Structural Properties Of Hexaphenyl Powder Under High Pressure
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arrangement shows outstanding optical properties[ 1,2]. The crystal structures are monoclinic with space group P2,/a. The lattice constants are determined to be a= 8.1A, and b z-5.6A, with the lattice constant c depending on the length of the oligomer ranging from 9.5A for biphenyl to 30.6A for septiphenyl[3,4]. The direction of the chains is chosen as the c-axis. b)The crystal structure of hexaphenyl Early attempts of structure determination of hexaphenyl (C 36 H26) were done by X-ray powder patterns and by Weissenberg photographs[5,6]. Their results are in reasonable agreement with single crystal investigations [4]: monoclinic unit cell, space group P2,/a with a=8.091A, b=5.568A, =98.170. The refinement factor of the structure solution was c=26.24A, and the monoclinic angle P3 0.062; since the molecule is centrosymmetric, the positions of only 18 C-atoms and 13 H-atoms have to be refined. The distances between the C-atoms vary between 1.322 - 1.507A , a value of 1.41 A is expected for aromatic bonds of carbon. The distances between a C- and the adjacent H-atom are determined as 0.95A in comparison to the expected distance of 1.08A; it is known that single crystal refinements slightly underestimate the C-H distances[7]. A sketch of the crystal structure of hexaphenyl is shown in Fig. 1. The positions of the atoms are taken from ref. [4]. The large circles represent carbon atoms and the small circles represent hydrogen. The long axis of the unit cell (c-axis) connects two layers of hexaphenyl molecules. Each layer is represented by two molecules: one in projection, the other stretched (Fig. 1 (a)). The plane of the two short crystallographic axes of the unit cell (ab-plane) is parallel to the hexaphenyl layers. 867
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
The monoclinic angle P3 is determined by the relative position of two hexaphenyl molecules of neighboring layers which are in the same orientation, that is, the angle of the ab-plane with the c-axis. The centrosymmetry of the space group P2,/a is expressed by the symmetry of the hexaphenyl molecule passing the origin of the unit cell with the center of the molecule (top-right molecule in Fig.1 (a)). The symmetry element 2,/a of the space group generates the second hexaphenyl molecule of the layer (bottom-right molecule). The arrangement of the molecules within one layer is shown in Fig. 1(b). The molecules are projected along their long axes; within one molecule the larger inner circles represent carbon atoms, the smaller outer circles are hydrogen atoms. The molecules are arranged in a herringbone pattern within one layer. Figures 1 (a) and (b) show that within the crystal structure solution the molecules are flat. However, molecular simulations on free hexaphenyl molecules reveal a force that tilts the phenyl rings relative to each other. It is caused by the repulsion of the ortho-hydrogens. On the other hand the 7t-electron system tends to planarize the molecule. An average tilt angle of 45' is determined for free biphenyl molecul
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