A Crystallographic Study of Solid Benzene by Neutron Diffraction

A study by neutron diffraction of crystals of solid benzene at — 55 and — 135 °C has extended the earlier picture of the molecular structure provided by Cox’s X-ray work at — 3 °C. Neutron reflexions in four crystal zones have been examined, leading to values of the atomic co-ordinates and thermal parameters. Superimposed upon the random motion of the carbon and hydrogen atoms, there is an oscillatory angular motion of the molecule in its own plane for which the root-mean-square amplitudes are 7.9°, 4.9° and 2.5°, respectively, at the three different temperatures. At the same time there are small changes in the orientation of the molecules as the temperature is reduced, of such a kind that they pack together more easily along the Z axis, thus accounting for the enhanced thermal expansion along Z compared with X.