Combination of Negative, Positive, and Near-Zero Thermal Expansion in Bis(imidazolium) Terephthalate with a Helical Hydrogen-Bonded Network
Sylwia Zięba, Andrzej Gzella, Alina T. Dubis, Andrzej Łapiński
Abstract
A variable-temperature single-crystal X-ray diffraction study revealed an unusual thermal expansion property of a bis(imidazolium) terephthalate salt with a helical hydrogen-bonded network. The anomalous thermal expansion was investigated by X-ray diffraction, infrared, and Raman spectroscopy techniques. Positive, negative, and near-zero thermal expansion were observed along the crystallographic b, c, and a axes, respectively. The anomalous thermal expansion can be explained at the atomic level by the "bond-rotation effect". The hydrogen-bonded, flexible network in the crystal structure resembles a folding trellis that undergoes a scissorlike motion, resulting in abnormal thermal behavior. Thermal expansion induced by a scissor motion of the helical hydrogen-bonded network in a crystalline organic compound has not been reported before.