Crystal structure of 4-chloro-2-methyl phenol and its infrared study on phase transition

The crystal structure of 4-chloro-2-methyl phenol was determined, revealing that the crystal belongs to the orthorhombic system with space group P2(1) 2(1) 2(1), a = 20.330 (3), b = 22.399 (4), c = 4.578 (1) A, and z = 12. Molecules stack with benzene rings almost parallel and form two types of hydrogen bond along the c axis. Infrared. spectra were studied in relation to the phase transition below room temperature. The bandwidth of COH bends change following the activation process above T-c and the critical feature below T-c, indicating disorder in COH groups and an ordering due to the long-range interaction, respectively. The infrared intensity change of OH torsions supports also the order-disorder type of phase transition of the second order and T-c = 188 K.