We will investigate the structure of 4-chloroimidazole, which undergoes extreme anisotropic thermal expansion. This is believed from our initial x-ray results to be due to a particular combination of weak C-H---Cl and strong N-H---N hydrogen bonds. Both the interatomic contacts and ADPs obtained from the SXD experiment will be used to provide a detailed description of the mechanism responsible for this behaviour. The ADPs will be used in a rigid body analysis to distinguish between the high-frequency internal modes from low-frequency external modes. and will be complemented by an off-line Raman study. In addition to the fundamental interest in further our understanding of extreme anisotropic expansion the knowledge obtained will be of use in designing functional materials, since one may tune the response of a thermally-driven actuator or switch.