Unusual Thermal Expansion in Hydrogen Bonded Network of an Organic Salt Induced by Large Transverse Vibration of Atoms

Thermal expansion of an organic salt, prepared by solvent-drop grinding of terephthalic acid and imidazole in a 1:2 ratio, has been studied over a wide range of temperature 100-430 K. Restricted motion of the components in the hydrogen-bonded network resembles a fence-like structure led to a uniaxial negative thermal expansion (NTE). Interestingly, N-H center dot center dot center dot O hydrogen bonding distances remain almost unchanged over the temperature of study. Large transverse vibration of atoms involved in the hydrogen bonding plays an unprecedented role to keep the distances constant.