Studies of the structure, thermal expansion and molecular vibrations of di- p -xylylene, C 16 H 16 , at 93 and 291 °K

Abstract

Recent accurate determinations of crystal structures have given Debye factors which show major differences between values for different atoms and in different directions. From these it has proved possible to deduce numerical data not only for the six principal translation and libration amplitudes, the molecule being treated as a rigid body, but also for the major distorting vibrations of the molecule. Di- p -xylylene is taken as an example. A refinement, at room and low temperatures, of the structure originally given by Brown, shows that there is a concertina movement of the two benzene rings towards and away from each other, accompanied by a corresponding up-and-down movement of the methylene bridges; and also a twisting movement of each benzene ring about its normal, out of parallelism with its opposite number. All vibration amplitudes are reduced at low temperature, but the pattern of vibrations is unchanged. The anisotropy of thermal expansion coefficients, measured by the X-ray method, is explained qualitatively; and diffuse scattering patterns are given. There is a discussion of the experimental accuracy necessary for the method to be applied, and the effect on bond-length measurements is considered.