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

Abstract

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 α- (high temperature phase, HTP) to β-form (low temperature phase, LTP) occurred was determined as −117 °C by differential scanning calorimetry (DSC). The diffraction experiments indicated a reversible single-crystal-to-single-crystal phase transition. The α-form crystallizes in the orthorhombic space group Pbca with cell parameters a = 17.0159(6), b = 10.8347(4), and c = 19.1795(9) Å at −100 °C. The β-form belongs to the monoclinic space group P2₁/a with cell parameters a = 10.860(2), b = 17.249(2), and c = 18.531(1) Å, and β = 96.916(4)° at −160 °C. The crystal structure of the α-form consists of one crystallographically independent molecule, whereas the β-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 °C and −160 °C. In the crystal there are weak interactions such as π–π, CH–π, and Cl–π interactions.