Quasistatic Disorder of NH···N Bonds and Elastic-Properties Relationship in 2-Phenylimidazole Crystals

Abstract

The molecular aggregation in 2-phenylimidazole is analogous to that observed in the new class of NH···N bonded ferroelectrics and relaxors. Disordered H-atoms in NH···N hydrogen-bonded chains in the average crystal structure of 2-phenylimidazole persist to 100 K, but the relaxation time of this process is very long even above 340 K. Above 200 K, a gradual increase of the electric permittivity along chains testifies to the activation of dipolar fluctuations, associated with proton transfers in the quasistatic disordered chains. However, the chemical shifts of nitrogen atoms (equivalent according to the average crystal symmetry) remain clearly distinguished as protonated and unprotonated in 15N NMR spectra up to 340 K at least. The 2-phenylimidazole crystal exhibits an unusual negative-linear thermal expansion (NTE) in the entire 140–340 K range, induced by subtle rotations of the NH···N bonded molecules. The NTE direction is the softest in the hydrostatically compressed crystal, which violates the inverse ...