Characterization of Complicated New Polymorphs of Chlorothalonil by X-ray Diffraction and Computer Crystal Structure Prediction

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 (P21/a). The structure of form 2 was solved and refined
from powder diffraction data, giving a disordered structure in the R3̄m (166) space group (Z = 3). It could
also be refined against a P1̄ 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 P21 (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.