MOLECULAR-STRUCTURE AND INTRAMOLECULAR MOTION OF (E)-STILBENES IN CRYSTALS - AN INTERPRETATION OF THE UNUSUALLY SHORT ETHYLENE BOND

Crystal structures of (E)-2,2'-dimethylstilbene (2), (E)-2,2',4,4'-tetramethylstilbene (3), (E)-2,2',5,5'-tetramethylstilbene (4), (E)-2,2',4,4',5,5'-hexamethylstilbene (5), and (E)-2,2',3,3-tetramethylstilbene (6) were determined at several temperatures by X-ray diffraction. Analyses of these results and also of those reported for (E)-stilbene (1) and its related compounds revealed that the X-ray structures of compounds having the (E)-stilbene skeleton commonly show an unusually short bond length for the ethylene bond and a strong temperature dependence for the molecular structure. No sign confirming these anomalies could be detected in solution by NMR or UV spectroscopy. It is concluded that the short ethylene bond in the X-ray structures of these compounds is an artifact caused by dynamic averaging originating from the torsional vibration of the C-Ph bonds, during which the movement of the benzene rings is restrained to be a minimum. The observed temperature dependence of the ethylene bond length and angles and of the torsion angles of the C-Ph bonds is ascribed to the slight energy difference between the conformers, which interconvert by the torsional vibration. It has also been revealed that the rotational vibration of the benzene rings around the normal axes through C1 and C1' is a characteristic motion of (E)-stilbenes in the crystalline state.