Polymorphs of 2,4,5,6-tetrachloro-1,3-benzenedicarbonitile and their transformations

The transformation of Polymorphs (α and β modifications) of 2,4,5,6-tetrachloro-1,3-benzenedicarbonitile (chlorothalonil) to each other was studied by DSC method. The transformation temperature for α to β was determined to be 150.5 ± 0.1 °C. The enthalpy, ΔHt, and activation energy, Ea, for the transformation α to β were found to be, respectively, 4.03 ± 0.03 kJ mol−1 (significance level = 0.02) and 6.5 ± 0.9 × 102 kJ mol−1 (95% confidence limits). The rate of transformation for α to β is fast, but the reverse is very slow. The X-ray powder diffraction (XRD) patterns of the two modifications were measured, and three of the strongest lines (2θ/° and relative intensity (I/I0)) for each modification are given, relatively, 25.52 (100), 14.38 (56), 27.95 (47); 26.58 (100), 14.33 (6), and 54.77 (3).