)0.0297 (6)
Atomic displacement parameters (Å2) top
O10.0262 (8)0.0295 (8)0.0745 (13)0.0014 (7)0.0085 (8)0.0023 (8)
O20.0369 (9)0.0239 (9)0.0777 (13)0.0049 (7)0.0019 (8)0.0031 (8)
O30.0312 (9)0.0322 (10)0.0980 (15)0.0042 (7)0.0110 (9)0.0009 (9)
O40.0320 (9)0.0216 (8)0.0738 (13)0.0026 (7)0.0008 (8)0.0002 (8)
O50.0285 (9)0.0382 (11)0.0849 (15)0.0002 (8)0.0005 (9)0.0002 (10)
N10.0260 (9)0.0200 (10)0.0527 (13)0.0009 (8)0.0026 (9)0.0018 (9)
N20.0228 (10)0.0272 (10)0.0543 (14)0.0019 (8)0.0025 (9)0.0012 (9)
N30.0282 (10)0.0257 (10)0.0512 (13)0.0018 (8)0.0017 (8)0.0004 (9)
C10.0275 (11)0.0276 (12)0.0447 (15)0.0003 (9)0.0030 (10)0.0002 (10)
C20.0280 (11)0.0262 (12)0.0463 (15)0.0040 (10)0.0016 (10)0.0020 (11)
C30.0262 (11)0.0264 (12)0.0498 (15)0.0016 (9)0.0024 (10)0.0012 (10)
C40.0253 (11)0.0237 (12)0.0395 (14)0.0007 (9)0.0038 (10)0.0001 (10)
Geometric parameters (Å, º) top
O1—C11.232 (3)N1—C11.364 (3)
O2—C21.210 (2)N1—C21.407 (3)
O3—C31.219 (3)N2—H20.83 (3)
O4—H40.96 (3)N2—C21.373 (3)
O4—N31.356 (2)N2—C31.367 (3)
O5—H5B0.93 (4)N3—C41.285 (3)
O5—H5A0.83 (3)C1—C41.492 (3)
N1—H10.89 (3)C3—C41.498 (3)
H4—O4—N3102 (2)N1—C1—C4115.8 (2)
H5B—O5—H5A111 (3)O2—C2—N1121.19 (19)
H1—N1—C1117.4 (15)O2—C2—N2121.9 (2)
H1—N1—C2117.5 (15)N1—C2—N2116.91 (19)
C1—N1—C2125.08 (19)O3—C3—N2121.2 (2)
H2—N2—C2116.4 (18)O3—C3—C4124.7 (2)
H2—N2—C3116.0 (18)N2—C3—C4114.12 (18)
C2—N2—C3127.5 (2)N3—C4—C1111.55 (19)
O4—N3—C4116.19 (18)N3—C4—C3127.9 (2)
O1—C1—N1123.0 (2)C1—C4—C3120.50 (18)
O1—C1—C4121.17 (19)
C2—N1—C1—O1178.5 (2)O4—N3—C4—C30.2 (3)
C2—N1—C1—C42.3 (3)O1—C1—C4—N31.4 (3)
C3—N2—C2—O2178.6 (2)O1—C1—C4—C3179.9 (2)
C3—N2—C2—N11.0 (3)N1—C1—C4—N3177.77 (19)
C1—N1—C2—O2178.8 (2)N1—C1—C4—C30.8 (3)
C1—N1—C2—N21.5 (3)O3—C3—C4—N30.3 (4)
C2—N2—C3—O3177.1 (2)O3—C3—C4—C1178.1 (2)
C2—N2—C3—C42.2 (3)N2—C3—C4—N3179.6 (2)
O4—N3—C4—C1178.69 (18)N2—C3—C4—C11.2 (3)
Hydrogen-bond geometry (Å, º) top
N1—H1···O1i0.89 (3)2.27 (3)3.077 (3)151 (2)
N1—H1···N3i0.89 (3)2.38 (2)3.124 (3)141 (2)
N2—H2···O5ii0.83 (3)2.02 (3)2.828 (3)164 (2)
O4—H4···O1iii0.96 (3)1.85 (3)2.722 (2)150 (3)
O4—H4···O2iv0.96 (3)2.30 (3)2.817 (2)113 (2)
O5—H5A···O30.83 (3)2.17 (3)2.988 (3)168 (3)
O5—H5A···O40.83 (3)2.61 (3)3.087 (3)118 (2)
O5—H5B···O2iv0.93 (4)2.00 (4)2.907 (3)163 (3)
Symmetry codes: (i) x+1/2, y+1/2, z+1/2; (ii) x+2, y+1, z+1; (iii) x+1/2, y1/2, z+1/2; (iv) x, y1, z.

Experimental details

Crystal data
Chemical formulaC4H3N3O4·H2O
Crystal system, space groupMonoclinic, P21/n
Temperature (K)150
a, b, c (Å)7.862 (5), 8.869 (5), 9.479 (5)
β (°) 100.25 (3)
V3)650.4 (6)
Radiation typeMo Kα
µ (mm1)0.17
Crystal size (mm)0.30 × 0.20 × 0.10
Data collection
DiffractometerNonius KappaCCD
Absorption correctionMulti-scan
(SADABS; Sheldrick, 2006)
Tmin, Tmax0.960, 0.985
No. of measured, independent and
observed [I > 2σ(I)] reflections
7836, 1266, 864
(sin θ/λ)max1)0.617
R[F2 > 2σ(F2)], wR(F2), S 0.045, 0.118, 1.05
No. of reflections1266
No. of parameters129
H-atom treatmentAll H-atom parameters refined
Δρmax, Δρmin (e Å3)0.21, 0.27

Computer programs: COLLECT (Nonius, 1998), EVALCCD (Duisenberg et al., 2003), EVALCCD, SHELXTL (Sheldrick, 2001), DIAMOND (Brandenburg & Putz, 2004), SHELXTL and local programs.

Hydrogen-bond geometry (Å, º) top
N1—H1···O1i0.89 (3)2.27 (3)3.077 (3)151 (2)
N1—H1···N3i0.89 (3)2.38 (2)3.124 (3)141 (2)
N2—H2···O5ii0.83 (3)2.02 (3)2.828 (3)164 (2)
O4—H4···O1iii0.96 (3)1.85 (3)2.722 (2)150 (3)
O4—H4···O2iv0.96 (3)2.30 (3)2.817 (2)113 (2)
O5—H5A···O30.83 (3)2.17 (3)2.988 (3)168 (3)
O5—H5A···O40.83 (3)2.61 (3)3.087 (3)118 (2)
O5—H5B···O2iv0.93 (4)2.00 (4)2.907 (3)163 (3)
Symmetry codes: (i) x+1/2, y+1/2, z+1/2; (ii) x+2, y+1, z+1; (iii) x+1/2, y1/2, z+1/2; (iv) x, y1, z.

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