What Molecules Are Likely or Unlikely To Undergo Pedal Motions in Crystals?

The crystal structures of several
(E)-stilbenes
and 1,2-diarylethane that have methyl or chloro substituents were
examined by variable-temperature X-ray diffraction analysis. All the
compounds showed disorders in their crystal structures at least at
room temperature. In the crystal structures of the molecules with ortho-methyl groups, the disorder observed at room temperature
disappeared at 90 K. The dynamic disorders in these crystals were
explained in terms of the conformational changes induced by pedal
motions, which were not obstructed by the ortho-methyl
groups. In the crystals of the stilbenes with α-substituents,
however, the populations of the two disordered conformers did not
change with temperature, which indicates that the disorders were static
and these molecules did not undergo the pedal motions in their crystals.
Theoretical calculations on the energy barriers for the pedal motions
indicated that the observed restriction of the pedal motion in crystals
was originated not only in the intramolecular steric repulsions involving
the α-substituents but mainly in the steric repulsions between
the α-substituents and the surrounding molecules in the crystals.
The pedal motion can proceed only when the shapes of the molecules
do not largely change during the process.