Low-temperature-induced reversible single-crystal-to-single-crystal phase transition of 3,4-dichloro-2 ',4 ',6 '-triethylbenzophenone

The polymorphic phase transition of 3,4-dichloro-2',4',6'-triethylbenzophenone (1) that occurs on cooling of the crystal has been investigated by single-crystal X-ray diffraction. The temperature at which transition from alpha-(high temperature phase, HTP) to beta-form (low temperature phase, LTP) occurred was determined as -117 degrees C by differential scanning calorimetry (DSC). The diffraction experiments indicated a reversible single-crystal-to-single-crystal phase transition. The a-form crystallizes in the orthorhombic space group Pbca with cell parameters a = 17.0159(6), b = 10.8347(4), and c = 19.1795(9) angstrom at -100 degrees C. The beta-form belongs to the monoclinic space group P2(1)/a with cell parameters a = 10.860(2), b = 17.249(2), and c = 18.531(1) angstrom, and b = 96.916(4)degrees at -160 degrees C. The crystal structure of the alpha-form consists of one crystallographically independent molecule, whereas the beta-form consists of two crystallographically independent molecules. X-Ray diffraction provides detailed information on the molecular structural changes and on the large rotation of the p- and o-ethyl groups between -100 degrees C and -160 degrees C. In the crystal there are weak interactions such as pi-pi, CH-pi, and Cl-pi interactions.