CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 6 425-428
Articles Structural Rationalization Directly from Powder Diffraction Data: Intermolecular Aggregation in 2-(Methylsulfonyl)ethyl Succinimidyl Carbonate David Albesa Jove´, Emilio Tedesco, Kenneth D. M. Harris,* Roy L. Johnston, and Eugene Y. Cheung School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom Received August 20, 2001
ABSTRACT: Structural rationalization of molecular materials has relied largely on the application of single crystal X-ray diffraction techniques, although clearly such studies are restricted to those materials that can be prepared in the form of single crystals of appropriate size and quality. To broaden our structural understanding beyond such cases, there is considerable scope for applying techniques that have been developed recently to allow structure determination of molecular materials directly from powder diffraction data. In this light, this paper reports and discusses the structural properties of 2-(methylsulfonyl)ethyl succinimidyl carbonate. The structure determination was carried out directly from powder X-ray diffraction data, using the genetic algorithm technique for structure solution, followed by Rietveld refinement. This material is an example of a molecular crystal that contains several conventional strong hydrogen bond acceptor groups but does not contain any conventional strong hydrogen bond donor groups. The structure is found to contain a number of short C-H‚‚‚O interactions. Introduction In the field of crystal engineering, the primary interest centers on developing an understanding and rationalization of preferred modes of intermolecular aggregation (for example, hydrogen bonding schemes) in crystals and subsequently exploiting this understanding to design materials with specific desired structures. Hitherto, the structural rationalization phase has been dependent largely on the availability of sufficient structural information from single crystal X-ray diffraction data, which clearly precludes the study of materials that are not available in the form of single crystals of sufficient size and/or quality for investigation using this technique. In recent years, however, considerable progress has been made in the availability of techniques and procedures for determining molecular crystal structures directly from powder diffraction data,1-5 and clearly such techniques have the potential to broaden significantly our knowledge of structural properties beyond those simply of materials for which singlecrystal X-ray diffraction studies are viable. * To whom correspondence should be addressed. Telephone: +44121-414-7474; FAX: +44-121-414-7473; E-mail: K.D.M.Harris@ bham.ac.uk.
In this paper, we demonstrate the utility of powder diffraction techniques in this field by reporting structural studies of 2-(methylsulfonyl)ethyl succinimidyl carbonate (abbreviated 2-MSESC; Figure 1a). This molecule contains seven different oxygen atoms (three CdO groups, two SdO groups, and two C-O-C groups) which may be expected to behave as hydrogen bond acceptors but does not contain any conventional strong hydrogen bond donor groups. It is therefore interesting to contemplate the crystal packing of this type of molecule. For example, as several C-H groups are present, the formation of C-H‚‚‚O interactions may be anticipated. In general terms, the packing arrangement observed in a molecular crystal is the outcome of a balance among several different intermolecular and intramolecular interactions, which may represent varying degrees between cooperativity and competition. To understand the optimal structural arrangement observed in a crystal,6-9 we are required to understand the interplay between short-range intermolecular interactions (such as hydrogen bonding, dispersion, and repulsion) between functional groups, longer range electrostatic interactions, and intramolecular (conformational) energies. The presence of certain functional groups can lead to predictable local interactions and
10.1021/cg010021k CCC: $20.00 © 2001 American Chemical Society Published on Web 10/05/2001
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approach for the complete structure determination of 2-MSESC directly from powder diffraction data. Experimental and Computational Details
Figure 1. (a) Molecular structure of 2-MSESC. (b) Structural fragment used in the GA structure solution calculation. The variable torsion angles are indicated by arrows, and the atom numbering scheme is defined.
may promote characteristic crystal packing motifs. In this regard, the use of conventional strong hydrogen bonding as a basis for the rationalization and subsequent design of crystal structures has been particularly exploited.10-15 However, other weaker hydrogen bonding type interactions are now also well established,16 including the C-H‚‚‚O hydrogen bond.17-21 Understanding the structural properties of molecular crystals containing this interaction is facilitated by structural studies of molecules that contain conventional strong hydrogen bond acceptor groups (e.g., CdO) but do not contain any conventional strong hydrogen bond donor groups (e.g., O-H, N-H), as in the material (2-MSESC) investigated here. Structure Determination from Powder Diffraction Data. Among recent developments of techniques for solving crystal structures directly from powder diffraction data, the “direct-space” strategy22 is particularly suitable in the case of molecular materials. In this strategy, trial structures are generated in direct space, with the quality of each trial structure assessed by comparing the powder diffraction pattern calculated for the trial structure and the experimental powder diffraction pattern (this comparison is made here using the powder profile R-factor Rwp, which implicitly takes peak overlap into consideration). In the present paper, the direct-space structure solution has been carried out using our genetic algorithm (GA) method23-27 to locate the trial structure corresponding to the global minimum in Rwp. In the GA method,23-28 a population of trial structures is allowed to evolve subject to rules and operations (mating, mutation, and natural selection) analogous to those that govern evolution in biological systems. Each structure is specified by its “genetic code”, which represents, for each molecule in the asymmetric unit, the position {x, y, z} and orientation {θ, φ, ψ} of the molecule and the molecular conformation (defined by variable torsion angles {τ1, τ2, ..., τn}). New structures are generated by the mating and mutation operations, and in our implementation used here,26 each new structure is subjected to local minimization of Rwp. In natural selection, only the structures of highest “fitness” (i.e., lowest Rwp) are allowed to pass from one generation to the next generation. In this paper, we apply this
The synchrotron X-ray powder diffraction pattern of 2MSESC was recorded at ambient temperature on Station 2.3 at the SRS, Daresbury Laboratory using a capillary sample holder and λ ) 1.3000 Å. The 2θ range was 2 to 60°, measured in steps of 0.01° over ca. 3.5 h. The powder diffraction pattern was indexed by the program DICVOL,29 giving a unit cell with monoclinic metric symmetry (a ) 20.45 Å; b ) 6.01 Å; c ) 11.76 Å; β ) 125.9°). From systematic absences, the space group was assigned as P21/n, and density considerations indicated that there are four molecules in the unit cell and thus one molecule in the asymmetric unit. Line shape and line width parameters were determined using the POWDERFIT program,30 which uses a modified Pawley fitting procedure.31 The Pawley fit gave Rwp ) 9.55%. We note that, while synchrotron X-ray powder diffraction data has been used in the present case, the majority of our work on structure determination of molecular materials from powder X-ray diffraction data has utilized data recorded on a conventional laboratory powder X-ray diffractometer. Structure solution was carried out using our GA method incorporating local minimization of Rwp,26 implemented in the program EAGER.32 In the structure solution calculation, all non-hydrogen atoms of the molecule were included (Figure 1b). Each structure was defined by a total of 12 variables {x, y, z, θ, φ, ψ, τ1, τ2, ..., τ6}, with all torsion angles varied freely. The GA calculation involved the evolution of a population of 100 structures, with a linear fitness function. In each generation, 100 offspring (50 pairs of parents) and 15 mutations were generated. In the mating operation, the variables defining each parent were divided into eight blocks, specified as follows: {x, y, z | θ, φ, ψ | τ1 | τ2 | τ3 | τ4 | τ5 | τ6}, and four complementary blocks were contributed by each parent to each offspring. The GA calculation was run for 50 generations, and the best structure solution (i.e., with lowest Rwp in the final generation) was taken as the starting structural model for Rietveld refinement, which was carried out using the GSAS program.33 In the Rietveld refinement, standard geometric restraints were applied to bond lengths and bond angles, and in the later stages, a preferred orientation parameter was refined. The final Rietveld refinement (Figure 2; Table 1) gave Rwp ) 12.32% and Rp ) 9.31% (number of variables, 68; number of reflections, 736; number of data points, 4012).
Results and Discussion The final refined crystal structure of 2-MSESC is shown in Figure 3 (in this figure, and in the discussion below, hydrogen atom positions have been normalized according to standard geometric features established from neutron diffraction results). In the molecular conformation, the central torsion angles [τ2 ) -171°; τ3 ) -164°; τ4 ) 164°] are close to trans, whereas those at the end of the chain [τ5 ) 56°; τ6 ) -43°] are close to gauche. The conformation of the ring with respect to the chain corresponds to τ1 ) 124°. In the crystal structure, recognizable intermolecular C-H‚‚‚O interactions are formed between molecules of 2-MSESC, but there is no evidence for any recognizable intramolecular C-H‚‚‚O interactions. The intermolecular C-H‚‚‚O hydrogen bonds include an interaction between a hydrogen atom of the ring (C2) and a CdO group (O6) in the ring of another molecule (H‚‚‚O, 2.18 Å; C-H‚‚‚O, 155°) and an interaction between a hydrogen atom of the CH3 group (C17) and a C-O-C oxygen atom (O8)
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Figure 2. Experimental (+ marks), calculated (solid line), and difference (lower line) powder X-ray diffraction profiles for the final Rietveld refinement of 2-MSESC.
Figure 3. Crystal structure of 2-MSESC viewed along the a-axis. The dashed lines indicate short C-H‚‚‚O contacts.
of the chain (H‚‚‚O, 2.30 Å; C-H‚‚‚O, 163°). Two other C-H‚‚‚O interactions share the same acceptor (O15) from the sulfone group and involve hydrogen atoms from CH2 groups (C12 and C13) in two different molecules (H‚‚‚O and C-H‚‚‚O: 2.25 Å, 122°; 2.14 Å, 137°). Other C-H‚‚‚O contacts with longer H‚‚‚O distances (in the range 2.3-2.6 Å) may also be identified, although such interactions clearly lie toward (or beyond) the upper limit of the definition of C-H‚‚‚O hydrogen bonding. On this basis, four oxygen atoms [O16 (sulfone group), O11 (C-O-C), O7 (ring CdO), and O10 (chain CdO)] in the molecule are not involved in significant C-H‚‚‚O inter-
actions. The structure does not contain any recognizable dipolar arrays involving the SdO groups and/or the CdO groups, unlike the structures found in other cases34,35 of molecules containing strongly dipolar hydrogen bond acceptor groups but a deficiency of strong hydrogen bond donor groups. The absence of dipolar arrays suggests that the crystal structure of this material may be dictated predominantly by optimization of the dispersion energy (van der Waals interactions), rather than by optimization of the long-range electrostatic energy (which would be enhanced by the formation of dipolar arrays).
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Table 1. Fractional Coordinates and Isotropic Displacement Parameters for the Non-Hydrogen Atoms in the Final Refined Crystal Structure of 2-MSESCa C(1) C(2) C(3) N(4) C(5) O(6) O(7) O(8) C(9) O(10) O(11) C(12) C(13) S(14) O(15) O(16) C(17)
x
y
z
Uiso/Å2
0.8536(9) 0.8822(10) 0.8924(7) 0.8636(4) 0.8377(5) 0.7920(6) 0.9118(7) 0.8329(3) 0.8957(4) 0.9641(4) 0.8789(1) 0.9445(4) 0.9125(6) 0.8525(2) 0.9116(4) 0.8092(5) 0.7788(6)
0.7743(19) 0.9716(15) 0.8681(13) 0.6558(11) 0.5990(14) 0.4420(16) 0.9720(14) 0.5595(14) 0.4674(20) 0.4752(18) 0.3316(15) 0.2937(14) 0.2030(11) -0.0487(7) -0.2475(11) -0.1081(12) -0.0843(16)
0.1987(10) 0.3011(11) 0.4257(8) 0.4018(5) 0.2694(7) 0.2072(8) 0.5306(10) 0.4670(7) 0.5936(9) 0.6238(9) 0.6718(6) 0.8137(7) 0.8931(6) 0.8064(4) 0.8598(10) 0.8748(9) 0.6169(4)
0.105 0.105 0.105 0.105 0.105 0.033 0.033 0.056 0.056 0.056 0.056 0.056 0.056 0.056 0.027 0.027 0.027
a P2 /n; refined unit cell: a ) 20.4373(6) Å, b ) 6.0037(2) Å, 1 c ) 11.7476(4) Å, β ) 125.927(1)°.
Nevertheless, several of the potential hydrogen bond acceptors (oxygen atoms) are engaged in recognizable C-H‚‚‚O type interactions, which undoubtedly also play a role in influencing the observed crystal structure. This work further illustrates the current scope and potential of techniques for the determination of molecular crystal structures from powder diffraction data and provides promise for the future application of these techniques to solve structures of increasing complexity in this field. Acknowledgment. We thank EPSRC, the University of Birmingham, Wyeth-Ayerst, and Purdue Pharma for financial support. References (1) Cheetham, A. K.; Wilkinson, A. P. Angew. Chemie, Int. Ed. Engl. 1992, 31, 1557. (2) Harris, K. D. M.; Tremayne, M. Chem. Mater. 1996, 8, 2554. (3) Poojary, D. M.; Clearfield, A. Acc. Chem. Res. 1997, 30, 414. (4) Meden, A. Croat. Chem. Acta 1998, 71, 615. (5) Harris, K. D. M.; Tremayne, M.; Kariuki, B. M. Angew. Chemie, Int. Ed. 2001, 40, 1626. (6) Kitaigorodsky, A. I. Molecular Crystals and Molecules, Academic Press: New York, 1973. (7) Gavezzotti, A., Ed., Theoretical Aspects and Computer Modeling of the Molecular Solid State; John Wiley & Sons: Chichester, 1997.
Albesa Jove´ et al. (8) Desiraju, G. R. Angew. Chemie, Int. Ed. Engl. 1995, 34, 2311. (9) Dunitz, J. D.; Gavezzotti, A. Acc. Chem. Res. 1999, 32, 677. (10) Leiserowitz, L. Acta Crystallogr. 1976, B32, 775. (11) Leiserowitz, L.; Hagler, A. T. Proc. Royal Soc. 1983, A388, 133. (12) Desiraju, G. R. Crystal Engineering: the Design of Organic Solids; Elsevier: Amsterdam, 1989. (13) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (14) MacDonald, J. C.; Whitesides, G. M. Chem. Rev. 1994, 94, 2383. (15) Desiraju, G. R. Angew. Chemie, Int. Ed. Engl. 1995, 34, 2311. (16) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; International Union of Crystallography and Oxford Science Publications: New York, 1999. (17) Berkovitch-Yellin, Z.; Leiserowitz, L. Acta Crystallogr. 1984, B40, 159. (18) Seiler, P.; Dunitz, J. D. Helv. Chim. Acta 1989, 72, 1125. (19) Desiraju, G. R. Acc. Chem. Res. 1996, 29, 441. (20) Steiner, T. Chem. Commun. 1997, 727. (21) Kariuki, B. M.; Harris, K. D. M.; Philp, D.; Robinson, J. M. A. J. Am. Chem. Soc. 1997, 119, 12679. (22) Harris, K. D. M.; Tremayne, M.; Lightfoot, P.; Bruce, P. G. J. Am. Chem. Soc. 1994, 116, 3543. (23) Kariuki, B. M.; Serrano-Gonza´lez, H.; Johnston, R. L.; Harris, K. D. M. Chem. Phys. Lett. 1997, 280, 189. (24) Harris, K. D. M.; Johnston, R. L.; Kariuki, B. M.; Tremayne, M. J. Chem. Res. (S) 1998, 390. (25) Harris, K. D. M.; Johnston, R. L.; Kariuki, B. M. Acta Crystallogr. 1998, A54, 632. (26) Turner, G. W.; Tedesco, E.; Harris, K. D. M.; Johnston, R. L.; Kariuki, B. M. Chem. Phys. Lett. 2000, 321, 183. (27) Tedesco, E.; Turner, G. W.; Harris, K. D. M.; Johnston, R. L.; Kariuki, B. M. Angew. Chemie, Int. Ed. 2000, 39, 4488. (28) Shankland, K.; David, W. I. F.; Csoka, T. Z. Kristallogr. 1997, 212, 550. (29) Boultif, A.; Loue¨r, D. J. Appl. Crystallogr. 1991, 24, 987. (30) Engel, G. E.; Wilke, S.; Ko¨nig, O.; Harris, K. D. M.; Leusen, F. J. J. J. Appl. Crystallogr. 1999, 32, 1169. (31) Pawley, G. S. J. Appl. Crystallogr. 1981, 14, 357. (32) Habershon, S.; Harris, K. D. M.; Johnston, R. L.; Kariuki, B. M.; Lanning, O. J.; Tedesco, E.; Turner, G. W. EAGER, University of Birmingham, 2000 [an extended version of the program GAPSS, Harris, K. D. M.; Johnston, R. L.; Kariuki, B. M. University of Birmingham, 1997]. (33) Larson, A. C.; Von Dreele, R. B. Los Alamos Lab. Report No. LA-UR-86-748, 1987. (34) Calcagno, P.; Kariuki, B. M.; Kitchin, S. J.; Robinson, J. M. A.; Philp, D.; Harris, K. D. M. Chem. Eur. J. 2000, 6, 2338. (35) Tedesco, E.; Kariuki, B. M.; Harris, K. D. M.; Johnston, R. L.; Pudova, O.; Barbarella, G.; Marseglia, E. A.; Gigli, G.; Cingolani, R. J. Solid State Chem., in press.
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