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Crystal Polymorph Prediction

M. Leslie
CSE Department, CLRC Daresbury Laboratory

J.B.O. Mitchell and S.L. Price
Department of Chemistry, University College London

D. Buttar and R.J.Roberts
AstraZeneca/Avicia

The program DMAREL [1] has found widespread usage in modelling the crystal structures of polar organic molecules, where the crystal packing is sensitive to the electrostatic forces [2]. Recent enhancements include the following:

The anisotropic short range potentials have been used to investigate cyanuric chloride (C3N3Cl3) and two other more complex chlorinated azaaromatics [3]. An overlap model was used to derive non-empirical parameters for short-range repulsive atom-atom potentials. The model assumes a power law relationship between the total repulsive energy Erep and the overlap Sr

Erep = K[Sr]g

where the exponent g is just less than 1. The overlap is subdivided into atom-atom contributions which are then fitted to an analytical model. The short range repulsions are then combined with an accurate distributed multipole electrostatic model and an atom-atom dispersion model to give totally non-empirical intermolecular pair potentials.

The crystal structure of cyanuric chloride has C-Cl...N interactions which geometrically resemble hydrogen bonds. The Cl...N intermolecular contacts are considerably shorter than the sum of the van der Waal radii. One of these bonds is linear and two others have an angle of 173o. A potential model based on an empirical fit gives all three bonds linear and a space group Rc rather than the experimentally observed C2/c. The models using non-empirical potentials, whether isotropic or anisotropic, all give the correct space group with the C-Cl...N bond angle about 173o. There is a marked improvement in the reproduction of the cell constants going from the isotropic to the anisotropic model, the former gives a r.m.s error of 2.3%, the latter 1.3%.

To test the transferability of the potential parameters, two other compounds were investigated. Some small changes were made to the potential. First, the multipole charge distribution was calculated using SCF rather than MP2. Secondly, the small anisotropy in the short range potential on the N and C atoms in cyanuric chloride was removed. This was to allow for the different environments for C and N atoms in the larger molecules. When cyanuric chloride was recalculated using this potential model a small improvement was found. (r.m.s. 0.99%). The crystal structure of one of the two new molecules was reproduced well, the other was not as successful.

References

[1] D.J. Willock, S.L. Price, M. Leslie, C.R.A. Catlow, J. Comput. Chem. 16 (1995) 628.

[2] D.S. Coombes, S.L. Price, D.J. Willock, M. Leslie, J. Phys. Chem. 100 (1996) 7352

[3] J. B. O. Mitchell, S. L. Price, M. Leslie, D. Buttar and R. J. Roberts, The Journal of Physical Chemistry A, 9961-9971 (2001).

 
For more information about the work of the Computational Chemistry Group please contact Paul Sherwood p.sherwood@dl.ac.uk.

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