Invisible but common motion in organic crystals: A pedal motion in stilbenes and azobenzenes

Some molecules that have a molecular skeleton similar to stilbenes and azob enzenes are known to show an orientational disorder in the crystals. Some o f the disorders are known to be dynamic and mediated by a pedal motion in c rystals. Dynamic processes of (E)-stilbene (1) and (E)-3,3',4,4'-tetramethy lazobenzene (2) in the crystals were investigated by X-ray diffraction anal yses. The dynamic disorder and the pedal motion were detected at higher tem perature, even in the molecules that showed no traces of the disorder at ro om temperature. The results demonstrated that the pedal motion should alway s be taken into account, even if no disorder is detected. The reasons for t he nonoccurrence of the disorder and for the prevalence of the pedal motion are also discussed.