1896 Acta Cryst. (1981). B37, 1896-1900 Structure of 1,3,5-Triehloro-2,4,6-trifluorobenzene BY S. L. CHAPLOT,* G. J. MclNXYRE,t A. MIERZEJEWSKI:~ AND G. S. PAWLEY Department of Physics, University of Edinburgh, The King's Buildings, Mayfield Road, Edinburgh EH9 3JZ, Scotland (Received 12 February 1981; accepted 25 March 1981) Abstract The crystal structure of the symmetric 1,3,5-trichloro- 2,4,6-trifluorobenzene, C6CIaF a, was determined at room temperature by single-crystal X-ray diffraction and at 4.2 K by neutron powder diffraction. The space group is P63/m with a hexagonal primitive cell of dimensions a = b = 8.541 (3) and c = 6.282 (2)/k [which become a = b = 8.441 (2) and c = 6.052 (2)/k at 4.2 K], V = 397/k 3 and Z = 2. The cell contains two planar molecules at 6 (2d) sites. The single-crystal R factor of 3.9% was reached with unconstrained refinement on 401 symmetry-independent Bragg intensities. The structure was solved using constrained refinement on the molecular orientation. Constrained refinement on the thermal parameters has indicated significant inter- nal vibrations in the molecule. The low-temperature study showed there to be no phase transition when the powder is cooled. Introduction The structure determination of sym-C6Cl3F3 was undertaken as part of a project to study the effect of intermolecular forces in systems comprising rigid molecules. In this paper we present the evidence for molecular distortion due to these forces, and show that the internal molecular vibrations contribute signifi- cantly to the thermal motion as evidenced through structural studies. A detailed study of the lattice vibrations is planned, for which a large single crystal has been prepared suitable for inelastic coherent neutron scattering measurements. The similarity of the molecular shape of sym- C6C13F 3 with that of sym-triazine suggests that the possibility of a phase transition should not be over- looked. Preliminary measurements by Raman scatter- ing and also measurements reported here of powder * On leave from Nuclear Physics Division, Bhabha Atomic Research Centre, Bombay400085, India (now returned). ~" Present address: Institute of Chemistry, Universityof Uppsala, Box 531, $751 21, Sweden. On leave from Department of Organic and PhysicalChemistry, Technical University,Wroctaw, Poland (now returned). neutron diffraction at helium temperatures, however, give no indication of a transition. The structure found for sym-C6ClaF3 is different from either phase of sym-triazine (Smith & Rae, 1978), although there are close similarities with the upper phase of sym-triazine in that some of the trigonal molecular symmetry is retained in the crystal structure. Experimental A commercial sample of C6C13Fa was first purified by zone refining. Since the solid, which melts at about 330 K, sublimes very quickly in open conditions, it was necessary to grow a crystal for the X-ray diffraction in confined cofiditions. The crystal was grown in a thin Lindemann-glass capillary tube of 0.5 mm diameter. The tube containing the sample was slowly moved through a small loop of heating wire in which the heating current was adjusted so as to melt a very small length of samp!e in the tube. When a long crystal was obtained of sufficient quality as could be judged with a polarizing microscope, material was allowed to sublime from both ends until a suitable small length of about 0.5 mm remained. The capillary tube and the heating loop were enclosed in a small transparent cover to reduce air convection currents and also to slow the sublimation process. The tube was actually mounted on a moving arm which facilitated the slow movement of the tube through the fixed heating loop. The tube containing the finished sample was sealed at both ends, but some material was left in the tube at a safe distance from the crystal. In the course of the experiment the trapped residue of material sublimed through imperfec- tions in the seals, and in this way gave protection to the sample proper. The measurements of Bragg intensities were made on an Enraf-Nonius CAD-4 four-circle X-ray diffractom- eter in the oy--20 mode. Mo Ka radiation was used with a graphite (002) monochromator. The primitive-cell dimensions were refined from measurements on 25 independent Bragg reflections. Then nearly 4000 reflections were measured in three zones of reciprocal space. The measurements of individual intensities were so timed that they all had about the same estimated 0567-7408/81/101896-05501.00 © 1981 InternationalUnion of Crystallography