Determination of crystal structures of polymorphic chlorothalonil using Monte Carlo simulated annealing and Rietveld refinement

Crystal structures of types II and III chlorothalonil, 2,4,5,6-tetrachloro-1,3-dicyanobenzene, (C8Cl4N2) were solved by applying Monte Carlo simulated annealing techniques to X-ray powder diffraction data and refined using the Rietveld method. Both types of chlorothalonil crystallize in monoclinic symmetry (space group P2(1) and two molecules per unit cell). Lattice parameters are: a=8.1615(18) Angstrom, b=9.4191(19) Angstrom, c=6.4728(14) Angstrom, beta=93.7307(64)degrees and V=497.8 Angstrom(3) for type II, and a=8.6003(10) Angstrom, b=9.2382(11) Angstrom, c=6.3024(7) Angstrom, beta=96.2152(60)degrees and V=498.5 Angstrom(3) for type III. The structures of both types of chlorothalonil are stacked by two coplanar molecular sheets paralleled to the b-axis. The adjacent two paralleled molecules in type II are on the same plane, while those in type III are on two different parallel planes. (C) 2004 International Centre for Diffraction Data.