Noncovalent Interactions and Transverse Vibration Induced Restricted Thermal Expansion of Organic Salts Derived from Imidazole and Carboxylic Acids

Design of material showing contraction upon heating is highly challenging due to varying mechanism. However, imidazole is found to be a potential molecule that may provide low CTE materials when incorporated in the matrix. Here we have reported thermal expansion property of imidazolium salts of five aliphatic alpha, omega-alkane dicarboxylic acids and three aromatic acids. Either uniaxial or biaxial negative thermal expansion (NTE) has been observed in most of the salts. In some cases, axial zero thermal expansion (ZTE) has been observed. The role of imidazolium moiety for the anomalous thermal expansion behaviour of the salts has been analyzed in this study. The controlled TE behaviour of the salts is attributed to the hydrogen bonding and transverse vibration in all imidazolium salts. Owing to the high transverse vibration observed in imidazolium ion as well as the heavier oxygen atoms of acids in each case, the distance between hydrogen bonded atoms decreases-which provides either low expansion or contraction along one of the principal axes. Thermal expansion is controlled by transverse vibration of atoms in imidazolium moiety, oxygen atoms of carboxylic acids and noncovalent interactions in the crystal structures of organic salts derived from imidazole and carboxylic acids. Either uniaxial or biaxial negative thermal expansion (NTE) has been observed in most of the salts. In some cases, axial zero thermal expansion (ZTE) has been observed. image