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The crystal structure of quaterphenyl has been refined from new data collected at room temperature on a single-crystal diffractometer with a program taking account of curvilinear motions in the structure factor expression. 777 independent reflexions were used to yield a final Rw value of 0-045. A comparison is made between thermal-motion tensors of quaterphenyl and p-terphenyl. Translational terms are very similar for the two molecules. Librational terms about the directions perpendicular to the long molecular axis are approximately inversely proportional to the moments of inertia. Those of quaterphenyl are half those of p-terphenyl. For librations about the long molecular axis, the quaterphenyl central rings exhibit a large amplitude, (~02)= 178(°) 2, which is however smaller than the exceptional value of (~02) = 260(0) 2 for p-terphenyl. This is interpreted as resulting from a dynamic disorder, the rings oscillating in a double-well potential. The observation at 110 K of a superstructure confirms this interpretation.

The crystal structure of the ordered phase ofp-quaterphenyl has been determined from X-ray diffraction data collected at 110 K on a single-crystal diffractometer. This superstructure is very similar to that of pterphenyl: the space group is also P1 (Z = 4) with a = 9.74 (2), b = 9.74 (2), c = 17.70 (3) A, a= 85.40 (10), fl = 94.60 (10), y = 110.11 (10) °. However, for comparison with the high-temperature monoclinic cell a', b', c',fl' (P2Ja, Z = 2) it is more convenient to choose a pseudomonoclinic cell a, b, c, fl centred on the face (001) with Z = 8 and a = 15.97 (3) (~_2a'), b = 11.16 (2) (~_2b'), c = 17.70 (3) A (_~c'), fl = 95.61 (8) ° (~fl'). However, unlike p-terphenyl molecules, the p-quaterphenyl molecules do not conserve their centre of symmetry at low temperature. The average torsion angles are 17.1 o between an external and an internal ring and 22.7 ° between two internal rings. All the long molecular axes remain parallel. As for pterphenyl, the interpretation of the experimental result I~k t = I~ t requires the existence of two sorts of domains in the crystal, related by a mirror or glide plane perpendicular to b and equally probable.

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