title: Characterization of complicated new polymorphs of chlorothalonil by X-ray diffraction and computer crystal structure prediction.
abstract: A simultaneous experimental and computational search for polymorphs of chlorothalonil (2,4,5,6-tetrachloro-1,3-benzenedicarbonitrile) has been conducted, leading to the first characterization of forms 2 and 3. The crystal structure prediction study, using a specifically developed anisotropic atom-atom potential for chlorothalonil, gave as the global minimum in the lattice energy a structure that was readily refined against powder diffraction data to the known form 1 (P2(1)/a). The structure of form 2 was solved and refined from powder diffraction data, giving a disordered structure in the Rm (166) space group (Z = 3). It could also be refined against a P1 ordered model, starting from a low-energy hypothetical sheet structure found in the computational search. This shows that the disorder could be associated with the stacking of ordered sheets. The disordered structure for form 2 was later confirmed by single-crystal X-ray diffraction. The structure of form 3, determined from single-crystal diffraction, contains three independent molecules in the asymmetric unit in P2(1) (4) (Z = 6). Powder diffraction showed that this single-herringbone structure was similar to two low-energy structures found in the search. Further analysis confirmed that form 3 has a similar lattice energy and contains elements from both these predicted structures, which can be considered as good approximations to the form 3 structure.
authors.affiliation: School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. m.tremayne@bham.ac.uk
authors.forename: Maryjane
authors.initials: M
authors.lastname: Tremayne
authors.forename: Leanne
authors.initials: L
authors.lastname: Grice
authors.forename: James C
authors.initials: JC
authors.lastname: Pyatt
authors.forename: Colin C
authors.initials: CC
authors.lastname: Seaton
authors.forename: Benson M
authors.initials: BM
authors.lastname: Kariuki
authors.forename: Helen H Y
authors.initials: HH
authors.lastname: Tsui
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Price
authors.forename: Julian C
authors.initials: JC
authors.lastname: Cherryman
mesh_terms: D003198:Computer Simulation; D003460:Crystallization; D005659:Fungicides, Industrial / Q000737:chemistry*; D015394:Molecular Structure; D009570:Nitriles / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D005659:Fungicides, Industrial; D009570:Nitriles; C005806:tetrachloroisophthalonitrile