The crystal structure of the high-pressure ζ-form of the explosive CL-20 has been determined using a combination of x-ray single crystal and powder diffraction techniques. Conformational changes in the orientation of the nitro groups of the CL-20 molecule were observed in the γ→ζ transition, such that molecules in the ζ-form adopt the conformation in which all of the nitro groups are exo with respect to the five- and six-membered rings. The level of complexity of this crystal structure extends the limits to which high-pressure techniques may be applied, and higlights the need for employing a range of different methods for structure solution. In addition to solving a long-standing problem of great significance to the energetics community, the experimental results presented here will be of particular value to computational chemists seeking to model structural changes in energetic materials under extreme conditions, and will allow validation of the intermolecular potentials used to describe this important class of nitramines.
The performance of energetic materials can depend on a number of factors that include: sensitivity to detonation by stimulus; the detonation velocity; the chemical reactivity; the thermal stability; and crystal density. Polymorphism and solid-state phase transitions in these materials may therefore have significant consequences and the performance of an energetic formulation may be highly dependent on the particular polymorph that is used. In order to effectively model the behaviour of energetic materials under operational conditions it is essential to obtain detailed structural information for these compounds, particularly under the extreme conditions of temperature and pressure experienced during detonation.
Figure 1: Powder diffraction image obtained for the ζ-form of CL-20 at a pressure of 1.4 GPa.
We recently investigated the effects of pressure on the polycyclic nitramine 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW), which is also known as CL-20 on account of its development at China Lake, USA [1]. CL-20 is the most powerful explosive in current use although some concerns remain over its sensitivity to detonation. Previous studies had indicated that compression of the γ-form above 0.7 GPa result in a phase transition to an uncharacterised ζ-form [2][3][4]. The aim of the study was to determine the structure of the high-pressure ζ-form and obtain information on its phase stability. In order to achieve this, it proved essential to use a combination of x-ray single crystal and powder diffraction techniques. A polycrystalline sample of γ-CL-20 was loaded into a diamond-anvil cell with Fluorinert (FC-77) as the pressure-transmitting medium. Using the Extreme Conditions Beamline I15, the γ-form was observed to persist on compression up to 0.72(5) GPa, but at the next pressure point (1.44(5) GPa) a dramatic change was observed in the powder diffraction pattern, indicative of a phase transition to the high-pressure ζ-form. Despite the high quality data, attempts to index these patterns gave several possible solutions, none of which allowed structure solution. For this reason we turned to single crystal x-ray diffraction and it proved possible to index a set of reflections to a monoclinic cell, space group P21/n. Due to the limitations of high-pressure data collections caused by shading from the steel body of the diamond-anvil cell, these data-sets suffered from low completeness (ca. 60%). This, along with the complexity of the molecular structure, meant that structure solution via direct methods was not possible. Instead the indexing solutions obtained from the single crystal studies were used as a starting point for structure solution from the powder diffraction data using the program FOX [5]. Further corroboration that the crystal structure was correct was obtained by performing full-profile Rietveld refinements on all of the collected powder diffraction patterns.
Comparison of the molecular structures in the γ- and ζ-forms shows that the isowurtzitane cage remains unchanged and it is the exo- and endo- spatial orientation of the nitro groups with respect to the five- and six-membered rings that varies. The figure clearly shows that the high-pressure ζ-form adopts the conformation in which all of the nitro groups are exo with respect to the five- and six-membered rings. DFT calculations have previously explored the relative energies of a range of conformations for the isolated molecule and have found that there are four conformations that are energetically favourable on steric grounds [6]. The relative energies of these are: ζ-form (9.63 kJ mol-1), ε-form (6.99 kJ mol-1), γ-and α-forms (4.73 kJ mol-1), and β-form (0.0 kJ mol-1). Given these relatively small energy barriers, it is perhaps not surprising that compression of the γ-form will induce a phase transition to a conformation that allows more efficient crystal packing, as is exemplified in this case by the contraction in volume across the γ→ζ transition. Based on the unit-cell volumes reported previously, our results show that over the pressure range 1.2-3.5 GPa the ζ-form has a lower density than the ε-form at any given pressure. On this basis, the detonation velocity of the ζ-form would be expected to be lower than that of the ε-form. As has been observed by other authors, all attempts to recover the ζ-form to ambient pressure were unsuccessful and instead resulted in the formation of the γ-form. This presumably reflects the relatively low barrier to interconversion between the two molecular conformations and so it seems unlikely that recovery could be achieved unless low temperatures were employed.
Figure 2: Conformation of molecules in the ζ-form compared with conformation in the γ-form.
The crystal structure of the high-pressure ζ-form of CL-20 has been determined using a combination of x-ray single crystal and powder diffraction techniques. Conformational changes in the orientation of the nitro groups of the CL-20 molecule were observed in the γ→ζ transition, such that molecules in the ζ-form adopt the conformation in which all of the nitro groups are exo with respect to the five- and six-membered rings. The level of complexity of this crystal structure extends the limits to which high-pressure techniques may be applied, and highlights the need for employing a range of different methods for structure solution. In addition to solving a long-standing problem of great significance to the energetics community, the experimental results presented here will be of particular value to computational chemists seeking to model structural changes in energetic materials under extreme conditions, and will allow validation of the intermolecular potentials used to describe this important class of nitramines.
References
[1] A.T. Nielsen, A.P. Chafin, S.L. Christian, et al., Tetrahedron, 54, 11793 (1998).
[2] J.C. Gump, C.A. Stoltz and S.M. Peiris, AIP Conf. Proc., 955, 127 (2007).
[3] S.M. Peiris and J.C. Gump, Static Compression of Energetic Materials (eds. S.M. Peiris and G.J. Piermarini) Springer, Berlin, Germany, (2008).
[4.] T.P. Russell, P.J. Miller, G.J. Piermarini and S. Block, J. Phys. Chem., 97, 1993-1997 (1993).
[5] V. Favre-Nicolin and R. Cerný, J. Appl. Crystallogr., 35, 734 (2002).
[6] Y. Kholod, S. Okovytyy, G. Kuramshina, M. Qasim, L. Gorb and J. Leszczynski, J. Mol. Struct., 843, 14 (2007).
Principal Publications and Authors
D. I. A. Millar, H. E. Maynard-Casely, A. K. Kleppe, W. G. Marshall, C. R. Pulham, and A. S. Cumming, Putting the squeeze on energetic materials – structural characterisation of a high-pressure phase of CL-20, CrystEngComm, (2010) . DOI: 10.1039/c002701d
Funding Acknowledgement
Defence Science and Technology Laboratory, the Engineering and Physical Research Council and Ministry of Defence, UK.
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