Benzene is the parent compound of organic solids based on C-H...pi interactions that have been used in organic field effect transistors. Any change in the C-H...pi interactions due to chemical environment or isotopic substitution can affect the electronic properties of these materials. Here we propose to study the effect of chemical substitution on the elastic properties of hexa-alogenated benzenes, by analysing the temperature-dependence of Atomic Displacement Parameters (ADP's) that allows the retrieval of lattice normal mode frequencies from diffraction data.