We will investigate the structure of 4-chloroimidazole a material, which undergoes extreme anisotropic thermal expansion,.as a function of pressure at a range of temperatures This expansion is believed to be the result of the combination of weak C-H---Cl and strong N-H---N hydrogen bonds. Data collected from PEARL will be used to correlate the changes at the atomic level in the strong N-H---N bonds and weak C-H---Cl bonds with the macroscopic changes in the lattice parameters. This will be achieved by carrying out a combination of structural refinements together with the measurement of the lattice parameters. In addition to the fundamental interest in the properties of hydrogen bonded systems with pressure the information gained from this experiment will be desirable in designing functional materials whose properties respond dramatically in response to external stimuli.