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.
Highlights
Many materials expand in all three directions when heated
This study presents an explanation of the negative thermal expansion (NTE) phenomenon observed for bis(imidazolium) terephthalate.[71,99]
Terephthalate and imidazolium ions are connected by hydrogen bonds N1b−H1b···O1a and N2b− H2b···O2ai into two-dimensional layers growing in parallel to the (−301) plane (Figure 1a)
Summary
Many materials expand in all three directions when heated. In contrast to isotropic materials, anisotropic materials can show different rates of thermal expansion in different directions. This phenomenon is called positive thermal expansion (PTE). Some materials show the opposite trend in thermal behavior.[1−4] This phenomenon is known as negative thermal expansion (NTE), caused by some structural peculiarities.[3,5] Some materials, such as a nitromethane solvate of 18-crown-6,6 involve contraction along with one (uniaxial)[7−9] or two (biaxial) (S,S)-octa-3,5-diyn-2,7-diol[10−12] principal axes of expansion, while other materials contract in overall volume.[13−15]
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