Pedal motion in crystals.

This tutorial review introduces pedal motion in crystals, where a pair of benzene rings in a molecule move like the pedals of a bicycle. The pedal motion triggers conformational interconversions, which result in disordered crystal structures. The pedal motion also plays important roles in solid-state reactions. This type of molecular motion occurs in a wide range of compounds, although the detection of the process is difficult in most cases. This review also describes how powerful X-ray diffraction analysis is in investigating dynamic processes in crystals, especially focusing on disorder analysis as a function of temperature.