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Overcoming poor particle statistics - low temperature structure of succinonitrile using a combination of simulated annealing and quantum optimization |
| Pamela S. Whitfield , Yvon Le Page , Ali Abouimrane , Isobel J. Davidson |
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Institute for Chemical Process and Environmental Technology National Research Council of Canada, Ottawa ONK1A06, Canada |
| Abstract |
| Succinonitrile at room temperature forms what is known as a plastic-crystal phase with positional ordering but rotational disorder yielding an I-centred cubic lattice. There is a phase transition on cooling at around 218K to an ordered crystalline phase which currently has an unknown crystal structure. Given that samples are normally mounted around room temperature, the morphology and properties of the plastic crystal phase become important for the quality of the sample. The plastic and ‘sticky’ nature of the succinonitrile makes producing a good powder almost impossible. Injecting the molten succinonitrile inside a capillary is the most practical way to mount a sample of succinonitrile, but its tendency towards dendritic and direction crystal growth may be problematic. The sample is likely to be prone to both preferential orientation and poor particle statistics, neither of which is conducive to lab-based structure solution techniques. This may explain why the low temperature structure of a very common laboratory chemical has not been solved previously. Data were taken from a capillary sample of succinonitrile at 173K using a Vantec PSD detector with a focussing primary mirror. After indexing using TOPAS, the structure solution was attempted using simulated annealing. Constraints were used to prevent errors in the relative intensities forcing the structure away from a chemically reasonable conformation. Succinonitrile can adopt the cis-, trans- or gauche- conformations. The SA was set up with a z-matrix rigid body such that the conformation of succinonitrile was randomly selected for each cycle before minimization with limited freedom in the torsion angle such that it had to remain in the selected conformation. A constrained March-Dollase correction was also included in the SA to help account for orientation. Quantum optimization was used to minimize the energy of the resultant succinonitrile structure before refining the energy-optimized structure as a rigid-body against the experimental data. Analysis of the spherical harmonic orientation coefficients from the final refinement confirm moderate texture with an orientation that can be rationalized in terms of the expected temperature gradients in the capillary on cooling. The structure is monoclinic in P21/a with a = 9.160(1), b =8.6085(9), c = 5.8568(4) Å and ß = 79.652(4)º with Z = 4. The succinonitrile is present solely in the gauche conformation. Although the data quality limits the residual that may be achieved in the final fit, the resulting structure is consistent with available literature spectroscopy results. |
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Presentation: Oral at 11th European Powder Diffraction Conference, Microsymposium 3, by Pamela S. WhitfieldSee On-line Journal of 11th European Powder Diffraction Conference Submitted: 2008-05-06 15:59 Revised: 2009-06-07 00:48 |