Lattice-Dynamical Study of the Abnormally Short Bond-Length in trans-Stilbene Molecules in a Crystal

Lattice-dynamics calculations have been made on crystalline trans-stilbene under an atom–atom potential approximation, using both flexible and rigid molecular models in order to clarify the existence of the molecular motion suggested for explaining the abnormal bond-length of the central C=C bond. The intramolecular twisting of the benzene rings strongly couples with the overall rotation of the molecule in some manner, including the suggested manner. The coupled modes form phonon branches and show little dispersion.