4-cyano-4'-ethyl-bipbenyl (2 CB) undergoes a reversible distortive phase transition at 290.8 K (transition enthalpy 0.9 kJ/mol). The crystal and molecular structures of both phases were determined. Below the transition point 2CB crystallizes in the monoclinic space group P2(1)/c (phase I) with a = 8.42(1), b = 5.94(1), c = 23.15(2) angstrom, beta = 92.93(1)-degrees-(T = 223 K), Z = 4,1328 non-zero reflections, final R = 0.052, Rw = 0.047. The higher temperature modification (phase II) is also monoclinic, space group P2(1) with a = 8.589(2), b = 5.864(1), c = 11.835(3) angstrom, beta = 92.42(1)-degrees-(T = 298 K), Z = 2,795 non-zero reflections, final R = 0.076, Rw = 0.067. The lattice parameters for both phases I and II have been determined as a function of temperature from 173 K to 308 K. The crystal structures of both modifications are very similar: The molecules are arranged in layers perpendicular to [001]. At the phase transition, alternating layers of biphenyl units undergo a approximately 55-degrees flip around the long axis of the molecules. Thus, in phase I every layer is related to the neighbouring ones through a center of symmetry. In phase II adjacent layers are identical, so the layer thickness equals the length of the unit axis c. The lattice parameters for 4-cyano-4'-methyl-biphenyl (1 CB, phase II) are a = 15.71(8), b = 14.38(8), c = 9.61(6) angstrom, beta = 95.0(2)-degrees, (T = 296 K), space group P2(1)/n, Z = 8. The compound is isostructural to some known 4,4'-disubstituted biphenyls.
