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organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

Journal logoSTRUCTURAL
CHEMISTRY
ISSN: 2053-2296
Volume 62| Part 7| July 2006| Pages o412-o415
doi:10.1107/S0108270106019688

5-Fluoro­uracil and thymine form a crystalline solid solution

CROSSMARK_Color_square_no_text.svg
Sarah A. Barnett,a Ashley T. Hulmea* and Derek A. Tochera

aChristopher Ingold Laboratory, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, England
*Correspondence e-mail: a.hulme@ucl.ac.uk

(Received 24 March 2006; accepted 25 May 2006; online 23 June 2006)

The crystal structure of a 5-fluoro­uracil–thymine [5-fluoro­pyrimidine-2,4(1H,3H)-dione–5-methyl­pyrimidine-2,4(1H,3H)-dione, C4H3FN2O2·C5H6N2O2] solid solution has been determined. Both of the crystallographically independent sites can accommodate either 5-fluoro­uracil or thymine mol­ecules, leading to occupational disorder [C5−xH6–3xFxN2O2·C5−yH6−3xFyN2O2, with x = 0.52 and y = 0.7 for determination (I), x = 0.55 and y = 0.69 for (II), and x = 0.67 and y = 0.76 for (III)]. The 5-fluoro­uracil–thymine ratio in the crystal structure is influenced by the 5-fluoro­uracil–thymine ratio in the crystallization solution, though it does not exactly mirror it. The crystal structure comprises inter­penetrating hydrogen-bonded nets, containing four independent hydrogen bonds.

Comment

F atoms and methyl groups have been identified as being capable of replacing one another in a mol­ecule to produce isomorphic crystal structures because of their similar size, shape and van der Waals inter­actions (Kuhnert-Brandstatter, 1982[Kuhnert-Brandstatter, M. (1982). Thermomicroscopy of Organic Compounds in Comprehensive Analytical Chemistry, Vol. XVI, edited by G. Svehla, pp. 329-491. Amsterdam: Elsevier Scientific Publishing.]). Attempts were made to exploit this inter­changeability as part of an ongoing study into the crystalline solid state of 5-fluoro­uracil (Hulme et al., 2005[Hulme, A. T., Price, S. L. & Tocher, D. A. (2005). J. Am. Chem. Soc. 127, 1116-1117.]; Hamad et al., 2006[Hamad, S., Moon, C., Catlow, C. R. A., Hulme, A. T. & Price, S. L. (2006). J. Phys. Chem. B, 110, 3323-3329.]), with the aim of growing 5-fluoro­uracil crystals isostructural with thymine (Portalone et al., 1999[Portalone, G., Bencivenni, L., Colapietro, M., Pieretti, A. & Ramondo, F. (1999). Acta Chem. Scand. 53, 57.]) or thymine crystals isostructural with 5-fluoro­uracil form 1 (Fallon, 1973[Fallon, L. III (1973). Acta Cryst. B29, 2549-2556.]). Instead of producing such isostructural cocrystals, an entirely new structure was discovered, grown from solution in 2,2,2-trifluoro­ethanol, containing 5-fluoro­uracil and thymine in a solid solution.

[Scheme 1]

A cocrystal can be defined as a crystal structure containing two (or more) mol­ecular species on separate crystallographic sites with a fixed stoichiometric ratio in the crystal structure, in contrast with a solid solution, which exhibits `a homogeneous crystalline phase in which some of the constituent mol­ecules are substituted by foreign mol­ecules that possess sufficient similarity that the lattice dimensions are changed only slightly' (Datta & Grant, 2004[Datta, S. & Grant, D. J. W. (2004). Nat. Rev. 3, 42-57.]). In the structure reported here, both of the crystallographically independent sites (Fig. 1[link]) can be occupied by either 5-fluoro­uracil or thymine mol­ecules, giving non-integer occupancies for both mol­ecules at each site and leading to the description of this structure as a solid solution rather than a cocrystal.

The structure adopts the monoclinic space group C2/c. The crystal structure, denoted (I)[link], of a crystal grown from a 1:1 solution of 5-fluoro­uracil and thymine, with the structure determined at 150 K, is reported; two further structure determinations are reported to exemplify the features of this system. (II)[link] denotes the crystal structure determination of a crystal grown from a 1:1 solution at 298 K and (III)[link] denotes the crystal structure determination of a crystal grown from a 2:1 solution of 5-fluoro­uracil and thymine at 150 K. Structure (I)[link] will be used exclusively for the purposes of the discussion of the crystal structure, with the other two determinations used to highlight features of the solid solution structure.

The only difference between 5-fluoro­uracil and thymine is the substituent bonded to the 5-position in the mol­ecular structure, and hence the only sign of the occupational disorder is the F:Me ratio at the 9- and 19-positions in the crystal structure. Both (I)[link] and (II)[link] were grown from 1:1 5-fluoro­uracil/thymine crystallization solutions and have similar F:Me ratios at the 9- and 19-postions [for (I)[link], 0.52 (1):0.48 (1) for the 9-position and 0.70 (1):0.30 (1) for the 19-position; for (II)[link], 0.55 (1):0.45 (1) for the 9-position and 0.69 (2):0.31 (2) for the 19-position]. This result indicates that the 5-fluoro­uracil/thymine ratio in the crystals is not simply a statistical distribution throughout the crystal but depends on the ratio in the crystallization solution. This fact is exemplified by the distinct preference for incorporating F at the 19-position, even though the original crystallization solution contained a 1:1 ratio. Structure (III)[link] was grown from a 2:1 solution and has a higher proportion of F at both positions [0.66 (1):0.34 (1) for the 9-position and 0.76 (1):0.24 (1) for the 19-position]. It can be concluded that altering the 5-fluoro­uracil/thymine ratio in the crystallization solution will alter the ratio at each of the crystallographically independent sites. Refinements of (I)[link] as either fully 5-fluoro­uracil or fully thymine did not prove satisfactory, yielding unacceptable displacement parameters at the 9- and 19-positions, and higher than expected R factors, thus confirming the disordered model.

Structure (II)[link], measured at room temperature, shows thermal expansion in the a axis of approximately 0.5 Å (2.6%) compared with structure (I)[link], determined at 150 K. No significant change is evident in either of the other cell axes or the β angle. The unit cell was determined at 298 K for the crystal used for (III)[link] at 150 K, and a similar expansion in the a axis was observed [a = 19.704 (11) Å at 298 K and a = 19.235 (3) Å at 150 K].

It should be noted that crystals with this structure could not be grown from solutions with 5-fluoro­uracil/thymine ratios of 3:1 or 1:2, and attempts to grow pure 5-fluoro­uracil crystals with this structure from seeded solutions also failed. This implies that the two compounds have a limited solubility range in this solid solution.

The crystal structure contains four independent N—H⋯O hydrogen bonds, and all hydrogen-bond donors and acceptors are used (Table 1[link][link]–3[link]). Two R22(8) hydrogen-bonded dimers are present (Bernstein et al., 1995[Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. 34, 1555-1573.]), with one dimer composed of two N3—H3⋯O8(−x + 1, −y + 2, −z + 1) hydrogen bonds and the other dimer composed of two N13—H13⋯O18(−x + [{3\over 2}], −y + [{1\over 2}], −z + 2) hydrogen bonds. Along with the dimers, two single D11(2) hydrogen bonds participate in the overall hydrogen-bond network (Fig. 2[link]), viz. N1—H1⋯O17(−x + [{3\over 2}], y − [{1\over 2}], −z + [{3\over 2}]) and N11—H11⋯O7.

The hydrogen bonds build a two-dimensional net, with the constituent rings of the net made up of 14 mol­ecules in an approximately rectangular conformation. Of the 14 mol­ecules, 12 are involved in six dimers joined by R22(8) hydrogen-bonded rings, and these dimers are connected together by single hydrogen bonds. The two remaining mol­ecules are at diagonally opposite corners of the rectangle; along with the two single hydrogen bonds that incorporate each of these mol­ecules into the ring, each participates in a dimer with the second mol­ecule part of the adjacent ring. These inter­actions hence produce the two-dimensional net motif (Fig. 3[link]).

Two subsets of nets are observed; within each subset, the planes of the nets are parallel to one another, but each of the subsets is parallel to different Miller planes, viz. [[\overline{5}]11] and [[\overline{5}][\overline{1}]1]. The two subsets inter­penetrate to give the overall three-dimensional hydrogen-bonded motif (Fig. 4[link]), with hydrogen bonding at the points of inter­penetration of the nets via single hydrogen bonds only.

[Figure 1]
Figure 1
The asymmetric unit of the title cocrystal. Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as spheres (only one component of the disordered methyl groups is shown). The dashed line indicates the inter­mol­ecular hydrogen bond.
[Figure 2]
Figure 2
The hydrogen bonding present in the crystal structure. Hydrogen bonds are shown as dotted lines. [Symmetry codes: (i) −x + [{3\over 2}], y − [{1\over 2}], −z + [{3\over 2}]; (ii) −x + 1, −y + 2, −z + 1; (iii) −x + [{3\over 2}], −y + [{1\over 2}], −z + 2.]
[Figure 3]
Figure 3
Four adjacent rings from a single net. Hydrogen bonds are shown as dotted lines.
[Figure 4]
Figure 4
A view of three nets parallel to the c axis, with two nets parallel to one another and inter­secting the third net. Separate nets are shown as single colours. Hydrogen bonds are shown as dotted lines.

Experimental

The crystals used for determinations (I)[link] and (II)[link] were produced from a saturated solution of 5-fluoro­uracil and thymine (1:1 molar ratio) in 2,2,2-trifluoro­ethanol by solvent evaporation. The crystal used for structure determination (III)[link] was produced from a saturated solution of 5-fluoro­uracil and thymine (2:1 molar ratio) in 2,2,2-trifluoro­ethanol by solvent evaporation.

Determination (I)[link]

Crystal data

  • C4.48H4.45F0.52N2O2·C4.30H3.91F0.70N2O2

  • Mr = 257.07

  • Monoclinic, C 2/c

  • a = 19.3785 (15) Å

  • b = 5.9918 (5) Å

  • c = 20.0293 (15) Å

  • β = 117.813 (1)°

  • V = 2057.0 (3) Å3

  • Z = 8

  • Dx = 1.660 Mg m−3

  • Mo Kα radiation

  • μ = 0.15 mm−1

  • T = 150 (2) K

  • Diamond tablet, colourless

  • 0.79 × 0.22 × 0.20 mm
Data collection

  • Bruker SMART APEX diffractometer

  • ω scans

  • Absorption correction: multi-scan (SADABS; Sheldrick, 1996[Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.]) Tmin = 0.893, Tmax = 0.971

  • 8568 measured reflections

  • 2459 independent reflections

  • 2232 reflections with I > 2σ(I)

  • Rint = 0.016

  • θmax = 28.3°
Refinement

  • Refinement on F2

  • R[F2 > 2σ(F2)] = 0.037

  • wR(F2) = 0.096

  • S = 1.06

  • 2459 reflections

  • 207 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • w = 1/[σ2(Fo2) + (0.0452P)2 + 1.6915P] where P = (Fo2 + 2Fc2)/3

  • (Δ/σ)max < 0.001

  • Δρmax = 0.35 e Å−3

  • Δρmin = −0.19 e Å−3

Table 1
Hydrogen-bond geometry (Å, °) for (I)[link]

D—H⋯A D—H H⋯A D⋯A D—H⋯A
N1—H1⋯O17i 0.894 (18) 1.895 (18) 2.7769 (15) 168.4 (16)
N3—H3⋯O8ii 0.901 (18) 1.910 (18) 2.8092 (14) 175.2 (15)
N11—H11⋯O7 0.876 (19) 1.96 (2) 2.7892 (14) 156.6 (16)
N13—H13⋯O18iii 0.875 (18) 1.956 (18) 2.8291 (14) 176.1 (16)
Symmetry codes: (i) [-x+{\script{3\over 2}}, y-{\script{1\over 2}}, -z+{\script{3\over 2}}]; (ii) -x+1, -y+2, -z+1; (iii) [-x+{\script{3\over 2}}], [-y+{\script{1\over 2}}, -z+2].

Determination (II)[link]

Crystal data

  • C4.45H4.36F0.55N2O2·C4.31H3.94F0.69N2O2

  • Mr = 257.13

  • Monoclinic, C 2/c

  • a = 19.856 (11) Å

  • b = 5.946 (3) Å

  • c = 20.073 (11) Å

  • β = 117.660 (8)°

  • V = 2099.0 (19) Å3

  • Z = 8

  • Dx = 1.627 Mg m−3

  • Mo Kα radiation

  • μ = 0.14 mm−1

  • T = 298 (2) K

  • Diamond tablet, colourless

  • 0.69 × 0.20 × 0.15 mm
Data collection

  • Bruker SMART APEX diffractometer

  • ω scans

  • Absorption correction: multi-scan (SADABS; Sheldrick, 1996[Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.]) Tmin = 0.907, Tmax = 0.979

  • 8509 measured reflections

  • 2492 independent reflections

  • 1856 reflections with I > 2σ(I)

  • Rint = 0.031

  • θmax = 28.4°
Refinement

  • Refinement on F2

  • R[F2 > 2σ(F2)] = 0.056

  • wR(F2) = 0.138

  • S = 1.09

  • 2492 reflections

  • 207 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • w = 1/[σ2(Fo2) + (0.0602P)2 + 1.3663P] where P = (Fo2 + 2Fc2)/3

  • (Δ/σ)max < 0.001

  • Δρmax = 0.24 e Å−3

  • Δρmin = −0.19 e Å−3

Table 2
Hydrogen-bond geometry (Å, °) for (II)[link]

D—H⋯A D—H H⋯A D⋯A D—H⋯A
N1—H1⋯O17i 0.86 (3) 1.94 (3) 2.792 (3) 171 (2)
N3—H3⋯O8ii 0.88 (3) 1.95 (3) 2.831 (2) 173 (2)
N11—H11⋯O7 0.89 (3) 1.97 (3) 2.805 (3) 156 (2)
N13—H13⋯O18iii 0.88 (2) 1.98 (2) 2.852 (3) 175 (2)
Symmetry codes: (i) [-x+{\script{3\over 2}}, y-{\script{1\over 2}}, -z+{\script{3\over 2}}]; (ii) -x+1, -y+2, -z+1; (iii) [-x+{\script{3\over 2}}], [-y+{\script{1\over 2}}], -z+2.

Determination (III)[link]

Crystal data

  • C4.33H3.99F0.67N2O2·C4.24H3.73F0.76N2O2

  • Mr = 257.87

  • Monoclinic, C 2/c

  • a = 19.235 (3) Å

  • b = 5.9683 (8) Å

  • c = 20.042 (3) Å

  • β = 118.216 (2)°

  • V = 2027.4 (5) Å3

  • Z = 8

  • Dx = 1.690 Mg m−3

  • Mo Kα radiation

  • μ = 0.15 mm−1

  • T = 150 (2) K

  • Colourless, diamond tablet

  • 0.45 × 0.37 × 0.34 mm
Data collection

  • Bruker SMART APEX diffractometer

  • ω scans

  • Absorption correction: multi-scan (SADABS; Sheldrick, 1996[Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.]) Tmin = 0.935, Tmax = 0.950

  • 8306 measured reflections

  • 2405 independent reflections

  • 2147 reflections with I > 2σ(I)

  • Rint = 0.030

  • θmax = 28.1°
Refinement

  • Refinement on F2

  • R[F2 > 2σ(F2)] = 0.042

  • wR(F2) = 0.110

  • S = 1.07

  • 2405 reflections

  • 207 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • w = 1/[σ2(Fo2) + (0.0584P)2 + 1.4434P] where P = (Fo2 + 2Fc2)/3

  • (Δ/σ)max < 0.001

  • Δρmax = 0.30 e Å−3

  • Δρmin = −0.25 e Å−3

Table 3
Hydrogen-bond geometry (Å, °) for (III)[link]

D—H⋯A D—H H⋯A D⋯A D—H⋯A
N1—H1⋯O17i 0.90 (2) 1.90 (2) 2.7790 (16) 165.9 (18)
N3—H3⋯O8ii 0.88 (2) 1.93 (2) 2.8054 (15) 173.3 (17)
N11—H11⋯O7 0.84 (2) 1.98 (2) 2.7856 (16) 159.7 (19)
N13—H13⋯O18iii 0.87 (2) 1.96 (2) 2.8205 (15) 174.1 (18)
Symmetry codes: (i) [-x+{\script{3\over 2}}, y-{\script{1\over 2}}, -z+{\script{3\over 2}}]; (ii) -x+1, -y+2, -z+1; (iii) [-x+{\script{3\over 2}}], [-y+{\script{1\over 2}}], -z+2.

In all three structures, the C5—C9 and C15—C19 bonds were restrained to 1.520 (2) Å, and the C5—F9 and C15—F19 bonds were restrained to 1.350 (2) Å. For each determination, all H atoms other than the methyl H atoms were located in a difference map and were refined isotropically. In determinations (I)[link] and (II)[link], methyl H atoms were modelled as idealized disordered methyl groups over two sites offset by 60°. For determination (III)[link], the C19 methyl group was modelled as an idealized disordered methyl group, and for the C9 methyl group the H atoms were located from a difference map and refined using a rigid rotor model. For structure determination (I)[link], the refined C—H bond lengths are 0.948 (18) and 0.968 (17) Å, with all methyl C—H bond lengths fixed at 0.98 Å; the range of N—H bond lengths is 0.875 (18)–0.901 (18) Å. For structure determination (II)[link], the C—H bond lengths are 0.95 (3) and 1.02 (3) Å, with all methyl C—H bond lengths fixed at 0.96 Å; the range of N—H bond lengths is 0.86 (3)–0.89 (3) Å. For structure determination (III)[link], the C—H bond lengths are 0.950 (19) and 0.96 (2) Å, with all methyl C—H bond lengths fixed at 0.98 Å; the range of N—H bond lengths is 0.84 (2)–0.90 (2) Å.

For all determinations, data collection: SMART (Bruker, 1998[Bruker (1998). SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 1998[Bruker (1998). SMART and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997[Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Göttingen, Germany.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997[Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Göttingen, Germany.]); molecular graphics and software used to prepare material for publication: CAMERON (Watkin et al., 1996[Watkin, D. J., Prout, C. K. & Pearce, L. J. (1996). CAMERON. Chemical Crystallography Laboratory, Oxford, England.]) and MERCURY (Macrae et al., 2006[Macrae, C. F., Edgington, P. R., McCabe, P., Pidcock, E., Shields, G. P., Taylor, R., Towler, M. & van de Streek, J. (2006). J. Appl. Cryst. 39, 453-457.]).

Supporting information

Crystal structure: contains datablocks I, II, III, global. DOI: 10.1107/S0108270106019688/gg3013sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S0108270106019688/gg3013Isup2.hkl

Structure factors: contains datablock II. DOI: 10.1107/S0108270106019688/gg3013IIsup3.hkl

Structure factors: contains datablock III. DOI: 10.1107/S0108270106019688/gg3013IIIsup4.hkl


Comment top

F atoms and methyl groups have been identified as being capable of replacing one another in a molecule to produce isomorphic crystal structures because of their similar size, shape and van der Waals interactions (Kuhnert-Brandstatter, 1982). Attempts were made to exploit this interchangeability as part of an ongoing study into the crystalline solid state of 5-fluorouracil (Hulme et al., 2005; Hamad et al., 2006) with the aim of growing 5-fluorouracil crystals isostructural with the structure of thymine (Portalone et al., 1999) or thymine crystals isostructural with 5-fluorouracil form 1 (Fallon, 1973). Instead of producing such isostructural cocrystals, an entirely new structure was discovered, grown from solution in 2,2,2-trifluoroethanol, containing 5-fluorouracil and thymine in a solid solution.

A cocrystal can be defined as a crystal structure containing two (or more) molecular species on separate crystallographic sites with a fixed stoichiometric ratio in the crystal structure, in contrast with a solid solution which exhibits `a homogeneous crystalline phase in which some of the constituent molecules are substituted by foreign molecules that possess sufficient similarity that the lattice dimensions are changed only slightly' (Datta & Grant, 2004). In the structure reported here, both of the crystallographically independent sites (Fig. 1) can be occupied by either 5-fluorouracil or thymine molecules, giving non-integer occupancies for both molecules at each site and leading to the description of this structure as a solid solution rather than a cocrystal.

The structure adopts the monoclinic space group C2/c. The crystal structure, (I), of a crystal grown from a 1:1 solution of 5-fluorouracil/thymine, with the structure determined at 150 K, is reported; two further structure determinations are reported to exemplify the features of this system. (II) denotes the crystal structure determination of a crystal grown from a 1:1 solution at 298 K and (III) denotes the crystal structure determination of a crystal grown grown from a 2:1 solution of 5-fluorouracil:thymine at 150 K. Structure (I) will be used exclusively for the purposes of the discussion of the crystal structure, with the other two determinations used to highlight features of the solid solution structure.

The only difference between 5-fluorouracil and thymine is the substituent bonded to the 5-position in the molecular structure, and hence the only sign of the occupational disorder is the ratio F:Me at positions 9 and 19 in the crystal structure. Both (I) and (II) were grown from 1:1 5-fluorouracil/thymine crystallization solutions and have similar F:Me ratios at postions 9 and 19 [for (I), 0.52 (1):0.48 (1) for the 9-position and 0.70 (1):0.30 (1) for the 19-position; for (II), 0.55 (1):0.45 (1) for the 9-position and 0.69 (2):0.31 (2) for the 19-position]. This result indicates that the 5-fluorouracil/thymine ratio in the crystals is not simply a statistical distribution throughout the crystal but depends on the ratio in the crystallization solution. This fact is exemplified by the distinct preference for incorporating F at the 19-position, even though the original crystallization solution contained a 1:1 ratio. Structure (III) was grown from a 2:1 solution and has a higher proportion of F at both positions [0.66 (1):0.34 (1) for the 9-position and 0.76 (1):0.24 (1) for the 19-position]. It can be concluded that altering the 5-fluorouracil/thymine ratio in the crystallization solution will alter the ratio at each of the crystallographically independent sites. Refinements of (I) as either fully 5-fluorouracil or fully thymine did not prove statisfactory, yielding unacceptable displacment parameters at the 9- and 19-positions, and higher than expected R factors, thus confirming the disordered model.

Structure (II), measured at room temperature, shows thermal expansion in the a axis of approximately 0.5 Å (2.6%) compared with structure (I), determined at 150 K. No significant change is evident in either of the other cell axes or the β angle. The unit cell was determined at 298 K for the crystal used for (III) at 150 K, and a similar expansion in the a axis was observed [a = 19.704 (11) Å at 298 K and a = 19.235 (3) Å at 150 K).

It should be noted that crystals with this structure could not be grown from solutions with 5-fluorouracil/thymine ratios of 75:25 or 33:66, and attempts to grow pure 5-fluorouracil crystals with this structure from seeded solutions also failed. This result implies that the two compounds have a limited solubility range in this solid solution.

The crystal structure contains four independent N—H···O hydrogen bonds, and all hydrogen-bond donors and acceptors are used. Two R22(8) hydrogen-bonded dimers are present (Bernstein et al., 1995), with one dimer composed of two N3—H3···O8(1 − x, 2 − y, 1 − z) hydrogen bonds and the other dimer composed of two N13—H13···O18(−x + 3/2, −y + 1/2, −z + 2) hydrogen bonds. Along with the dimers, two single D11(2) hydrogen bonds participate in the overall hydrogen-bond network (Fig. 2), N1—H1···O17(−x + 3/2, −1/2 + y, −z + 3/2) and N11—H11···O7.

The hydrogen bonds build a two-dimensional net, with the constituent rings of the net made up of 14 molecules in an approximately rectangular conformation. Of the 14 molecules, 12 are involved in six dimers joined by R22(8) hydrogen-bonded rings, and these dimers are connected together by single hydrogen bonds. The two remaining molecules are at diagonally opposite corners of the rectangle; along with the two single hydrogen bonds that incorporate each of these molecules into the ring, each participates in a dimer with the second molecule part of the adjacent ring. These interactions hence produce the two-dimensional net motif (Fig. 3).

Two subsets of nets are observed; within each subset, the planes of the nets are parallel to one another, but each of the subsets is parallel to different Miller planes, viz. [511] and [511]. The two subsets interpenetrate to give the overall three-dimensional hydrogen-bonded motif (Fig. 4), with hydrogen bonding at the points of interpenetration of the nets via single hydrogen bonds only.

Experimental top

The crystals used for determinations (I) and (II) were produced from a saturated solution of 5-fluorouracil and thymine (1:1 molar ratio) in 2,2,2-trifluoroethanol by solvent evaporation. The crystal used for structure determination (III) was produced from a saturated solution of 5-fluorouracil and thymine (2:1 molar ratio) in 2,2,2-trifluoroethanol by solvent evaporation.

Refinement top

In all three structures the bonds C5—C9 and C15—C19 were restrained to 1.520 (2) Å and the bonds C5—F9 and C15—F19 were restrained to 1.350 (2) Å. For each determination, all H atoms other than the methyl H atoms were located in the difference map and were refined isotropically. In determinations (I) and (II), methyl H atoms were modelled as idealized disordered methyl groups over two sites, offset by 60°. For determination (III), the methyl group at C19 was modelled as an idealized disordered methyl group and for the methyl group at C9 the H atoms were located from ΔF and refined using a rigid rotor model. For structure (I), the refined C—H bond lengths are 0.95</span>(2) and 0.97 (2) Å, with all methyl C—H lengths fixed at 0.98 Å; the range of N—H bond lengths is 0.88 (2) to 0.90 (2) Å. For structure (II) the C—H bond lengths are 0.95 (3) and 1.02 (3) Å, with all methyl C—H lengths fixed at 0.96 Å; the range of N—H bond lengths is 0.86 (2) to 0.89 (3) Å. For structure (III), the C—H bond lengths are 0.95 (2) and 0.96 (2) Å, with all methyl C—H lengths fixed at 0.98 Å; the range of N—H bond lengths is 0.84 (2) to 0.90<span style=" font-weight:600;">(2) Å.

Computing details top

For all compounds, data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1998); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); software used to prepare material for publication: CAMERON (Watkin et al., 1996) and Mercury (Macrae et al., 2006).

Figures top
[Figure 1] Fig. 1. The asymmetric unit of the title cocrystal. Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as spheres (only one component of the disordered methyl groups is shown). The dashed line indicates a hydrogen bond.
[Figure 2] Fig. 2. The hydrogen bonding present in the crystal structure. Hydrogen bonds are shown as dotted lines. [Symmetry codes: (i) 3/2 − x, y − 1/2, 3/2 − z; (ii) 1 − x, 2 − y, 1 − z; (iii) 3/2 − x, 1/2 − y, 2 − z.]
[Figure 3] Fig. 3. Four adjacent rings from a single net. Hydrogen bonds are shown as dotted lines.
[Figure 4] Fig. 4. A view of three nets parallel to the c axis, with two nets parallel to one another and intersecting the third net. Separate nets are shown as single colours. Hydrogen bonds are shown as dotted lines.
(I) 5-fluoropyrimidine-2,4(1H,3H)-dione–5-methylpyrimidine-2,4(1H,3H)-dione top
Crystal data top
C4.48H4.45F0.52N2O2·C4.30H3.91F0.70N2O2F(000) = 1056
Mr = 257.07Dx = 1.660 Mg m−3
Monoclinic, C2/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2ycCell parameters from 4174 reflections
a = 19.3785 (15) Åθ = 2.3–27.9°
b = 5.9918 (5) ŵ = 0.15 mm−1
c = 20.0293 (15) ÅT = 150 K
β = 117.813 (1)°Diamond tablet, colourless
V = 2057.0 (3) Å30.79 × 0.22 × 0.20 mm
Z = 8
Data collection top
Bruker SMART APEX
diffractometer		2459 independent reflections
Radiation source: fine-focus sealed tube2232 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.016
ω rotation with narrow frames scansθmax = 28.3°, θmin = 2.3°
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)		h = −25→25
Tmin = 0.893, Tmax = 0.971k = −7→7
8568 measured reflectionsl = −26→26
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)] = 0.037Hydrogen site location: mixed
wR(F2) = 0.096H atoms treated by a mixture of independent and constrained refinement
S = 1.06 w = 1/[σ2(Fo2) + (0.0452P)2 + 1.6915P]
where P = (Fo2 + 2Fc2)/3
2459 reflections(Δ/σ)max < 0.001
207 parametersΔρmax = 0.35 e Å−3
4 restraintsΔρmin = −0.19 e Å−3
Crystal data top
C4.48H4.45F0.52N2O2·C4.30H3.91F0.70N2O2V = 2057.0 (3) Å3
Mr = 257.07Z = 8
Monoclinic, C2/cMo Kα radiation
a = 19.3785 (15) ŵ = 0.15 mm−1
b = 5.9918 (5) ÅT = 150 K
c = 20.0293 (15) Å0.79 × 0.22 × 0.20 mm
β = 117.813 (1)°
Data collection top
Bruker SMART APEX
diffractometer		2459 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)		2232 reflections with I > 2σ(I)
Tmin = 0.893, Tmax = 0.971Rint = 0.016
8568 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0374 restraints
wR(F2) = 0.096H atoms treated by a mixture of independent and constrained refinement
S = 1.06Δρmax = 0.35 e Å−3
2459 reflectionsΔρmin = −0.19 e Å−3
207 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
F90.5287 (5)0.5725 (16)0.34143 (12)0.0362 (14)0.524 (8)
C90.5169 (9)0.573 (4)0.33083 (14)0.0347 (17)0.476 (8)
H9A0.48460.70640.31110.052*0.238 (4)
H9B0.48420.43930.31120.052*0.238 (4)
H9C0.55760.57180.31490.052*0.238 (4)
H9D0.53300.43860.31370.052*0.238 (4)
H9E0.53340.70570.31360.052*0.238 (4)
H9F0.46000.57320.30990.052*0.238 (4)
O70.63996 (6)0.59939 (16)0.64191 (5)0.0302 (2)
O80.48879 (6)0.91031 (16)0.41262 (5)0.0298 (2)
N10.63001 (6)0.41914 (18)0.53753 (6)0.0252 (2)
H10.6629 (10)0.314 (3)0.5667 (10)0.034 (4)*
N30.56482 (6)0.75029 (18)0.52639 (6)0.0227 (2)
H30.5504 (9)0.860 (3)0.5481 (9)0.032 (4)*
C20.61375 (7)0.5881 (2)0.57345 (7)0.0224 (3)
C40.53270 (7)0.7575 (2)0.44885 (7)0.0225 (3)
C50.55443 (7)0.5728 (2)0.41635 (6)0.0245 (3)
C60.60150 (8)0.4126 (2)0.46055 (8)0.0262 (3)
H60.6180 (9)0.287 (3)0.4412 (9)0.032 (4)*
F190.6142 (3)−0.1573 (5)0.8072 (3)0.0361 (8)0.695 (8)
C190.5927 (9)−0.151 (2)0.7950 (12)0.045 (4)0.305 (8)
H19A0.5983−0.22510.84080.067*0.152 (4)
H19B0.6029−0.25780.76370.067*0.152 (4)
H19C0.5395−0.09240.76660.067*0.152 (4)
H19D0.5622−0.15850.73990.067*0.152 (4)
H19E0.5576−0.12570.81700.067*0.152 (4)
H19F0.6210−0.29110.81420.067*0.152 (4)
O170.77445 (6)0.60761 (16)0.85727 (5)0.0322 (2)
O180.69016 (6)0.02966 (17)0.94790 (5)0.0296 (2)
N110.69425 (7)0.3489 (2)0.77421 (6)0.0285 (3)
H110.6911 (10)0.425 (3)0.7355 (11)0.042 (5)*
N130.73097 (6)0.31769 (18)0.90113 (6)0.0222 (2)
H130.7564 (10)0.370 (3)0.9471 (10)0.033 (4)*
C120.73583 (7)0.4364 (2)0.84456 (7)0.0239 (3)
C140.69056 (7)0.1218 (2)0.89290 (7)0.0233 (3)
C150.65056 (8)0.0408 (2)0.81649 (7)0.0288 (3)
C160.65294 (8)0.1548 (3)0.76039 (7)0.0306 (3)
H160.6261 (10)0.109 (3)0.7093 (10)0.038 (5)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
F90.051 (3)0.038 (2)0.0171 (11)0.016 (2)0.0143 (15)0.0031 (16)
C90.036 (3)0.046 (4)0.0178 (19)0.011 (2)0.009 (2)−0.005 (2)
O70.0357 (5)0.0319 (5)0.0206 (4)0.0054 (4)0.0111 (4)0.0049 (4)
O80.0356 (5)0.0283 (5)0.0199 (4)0.0106 (4)0.0083 (4)0.0012 (4)
N10.0273 (5)0.0216 (5)0.0263 (5)0.0050 (4)0.0121 (4)0.0043 (4)
N30.0257 (5)0.0221 (5)0.0198 (5)0.0036 (4)0.0101 (4)−0.0001 (4)
C20.0219 (6)0.0226 (6)0.0224 (6)0.0000 (5)0.0100 (5)0.0034 (4)
C40.0229 (6)0.0233 (6)0.0202 (6)0.0006 (5)0.0092 (5)−0.0004 (4)
C50.0280 (6)0.0256 (6)0.0205 (6)0.0014 (5)0.0118 (5)−0.0017 (5)
C60.0285 (6)0.0238 (6)0.0286 (6)0.0011 (5)0.0153 (5)−0.0025 (5)
F190.045 (2)0.0339 (10)0.0257 (16)−0.0228 (8)0.0133 (15)−0.0097 (7)
C190.050 (9)0.053 (6)0.026 (5)−0.041 (6)0.014 (6)−0.014 (4)
O170.0451 (6)0.0279 (5)0.0235 (5)−0.0091 (4)0.0160 (4)−0.0007 (4)
O180.0371 (5)0.0330 (5)0.0197 (4)−0.0105 (4)0.0142 (4)−0.0022 (4)
N110.0350 (6)0.0327 (6)0.0137 (5)−0.0016 (5)0.0080 (4)0.0031 (4)
N130.0257 (5)0.0248 (5)0.0145 (5)−0.0023 (4)0.0082 (4)−0.0017 (4)
C120.0279 (6)0.0249 (6)0.0183 (6)0.0017 (5)0.0102 (5)0.0017 (5)
C140.0241 (6)0.0264 (6)0.0194 (6)−0.0026 (5)0.0101 (5)−0.0021 (5)
C150.0315 (7)0.0315 (7)0.0218 (6)−0.0107 (5)0.0111 (5)−0.0061 (5)
C160.0301 (7)0.0404 (8)0.0159 (6)−0.0052 (6)0.0061 (5)−0.0047 (5)
Geometric parameters (Å, º) top
F9—C51.3426 (18)F19—C151.3488 (17)
C9—C51.517 (2)C19—C151.519 (2)
O7—C21.2218 (15)O17—C121.2248 (16)
O8—C41.2301 (15)O18—C141.2355 (15)
N1—C21.3611 (17)N11—C121.3600 (16)
N1—C61.3745 (17)N11—C161.3651 (19)
N1—H10.894 (18)N11—H110.876 (19)
N3—C21.3779 (16)N13—C141.3774 (16)
N3—C41.3781 (15)N13—C121.3775 (16)
N3—H30.901 (18)N13—H130.875 (18)
C4—C51.4426 (17)C14—C151.4392 (17)
C5—C61.3355 (18)C15—C161.3340 (19)
C6—H60.968 (17)C16—H160.948 (18)
C2—N1—C6122.90 (11)C12—N11—C16123.11 (11)
C2—N1—H1116.8 (11)C12—N11—H11118.7 (13)
C6—N1—H1120.3 (11)C16—N11—H11118.1 (12)
C2—N3—C4126.71 (11)C14—N13—C12126.48 (11)
C2—N3—H3116.9 (10)C14—N13—H13116.1 (11)
C4—N3—H3116.3 (10)C12—N13—H13117.4 (12)
O7—C2—N1123.85 (12)O17—C12—N11123.18 (12)
O7—C2—N3121.44 (12)O17—C12—N13122.08 (11)
N1—C2—N3114.71 (11)N11—C12—N13114.74 (11)
O8—C4—N3120.82 (11)O18—C14—N13120.92 (11)
O8—C4—C5124.90 (11)O18—C14—C15124.96 (12)
N3—C4—C5114.27 (11)N13—C14—C15114.12 (11)
C6—C5—F9121.5 (4)C16—C15—F19123.7 (2)
C6—C5—C4120.34 (11)C16—C15—C14120.66 (12)
F9—C5—C4118.1 (4)F19—C15—C14115.6 (2)
C6—C5—C9124.6 (7)C16—C15—C19117.3 (8)
C4—C5—C9115.0 (7)C14—C15—C19121.1 (9)
C5—C6—N1121.05 (12)C15—C16—N11120.86 (12)
C5—C6—H6123.1 (10)C15—C16—H16122.9 (11)
N1—C6—H6115.9 (10)N11—C16—H16116.2 (11)
C6—N1—C2—O7−178.26 (12)C16—N11—C12—O17178.17 (13)
C6—N1—C2—N31.43 (17)C16—N11—C12—N13−2.17 (19)
C4—N3—C2—O7178.96 (12)C14—N13—C12—O17−178.62 (12)
C4—N3—C2—N1−0.74 (18)C14—N13—C12—N111.72 (19)
C2—N3—C4—O8179.07 (12)C12—N13—C14—O18−179.86 (12)
C2—N3—C4—C5−0.14 (18)C12—N13—C14—C15−0.01 (18)
O8—C4—C5—C6−178.76 (13)O18—C14—C15—C16178.45 (14)
N3—C4—C5—C60.41 (18)N13—C14—C15—C16−1.4 (2)
O8—C4—C5—F94.0 (5)O18—C14—C15—F19−3.8 (3)
N3—C4—C5—F9−176.8 (5)N13—C14—C15—F19176.3 (3)
O8—C4—C5—C9−1.7 (9)O18—C14—C15—C1910.0 (10)
N3—C4—C5—C9177.5 (9)N13—C14—C15—C19−169.9 (10)
F9—C5—C6—N1177.4 (5)F19—C15—C16—N11−176.5 (3)
C4—C5—C6—N10.2 (2)C14—C15—C16—N111.0 (2)
C9—C5—C6—N1−176.5 (9)C19—C15—C16—N11169.9 (10)
C2—N1—C6—C5−1.2 (2)C12—N11—C16—C150.9 (2)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O17i0.894 (18)1.895 (18)2.7769 (15)168.4 (16)
N3—H3···O8ii0.901 (18)1.910 (18)2.8092 (14)175.2 (15)
N11—H11···O70.876 (19)1.96 (2)2.7892 (14)156.6 (16)
N13—H13···O18iii0.875 (18)1.956 (18)2.8291 (14)176.1 (16)
Symmetry codes: (i) −x+3/2, y−1/2, −z+3/2; (ii) −x+1, −y+2, −z+1; (iii) −x+3/2, −y+1/2, −z+2.
(II) 5-fluoropyrimidine-2,4(1H,3H)-dione–5-methylpyrimidine-2,4(1H,3H)-dione top
Crystal data top
C4.45H4.36F0.55N2O2·C4.31H3.94F0.69N2O2F(000) = 1056
Mr = 257.13Dx = 1.627 Mg m−3
Monoclinic, C2/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2ycCell parameters from 1826 reflections
a = 19.856 (11) Åθ = 2.3–23.5°
b = 5.946 (3) ŵ = 0.14 mm−1
c = 20.073 (11) ÅT = 298 K
β = 117.660 (8)°Diamond tablet, colourless
V = 2099.0 (19) Å30.69 × 0.20 × 0.15 mm
Z = 8
Data collection top
Bruker SMART APEX
diffractometer		2492 independent reflections
Radiation source: fine-focus sealed tube1856 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.031
ω rotation with narrow frames scansθmax = 28.4°, θmin = 2.3°
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)		h = −25→25
Tmin = 0.907, Tmax = 0.979k = −7→7
8509 measured reflectionsl = −25→26
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)] = 0.056Hydrogen site location: mixed
wR(F2) = 0.138H atoms treated by a mixture of independent and constrained refinement
S = 1.09 w = 1/[σ2(Fo2) + (0.0602P)2 + 1.3663P]
where P = (Fo2 + 2Fc2)/3
2492 reflections(Δ/σ)max < 0.001
207 parametersΔρmax = 0.24 e Å−3
4 restraintsΔρmin = −0.19 e Å−3
Crystal data top
C4.45H4.36F0.55N2O2·C4.31H3.94F0.69N2O2V = 2099.0 (19) Å3
Mr = 257.13Z = 8
Monoclinic, C2/cMo Kα radiation
a = 19.856 (11) ŵ = 0.14 mm−1
b = 5.946 (3) ÅT = 298 K
c = 20.073 (11) Å0.69 × 0.20 × 0.15 mm
β = 117.660 (8)°
Data collection top
Bruker SMART APEX
diffractometer		2492 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)		1856 reflections with I > 2σ(I)
Tmin = 0.907, Tmax = 0.979Rint = 0.031
8509 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0564 restraints
wR(F2) = 0.138H atoms treated by a mixture of independent and constrained refinement
S = 1.09Δρmax = 0.24 e Å−3
2492 reflectionsΔρmin = −0.19 e Å−3
207 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
F90.5251 (6)0.5622 (17)0.34149 (15)0.074 (3)0.549 (13)
C90.5188 (10)0.568 (4)0.33157 (17)0.054 (4)0.451 (13)
H9A0.48700.69840.31180.081*0.226 (6)
H9B0.48880.43480.31200.081*0.226 (6)
H9C0.55850.57170.31710.081*0.226 (6)
H9D0.53590.43820.31550.081*0.226 (6)
H9E0.53400.70180.31520.081*0.226 (6)
H9F0.46430.56490.31020.081*0.226 (6)
O70.64097 (10)0.6061 (3)0.64158 (9)0.0512 (4)
O80.48828 (9)0.9043 (3)0.41302 (8)0.0496 (4)
N10.63040 (11)0.4223 (3)0.53755 (11)0.0410 (5)
H10.6616 (14)0.322 (5)0.5660 (14)0.051 (7)*
N30.56521 (10)0.7510 (3)0.52624 (10)0.0357 (4)
H30.5521 (13)0.858 (4)0.5486 (13)0.043 (6)*
C20.61447 (11)0.5919 (3)0.57359 (11)0.0350 (5)
C40.53236 (12)0.7541 (3)0.44906 (11)0.0344 (5)
C50.55361 (12)0.5688 (4)0.41683 (11)0.0389 (5)
C60.60124 (13)0.4121 (4)0.46085 (13)0.0422 (5)
H60.6206 (14)0.283 (4)0.4411 (14)0.053 (7)*
F190.6146 (4)−0.1532 (7)0.8070 (3)0.0568 (16)0.685 (14)
C190.5897 (10)−0.134 (3)0.7940 (16)0.074 (9)0.315 (14)
H19A0.5944−0.20920.83840.111*0.158 (7)
H19B0.5960−0.24170.76160.111*0.158 (7)
H19C0.5402−0.06670.76800.111*0.158 (7)
H19D0.5594−0.13590.74030.111*0.158 (7)
H19E0.5578−0.10340.81710.111*0.158 (7)
H19F0.6135−0.27830.81060.111*0.158 (7)
O170.77299 (10)0.6112 (3)0.85737 (9)0.0522 (5)
O180.68974 (10)0.0316 (3)0.94708 (8)0.0491 (4)
N110.69425 (11)0.3544 (3)0.77465 (10)0.0446 (5)
H110.6916 (14)0.439 (5)0.7371 (16)0.060 (8)*
N130.73006 (10)0.3201 (3)0.90068 (9)0.0346 (4)
H130.7572 (12)0.366 (4)0.9473 (14)0.038 (6)*
C120.73498 (13)0.4410 (3)0.84481 (11)0.0369 (5)
C140.69017 (12)0.1249 (4)0.89244 (11)0.0365 (5)
C150.65032 (13)0.0467 (3)0.81615 (12)0.0439 (6)
C160.65290 (13)0.1612 (4)0.76069 (12)0.0460 (6)
H160.6264 (14)0.114 (4)0.7095 (15)0.050 (7)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
F90.111 (5)0.073 (4)0.032 (2)0.045 (3)0.028 (3)0.001 (2)
C90.068 (7)0.061 (8)0.037 (4)0.005 (6)0.028 (5)0.000 (5)
O70.0643 (11)0.0513 (10)0.0330 (9)0.0119 (8)0.0183 (8)0.0101 (7)
O80.0643 (10)0.0450 (9)0.0314 (8)0.0224 (8)0.0153 (8)0.0014 (7)
N10.0461 (11)0.0315 (9)0.0434 (11)0.0112 (8)0.0192 (9)0.0076 (8)
N30.0449 (10)0.0312 (9)0.0307 (9)0.0073 (8)0.0174 (8)−0.0002 (7)
C20.0370 (11)0.0329 (10)0.0347 (11)0.0018 (9)0.0163 (9)0.0070 (8)
C40.0383 (11)0.0331 (11)0.0309 (10)0.0041 (9)0.0152 (9)0.0007 (8)
C50.0463 (12)0.0386 (11)0.0334 (11)0.0047 (10)0.0199 (10)−0.0033 (9)
C60.0490 (13)0.0348 (11)0.0469 (13)0.0047 (10)0.0258 (11)−0.0039 (10)
F190.072 (4)0.0523 (18)0.040 (2)−0.0371 (17)0.021 (3)−0.0139 (14)
C190.063 (13)0.071 (10)0.053 (9)−0.045 (9)−0.003 (8)−0.002 (6)
O170.0739 (11)0.0416 (9)0.0401 (9)−0.0203 (8)0.0256 (8)−0.0024 (7)
O180.0672 (11)0.0505 (10)0.0312 (8)−0.0206 (8)0.0242 (8)−0.0019 (7)
N110.0587 (12)0.0455 (11)0.0242 (9)−0.0065 (9)0.0147 (9)0.0043 (8)
N130.0438 (10)0.0357 (9)0.0220 (9)−0.0068 (8)0.0133 (8)−0.0034 (7)
C120.0460 (12)0.0346 (11)0.0296 (10)0.0008 (10)0.0171 (9)0.0027 (8)
C140.0413 (11)0.0369 (11)0.0320 (11)−0.0059 (9)0.0178 (9)−0.0018 (9)
C150.0522 (13)0.0426 (12)0.0349 (12)−0.0160 (11)0.0185 (10)−0.0073 (9)
C160.0475 (13)0.0565 (15)0.0244 (11)−0.0064 (11)0.0087 (10)−0.0050 (10)
Geometric parameters (Å, º) top
F9—C51.347 (2)F19—C151.352 (2)
C9—C51.519 (2)C19—C151.520 (2)
O7—C21.217 (3)O17—C121.217 (3)
O8—C41.224 (2)O18—C141.233 (2)
N1—C21.361 (3)N11—C121.358 (3)
N1—C61.372 (3)N11—C161.363 (3)
N1—H10.86 (3)N11—H110.89 (3)
N3—C41.374 (3)N13—C141.371 (3)
N3—C21.376 (3)N13—C121.373 (3)
N3—H30.88 (3)N13—H130.88 (2)
C4—C51.437 (3)C14—C151.436 (3)
C5—C61.329 (3)C15—C161.326 (3)
C6—H61.02 (3)C16—H160.95 (3)
C2—N1—C6123.23 (19)C12—N11—C16122.97 (19)
C2—N1—H1115.7 (17)C12—N11—H11115.9 (18)
C6—N1—H1121.0 (17)C16—N11—H11120.8 (18)
C4—N3—C2126.97 (18)C14—N13—C12126.92 (18)
C4—N3—H3117.8 (15)C14—N13—H13115.0 (15)
C2—N3—H3115.1 (15)C12—N13—H13117.9 (15)
O7—C2—N1124.15 (19)O17—C12—N11122.92 (19)
O7—C2—N3121.7 (2)O17—C12—N13122.58 (19)
N1—C2—N3114.11 (19)N11—C12—N13114.50 (19)
O8—C4—N3120.72 (18)O18—C14—N13121.04 (19)
O8—C4—C5124.90 (18)O18—C14—C15125.12 (19)
N3—C4—C5114.37 (18)N13—C14—C15113.83 (17)
C6—C5—F9121.6 (5)C16—C15—F19124.0 (3)
C6—C5—C4120.34 (19)C16—C15—C14120.80 (19)
F9—C5—C4118.0 (5)F19—C15—C14115.1 (3)
C6—C5—C9124.0 (9)C16—C15—C19116.9 (11)
C4—C5—C9115.7 (9)C14—C15—C19120.7 (12)
C5—C6—N1121.0 (2)C15—C16—N11121.0 (2)
C5—C6—H6123.5 (14)C15—C16—H16122.4 (15)
N1—C6—H6115.5 (14)N11—C16—H16116.6 (15)
C6—N1—C2—O7−178.5 (2)C16—N11—C12—O17178.6 (2)
C6—N1—C2—N31.2 (3)C16—N11—C12—N13−1.2 (3)
C4—N3—C2—O7179.4 (2)C14—N13—C12—O17−178.6 (2)
C4—N3—C2—N1−0.3 (3)C14—N13—C12—N111.1 (3)
C2—N3—C4—O8178.9 (2)C12—N13—C14—O18−179.7 (2)
C2—N3—C4—C5−0.3 (3)C12—N13—C14—C15−0.1 (3)
O8—C4—C5—C6−179.1 (2)O18—C14—C15—C16178.6 (2)
N3—C4—C5—C60.1 (3)N13—C14—C15—C16−0.9 (3)
O8—C4—C5—F92.3 (6)O18—C14—C15—F19−4.7 (5)
N3—C4—C5—F9−178.6 (5)N13—C14—C15—F19175.8 (4)
O8—C4—C5—C91.4 (8)O18—C14—C15—C1913.4 (13)
N3—C4—C5—C9−179.5 (8)N13—C14—C15—C19−166.1 (12)
F9—C5—C6—N1179.4 (6)F19—C15—C16—N11−175.5 (4)
C4—C5—C6—N10.7 (3)C14—C15—C16—N110.9 (4)
C9—C5—C6—N1−179.7 (8)C19—C15—C16—N11166.6 (13)
C2—N1—C6—C5−1.4 (3)C12—N11—C16—C150.2 (4)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O17i0.86 (3)1.94 (3)2.792 (3)171 (2)
N3—H3···O8ii0.88 (3)1.95 (3)2.831 (2)173 (2)
N11—H11···O70.89 (3)1.97 (3)2.805 (3)156 (2)
N13—H13···O18iii0.88 (2)1.98 (2)2.852 (3)175 (2)
Symmetry codes: (i) −x+3/2, y−1/2, −z+3/2; (ii) −x+1, −y+2, −z+1; (iii) −x+3/2, −y+1/2, −z+2.
(III) 5-fluoropyrimidine-2,4(1H,3H)-dione–5-methylpyrimidine-2,4(1H,3H)-dione top
Crystal data top
C4.33H3.99F0.67N2O2·C4.24H3.73F0.76N2O2F(000) = 1056
Mr = 257.87Dx = 1.690 Mg m−3
Monoclinic, C2/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2ycCell parameters from 4138 reflections
a = 19.235 (3) Åθ = 2.3–28.1°
b = 5.9683 (8) ŵ = 0.15 mm−1
c = 20.042 (3) ÅT = 150 K
β = 118.216 (2)°Colourless, diamond tablet
V = 2027.4 (5) Å30.45 × 0.37 × 0.34 mm
Z = 8
Data collection top
Bruker SMART APEX
diffractometer		2405 independent reflections
Radiation source: fine-focus sealed tube2147 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.030
ω rotation with narrow frames scansθmax = 28.1°, θmin = 2.3°
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)		h = −25→25
Tmin = 0.935, Tmax = 0.950k = −7→7
8306 measured reflectionsl = −26→26
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)] = 0.042Hydrogen site location: mixed
wR(F2) = 0.110H atoms treated by a mixture of independent and constrained refinement
S = 1.07 w = 1/[σ2(Fo2) + (0.0584P)2 + 1.4434P]
where P = (Fo2 + 2Fc2)/3
2405 reflections(Δ/σ)max < 0.001
207 parametersΔρmax = 0.30 e Å−3
4 restraintsΔρmin = −0.25 e Å−3
Crystal data top
C4.33H3.99F0.67N2O2·C4.24H3.73F0.76N2O2V = 2027.4 (5) Å3
Mr = 257.87Z = 8
Monoclinic, C2/cMo Kα radiation
a = 19.235 (3) ŵ = 0.15 mm−1
b = 5.9683 (8) ÅT = 150 K
c = 20.042 (3) Å0.45 × 0.37 × 0.34 mm
β = 118.216 (2)°
Data collection top
Bruker SMART APEX
diffractometer		2405 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)		2147 reflections with I > 2σ(I)
Tmin = 0.935, Tmax = 0.950Rint = 0.030
8306 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0424 restraints
wR(F2) = 0.110H atoms treated by a mixture of independent and constrained refinement
S = 1.07Δρmax = 0.30 e Å−3
2405 reflectionsΔρmin = −0.25 e Å−3
207 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
F90.5279 (5)0.5785 (15)0.34048 (12)0.0345 (13)0.665 (9)
C90.5158 (17)0.573 (6)0.32973 (16)0.038 (3)0.335 (9)
H9A0.48320.70800.30980.056*0.167 (4)
H9B0.48260.43980.31020.056*0.167 (4)
H9C0.55670.57190.31370.056*0.167 (4)
H9D0.53180.43850.31270.056*0.167 (4)
H9E0.53240.70660.31230.056*0.167 (4)
H9F0.45830.57460.30880.056*0.167 (4)
O70.63948 (6)0.59234 (18)0.64111 (6)0.0286 (3)
O80.48920 (6)0.91451 (17)0.41236 (5)0.0275 (3)
N10.62952 (7)0.4148 (2)0.53618 (7)0.0236 (3)
H10.6633 (11)0.309 (3)0.5651 (11)0.033 (5)*
N30.56477 (7)0.74840 (19)0.52591 (6)0.0212 (3)
H30.5518 (11)0.856 (3)0.5480 (10)0.032 (5)*
C20.61329 (8)0.5833 (2)0.57262 (7)0.0210 (3)
C40.53269 (8)0.7596 (2)0.44817 (7)0.0208 (3)
C50.55438 (8)0.5747 (2)0.41560 (7)0.0234 (3)
C60.60107 (9)0.4111 (2)0.45916 (8)0.0245 (3)
H60.6154 (11)0.287 (3)0.4388 (11)0.030 (4)*
F190.6120 (2)−0.1625 (4)0.8077 (2)0.0352 (8)0.756 (9)
C190.5876 (9)−0.149 (3)0.7930 (12)0.043 (4)0.244 (9)
H19A0.5922−0.22600.83800.064*0.122 (5)
H19B0.5960−0.25630.76050.064*0.122 (5)
H19C0.5349−0.08280.76500.064*0.122 (5)
H19D0.5565−0.15070.73770.064*0.122 (5)
H19E0.5527−0.12040.81520.064*0.122 (5)
H19F0.6138−0.29390.81070.064*0.122 (5)
O170.77433 (7)0.60358 (18)0.85675 (6)0.0308 (3)
O180.68982 (6)0.02760 (18)0.94850 (5)0.0282 (3)
N110.69302 (8)0.3432 (2)0.77386 (6)0.0275 (3)
H110.6890 (11)0.422 (4)0.7375 (13)0.042 (6)*
N130.73048 (7)0.3148 (2)0.90106 (6)0.0214 (3)
H130.7555 (11)0.372 (3)0.9461 (11)0.030 (4)*
C120.73522 (8)0.4327 (2)0.84422 (7)0.0228 (3)
C140.68991 (8)0.1187 (2)0.89316 (7)0.0222 (3)
C150.64939 (9)0.0352 (2)0.81688 (8)0.0278 (3)
C160.65137 (9)0.1487 (3)0.76029 (8)0.0298 (3)
H160.6243 (12)0.101 (3)0.7082 (12)0.041 (5)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
F90.054 (3)0.0353 (17)0.0161 (9)0.0142 (19)0.0183 (14)0.0026 (13)
C90.038 (5)0.050 (6)0.022 (3)0.011 (4)0.012 (4)−0.005 (4)
O70.0367 (6)0.0307 (6)0.0176 (5)0.0049 (4)0.0122 (4)0.0041 (4)
O80.0359 (6)0.0270 (5)0.0170 (5)0.0101 (4)0.0104 (4)0.0016 (4)
N10.0276 (6)0.0206 (6)0.0230 (6)0.0048 (5)0.0123 (5)0.0036 (4)
N30.0262 (6)0.0212 (6)0.0172 (5)0.0040 (5)0.0111 (5)0.0005 (4)
C20.0215 (6)0.0221 (6)0.0201 (6)0.0004 (5)0.0104 (5)0.0037 (5)
C40.0232 (6)0.0227 (6)0.0177 (6)0.0005 (5)0.0106 (5)−0.0004 (5)
C50.0301 (7)0.0248 (7)0.0179 (6)0.0019 (5)0.0134 (5)−0.0010 (5)
C60.0294 (7)0.0224 (6)0.0255 (7)0.0018 (5)0.0160 (6)−0.0017 (5)
F190.0454 (19)0.0351 (9)0.0212 (16)−0.0219 (9)0.0126 (14)−0.0079 (7)
C190.046 (9)0.048 (6)0.020 (6)−0.031 (6)0.004 (6)−0.007 (4)
O170.0452 (6)0.0272 (5)0.0215 (5)−0.0086 (5)0.0169 (5)−0.0006 (4)
O180.0371 (6)0.0316 (6)0.0181 (5)−0.0101 (4)0.0149 (4)−0.0018 (4)
N110.0366 (7)0.0318 (7)0.0119 (5)−0.0024 (5)0.0097 (5)0.0033 (5)
N130.0266 (6)0.0243 (6)0.0128 (5)−0.0027 (5)0.0089 (4)−0.0017 (4)
C120.0287 (7)0.0244 (7)0.0160 (6)0.0013 (5)0.0112 (5)0.0021 (5)
C140.0240 (6)0.0257 (7)0.0175 (6)−0.0022 (5)0.0104 (5)−0.0018 (5)
C150.0325 (7)0.0307 (8)0.0199 (7)−0.0099 (6)0.0121 (6)−0.0059 (5)
C160.0314 (7)0.0396 (8)0.0138 (6)−0.0068 (6)0.0070 (5)−0.0044 (6)
Geometric parameters (Å, º) top
F9—C51.3423 (18)F19—C151.3485 (17)
C9—C51.518 (2)C19—C151.519 (2)
O7—C21.2195 (17)O17—C121.2207 (17)
O8—C41.2257 (16)O18—C141.2360 (17)
N1—C21.3629 (18)N11—C121.3603 (18)
N1—C61.3726 (19)N11—C161.363 (2)
N1—H10.90 (2)N11—H110.84 (2)
N3—C21.3756 (17)N13—C141.3739 (18)
N3—C41.3796 (17)N13—C121.3782 (17)
N3—H30.88 (2)N13—H130.87 (2)
C4—C51.4408 (19)C14—C151.4372 (18)
C5—C61.334 (2)C15—C161.337 (2)
C6—H60.950 (19)C16—H160.96 (2)
C2—N1—C6122.95 (12)C12—N11—C16123.22 (12)
C2—N1—H1117.0 (12)C12—N11—H11116.4 (15)
C6—N1—H1119.8 (12)C16—N11—H11120.0 (15)
C2—N3—C4126.91 (12)C14—N13—C12126.57 (12)
C2—N3—H3116.3 (12)C14—N13—H13117.5 (12)
C4—N3—H3116.8 (12)C12—N13—H13115.9 (13)
O7—C2—N1123.95 (12)O17—C12—N11123.30 (13)
O7—C2—N3121.38 (13)O17—C12—N13122.11 (12)
N1—C2—N3114.67 (12)N11—C12—N13114.58 (12)
O8—C4—N3120.98 (12)O18—C14—N13120.87 (12)
O8—C4—C5125.19 (12)O18—C14—C15124.89 (13)
N3—C4—C5113.82 (12)N13—C14—C15114.24 (11)
C6—C5—F9121.8 (4)C16—C15—F19123.71 (19)
C6—C5—C4120.89 (12)C16—C15—C14120.54 (13)
F9—C5—C4117.2 (4)F19—C15—C14115.7 (2)
C6—C5—C9123.7 (13)C16—C15—C19115.5 (8)
C4—C5—C9115.2 (13)C14—C15—C19122.4 (10)
C5—C6—N1120.75 (13)C15—C16—N11120.80 (13)
C5—C6—H6122.2 (11)C15—C16—H16123.3 (13)
N1—C6—H6117.0 (11)N11—C16—H16115.9 (12)
C6—N1—C2—O7−178.19 (13)C16—N11—C12—O17177.91 (14)
C6—N1—C2—N31.13 (19)C16—N11—C12—N13−2.1 (2)
C4—N3—C2—O7178.72 (13)C14—N13—C12—O17−178.24 (13)
C4—N3—C2—N1−0.6 (2)C14—N13—C12—N111.8 (2)
C2—N3—C4—O8179.05 (13)C12—N13—C14—O18−179.94 (13)
C2—N3—C4—C5−0.1 (2)C12—N13—C14—C150.0 (2)
O8—C4—C5—C6−178.75 (14)O18—C14—C15—C16178.33 (15)
N3—C4—C5—C60.4 (2)N13—C14—C15—C16−1.6 (2)
O8—C4—C5—F93.5 (5)O18—C14—C15—F19−3.1 (3)
N3—C4—C5—F9−177.4 (4)N13—C14—C15—F19177.0 (2)
O8—C4—C5—C9−3.0 (15)O18—C14—C15—C1913.3 (11)
N3—C4—C5—C9176.1 (15)N13—C14—C15—C19−166.6 (11)
F9—C5—C6—N1177.8 (4)F19—C15—C16—N11−177.2 (2)
C4—C5—C6—N10.1 (2)C14—C15—C16—N111.3 (2)
C9—C5—C6—N1−175.3 (16)C19—C15—C16—N11167.3 (12)
C2—N1—C6—C5−0.9 (2)C12—N11—C16—C150.7 (2)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O17i0.90 (2)1.90 (2)2.7790 (16)165.9 (18)
N3—H3···O8ii0.88 (2)1.93 (2)2.8054 (15)173.3 (17)
N11—H11···O70.84 (2)1.98 (2)2.7856 (16)159.7 (19)
N13—H13···O18iii0.87 (2)1.96 (2)2.8205 (15)174.1 (18)
Symmetry codes: (i) −x+3/2, y−1/2, −z+3/2; (ii) −x+1, −y+2, −z+1; (iii) −x+3/2, −y+1/2, −z+2.

Experimental details

(I)(II)(III)
Crystal data
Chemical formulaC4.48H4.45F0.52N2O2·C4.30H3.91F0.70N2O2C4.45H4.36F0.55N2O2·C4.31H3.94F0.69N2O2C4.33H3.99F0.67N2O2·C4.24H3.73F0.76N2O2
Mr257.07257.13257.87
Crystal system, space groupMonoclinic, C2/cMonoclinic, C2/cMonoclinic, C2/c
Temperature (K)150298150
a, b, c (Å)19.3785 (15), 5.9918 (5), 20.0293 (15)19.856 (11), 5.946 (3), 20.073 (11)19.235 (3), 5.9683 (8), 20.042 (3)
β (°) 117.813 (1) 117.660 (8) 118.216 (2)
V (Å3)2057.0 (3)2099.0 (19)2027.4 (5)
Z888
Radiation typeMo KαMo KαMo Kα
µ (mm−1)0.150.140.15
Crystal size (mm)0.79 × 0.22 × 0.200.69 × 0.20 × 0.150.45 × 0.37 × 0.34
Data collection
DiffractometerBruker SMART APEX
diffractometer		Bruker SMART APEX
diffractometer		Bruker SMART APEX
diffractometer
Absorption correctionMulti-scan
(SADABS; Sheldrick, 1996)		Multi-scan
(SADABS; Sheldrick, 1996)		Multi-scan
(SADABS; Sheldrick, 1996)
Tmin, Tmax0.893, 0.9710.907, 0.9790.935, 0.950
No. of measured, independent and
observed [I > 2σ(I)] reflections		8568, 2459, 2232 8509, 2492, 1856 8306, 2405, 2147
Rint0.0160.0310.030
(sin θ/λ)max (Å−1)0.6660.6680.664
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.037, 0.096, 1.06 0.056, 0.138, 1.09 0.042, 0.110, 1.07
No. of reflections245924922405
No. of parameters207207207
No. of restraints444
H-atom treatmentH atoms treated by a mixture of independent and constrained refinementH atoms treated by a mixture of independent and constrained refinementH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)0.35, −0.190.24, −0.190.30, −0.25

Computer programs: SMART (Bruker, 1998), SAINT (Bruker, 1998), SAINT, SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), CAMERON (Watkin et al., 1996) and Mercury (Macrae et al., 2006).

Hydrogen-bond geometry (Å, º) for (I) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O17i0.894 (18)1.895 (18)2.7769 (15)168.4 (16)
N3—H3···O8ii0.901 (18)1.910 (18)2.8092 (14)175.2 (15)
N11—H11···O70.876 (19)1.96 (2)2.7892 (14)156.6 (16)
N13—H13···O18iii0.875 (18)1.956 (18)2.8291 (14)176.1 (16)
Symmetry codes: (i) −x+3/2, y−1/2, −z+3/2; (ii) −x+1, −y+2, −z+1; (iii) −x+3/2, −y+1/2, −z+2.
Hydrogen-bond geometry (Å, º) for (II) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O17i0.86 (3)1.94 (3)2.792 (3)171 (2)
N3—H3···O8ii0.88 (3)1.95 (3)2.831 (2)173 (2)
N11—H11···O70.89 (3)1.97 (3)2.805 (3)156 (2)
N13—H13···O18iii0.88 (2)1.98 (2)2.852 (3)175 (2)
Symmetry codes: (i) −x+3/2, y−1/2, −z+3/2; (ii) −x+1, −y+2, −z+1; (iii) −x+3/2, −y+1/2, −z+2.
Hydrogen-bond geometry (Å, º) for (III) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O17i0.90 (2)1.90 (2)2.7790 (16)165.9 (18)
N3—H3···O8ii0.88 (2)1.93 (2)2.8054 (15)173.3 (17)
N11—H11···O70.84 (2)1.98 (2)2.7856 (16)159.7 (19)
N13—H13···O18iii0.87 (2)1.96 (2)2.8205 (15)174.1 (18)
Symmetry codes: (i) −x+3/2, y−1/2, −z+3/2; (ii) −x+1, −y+2, −z+1; (iii) −x+3/2, −y+1/2, −z+2.
 

Acknowledgements

The authors acknowledge the Research Councils UK Basic Technology Programme for supporting `Control and Prediction of the Organic Solid State'. For further information, please visit https://www.cposs.org.uk .

References

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ISSN: 2053-2296
Volume 62| Part 7| July 2006| Pages o412-o415
doi:10.1107/S0108270106019688
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