MONOTROPIC CHARACTER OF POLYMORPHISM IN PENTACHLOROPYRIDINE CRYSTAL STUDIED BY CALORIMETRIC AND SPECTROSCOPIC METHODS

Pentachloropyridine crystals in two polymorphic modifications: tetrago nal and monoclinic were studied by differential scanning calorimetry a nd polarized IR and Raman spectroscopy in 20 to 200 cm(-1) region and at different temperatures from melting point down to the liquid nitrog en temperature. The 4 K difference in melting points of both forms ind icates the occurrence of monotropy. The spectra were discussed in term s of positional disorder in the studied crystals which often happen to be mixtures of the monoclinic crystal and the L- and D-forms of the t etragonal one. The monotropic transition of the metastable tetragonal crystal into the stable monoclinic one was observed in the Raman spect ra and under the polarizing microscope at temperatures ranging from 17 0 to 185 K and as an anomaly on the differential scanning calorymetry diagram at 193 K.