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Volume 67 
Part 6 
Pages o206-o208  
June 2011  

Received 14 March 2011
Accepted 4 May 2011
Online 12 May 2011

The molecular conformation of pentan-3-one studied in cholic acid pentan-3-one solvate

aInstitute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
Correspondence e-mail: ullrich.englert@ac.rwth-aachen.de

The crystal structure of cholic acid-pentan-3-one (1/1), C5H10O·C24H40O5, has been determined in order to deduce the molecular conformation of the small volatile ketone. Data were collected at 100 K to a resolution of (sin [theta])/[lambda] = 0.91 Å-1. The structure contains a hydrogen-bonded cholic acid host network, forming only van der Waals interactions with the guest pentan-3-one molecules. The ketone molecules are disordered on general positions, with two clearly identifiable conformations. The majority conformer exhibits approximate C2 symmetry and is similar to that recently observed by microwave spectroscopy in the gas phase.

Comment

Many solvates of cholic acid have been studied, often with the focus on crystal engineering: Version 5.31 of the Cambridge Structural Database (CSD; Allen, 2002[Allen, F. H. (2002). Acta Cryst. B58, 380-388.]) contains 247 error-free organic structures in which the steroid acts as a guest for small organic host molecules. We have recently identified cocrystallization with cholic acid as an effective strategy to obtain high-resolution data sets on low-melting compounds, provided the intensity data collections are performed at reasonably low temperatures. Crystals obtained with the odorants allyl acetate and isopropyl acetate have been of sufficient quality even for charge-density studies (Mouhib et al., 2011[Mouhib, H., Jelisavac, D., Stahl, W., Wang, R., Kalf, I. & Englert, U. (2011). ChemPhysChem, 12, 761-764.]). We intended to obtain information about possible conformations of the small volatile molecule pentan-3-one, also known as diethyl ketone, in the solid state. In view of its relatively low melting point of ca 233 K and our previous successful experiments, we decided to grow cocrystals with cholic acid and to study the crystal structure of the title cocrystal, (I)[link], at low temperature.

The structure of (I)[link] at room temperature has been reported previously (Caira et al., 1994a[Caira, M. R., Nassimbeni, L. R. & Scott, J. L. (1994a). J. Chem. Crystallogr. 24, 783-791.]). The compound crystallizes in the space group P21 with lattice parameters similar to those of related compounds (Caira et al., 1994b[Caira, M. R., Nassimbeni, L. R. & Scott, J. L. (1994b). J. Chem. Soc. Perkin Trans. 2, pp. 623-628.]). A displacement ellipsoid plot of the contents of the asymmetric unit is shown in Fig. 1[link]. Our redetermination fully confirms the conclusions of the earlier authors with respect to the host structure, but the previously published data cannot provide details about the conformation of the guest molecule. In contrast, our intensity data collected at low temperature and up to high resolution

[Scheme 1]
allow us to compare the conformation of the ketone in the solid to that in the gas phase. Although single crystals of (I)[link] were well diffracting up to a resolution (sin [theta])/[lambda] = 0.91 Å-1, the resulting intensity data are not suitable for a charge-density study. After completion of the conventional refinement reported here, a difference Fourier synthesis shows local electron-density maxima centred on covalent bonds of the steroid host, in agreement with the expectation and in favour of a potential charge-density study. In the neighbourhood of the guest molecule, however, a less clear-cut and chemically not meaningful distribution is observed for the residual maxima, which precludes a more demanding interpretation of the electron density.

Classical O-H...O hydrogen bonds link the cholic acid molecules into layers parallel to the (10[\overline{1}]) planes (Fig. 2[link] and Table 2[link]). Pairs of the host molecules reside in a comparatively large cavity with a volume of 366 Å3 (Spek, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]). One of the ethyl substituents in the pentan-3-one molecule is disordered over two conformations with a 0.545 (12):0.455 (12) ratio between the site occupancies. The majority conformation shown in Fig. 3[link](a) deviates only slightly from C2 symmetry, in good agreement with the experimental results for the gas-phase conformation obtained from Fourier transform microwave spectroscopy, and also with the expectation from theory (Nguyen & Stahl, 2011[Nguyen, H. V. L. & Stahl, W. (2011). ChemPhysChem, doi:10.1002/cphc.201001021.]). The minority conformer (Fig. 3[link]b) is less symmetrical and belongs to point group C1. The only previous low-temperature crystal structure containing pentan-3-one (Goldup et al., 2008[Goldup, S. M., Leigh, D. A., Lusby, P. J., McBurney, R. T. & Slawin, A. M. Z. (2008). Angew. Chem. Int. Ed. 47, 3381-3384.]) shows an unexpected C-C-C bond angle of 141° in one of the ethyl substituents and therefore does not allow one to deduce the conformation of this molecule reliably.

[Figure 1]
Figure 1
The molecular structure of (I)[link], with displacement ellipsoids drawn at the 50% probability level. Atoms belonging to the minor conformer of pentan-3-one and all H atoms have been omitted for clarity.
[Figure 2]
Figure 2
Packing diagram of (I)[link], viewed along [010], showing the hydrogen-bonded layers of cholic acid host molecules in black and the pentan-3-one guest molecules in grey.
[Figure 3]
Figure 3
The molecular conformations of pentan-3-one in (I)[link], showing (a) the major conformer, similar to the gas-phase structure, and (b) the minor conformer.

Experimental

Single crystals of (I)[link] were obtained by dissolving cholic acid (30 mg, 0.07 mmol) in pentan-3-one (10 ml) at 313 K. Slow evaporation over a period of ca 7 d at room temperature afforded colourless plates. The crystal fragment chosen for the diffraction experiment was cut to an appropriate size while covered with mother liquor and transferred directly to the stream of cold N2 (100 K) on the diffractometer.

Crystal data
  • C5H10O·C24H40O5

  • Mr = 494.69

  • Monoclinic, P 21

  • a = 12.5606 (7) Å

  • b = 8.0425 (5) Å

  • c = 13.9139 (8) Å

  • [beta] = 102.002 (3)°

  • V = 1374.84 (14) Å3

  • Z = 2

  • Mo K[alpha] radiation

  • [mu] = 0.08 mm-1

  • T = 100 K

  • 0.16 × 0.13 × 0.11 mm

Data collection
  • Bruker SMART APEX CCD diffractometer

  • 33615 measured reflections

  • 13745 independent reflections

  • 9042 reflections with I > 2[sigma](I)

  • Rint = 0.070

Refinement
  • R[F2 > 2[sigma](F2)] = 0.081

  • wR(F2) = 0.200

  • S = 1.06

  • 13745 reflections

  • 357 parameters

  • 26 restraints

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

  • [Delta][rho]max = 0.69 e Å-3

  • [Delta][rho]min = -0.44 e Å-3

Table 1
Selected geometric parameters (Å, °)

O1-C1 1.321 (2)
O2-C1 1.213 (2)
O3-C7 1.446 (2)
O4-C11 1.435 (2)
O5-C15 1.438 (2)
C1-C2 1.512 (3)
C2-C3 1.519 (3)
C3-C4 1.540 (3)
O6-C25 1.216 (4)
C25-C26 1.493 (5)
C25-C28A 1.507 (5)
C25-C28B 1.566 (7)
C26-C27 1.519 (5)
C28A-C29A 1.512 (10)
C28B-C29B 1.531 (12)
O6-C25-C26 123.0 (3)
O6-C25-C28A 124.6 (3)
C26-C25-C28A 111.2 (3)
O6-C25-C28B 111.1 (5)
C26-C25-C28B 122.8 (5)
C25-C26-C27 115.1 (2)
C25-C28A-C29A 113.1 (7)
C29B-C28B-C25 107.4 (8)
O6-C25-C26-C27 16.7 (5)
O6-C25-C28A-C29A 17.7 (9)
O6-C25-C28B-C29B -66.6 (12)

Table 2
Hydrogen-bond geometry (Å, °)

D-H...A D-H H...A D...A D-H...A
O1-H1...O4i 0.87 (3) 1.84 (3) 2.672 (2) 160 (4)
O3-H3...O2i 0.83 (2) 2.08 (2) 2.879 (2) 160 (2)
O4-H4...O5ii 0.86 (2) 1.81 (2) 2.6499 (19) 165 (2)
O5-H5...O3ii 0.82 (2) 2.10 (3) 2.869 (2) 157 (2)
Symmetry codes: (i) [-x+1, y+{\script{1\over 2}}, -z+2]; (ii) [-x, y-{\script{1\over 2}}, -z+1].

A total of 22 similarity restraints (C-C distances, C-C-C angles and displacement parameters for split positions) were used to model the disordered ethyl group of the solvent molecule. Coordinates and isotropic displacement parameters were refined for H atoms attached to O atoms, with O-H distances restrained to 0.85 (2) Å. H atoms attached to C atoms were treated as riding, with C-H = 0.98 Å for CH3, C-H = 0.99 Å for CH2 and C-H = 1.00 Å for CH groups, and with Uiso(H) = 1.2 or 1.5Ueq(C). Methyl groups were allowed to rotate about their local threefold axes. In the absence of significant anomalous scattering effects, the Flack (1983[Flack, H. D. (1983). Acta Cryst. A39, 876-881.]) parameter is indeterminate. The absolute structure is assigned on the basis of the known configuration of the chiral cholic acid molecule. Friedel pairs were not merged.

Data collection: SMART (Bruker, 2001[Bruker (2001). SMART. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SMART; data reduction: SAINT-Plus (Bruker, 1999[Bruker (1999). SAINT-Plus. Bruker AXS Inc., Madison, Wisconsin, USA.]); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: PLATON (Spek, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]); software used to prepare material for publication: SHELXL97.


Supplementary data for this paper are available from the IUCr electronic archives (Reference: BI3016 ). Services for accessing these data are described at the back of the journal.


Acknowledgements

Halima Mouhib and Wolfgang Stahl are acknowledged for the fruitful co-operation in our project dedicated to the conformation of volatile compounds in the gas and solid phases.

References

Allen, F. H. (2002). Acta Cryst. B58, 380-388.  [ISI] [CrossRef] [details]
Bruker (1999). SAINT-Plus. Bruker AXS Inc., Madison, Wisconsin, USA.
Bruker (2001). SMART. Bruker AXS Inc., Madison, Wisconsin, USA.
Caira, M. R., Nassimbeni, L. R. & Scott, J. L. (1994a). J. Chem. Crystallogr. 24, 783-791.  [ChemPort]
Caira, M. R., Nassimbeni, L. R. & Scott, J. L. (1994b). J. Chem. Soc. Perkin Trans. 2, pp. 623-628.
Flack, H. D. (1983). Acta Cryst. A39, 876-881.  [CrossRef] [details]
Goldup, S. M., Leigh, D. A., Lusby, P. J., McBurney, R. T. & Slawin, A. M. Z. (2008). Angew. Chem. Int. Ed. 47, 3381-3384.  [ChemPort]
Mouhib, H., Jelisavac, D., Stahl, W., Wang, R., Kalf, I. & Englert, U. (2011). ChemPhysChem, 12, 761-764.  [ChemPort] [PubMed]
Nguyen, H. V. L. & Stahl, W. (2011). ChemPhysChem, doi:10.1002/cphc.201001021.
Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.  [CrossRef] [details]
Spek, A. L. (2009). Acta Cryst. D65, 148-155.  [ISI] [CrossRef] [details]


Acta Cryst (2011). C67, o206-o208   [ doi:10.1107/S0108270111016933 ]