UNUSUALLY SHORT ETHYLENE BOND AND LARGE-AMPLITUDE TORSIONAL MOTION OF (E)-STILBENES IN CRYSTALS - X-RAY CRYSTALLOGRAPHIC STUDY OF STIFF STILBENES

Crystal structures of (E)-4,4'-dimethyl-1,1'-biindanylidene, (E)-5,5'-dimethyl-1,1'-biindanylidene, (E)-6,6'-dimethyl-1,1'-biindanylidene, (E)-7,7'-dimethyl-1,1'-biindanylidene and (E)-2,2,2',2'-tetramethyl-1,1'-biindanylidene were determined by X-ray diffraction at 296 and 120 K. The observed ethylene bond length is 1.35-1.36 angstrom, independent of the temperature and the compound, which agrees well with the length estimated from the molecular mechanics calculations. The results make a sharp contrast with those for ordinary (E)-stilbenes and strongly support the interpretation that the unusual short ethylene bond in the X-ray structures of (E)-stilbenes is an artifact which originates from the large-amplitude torsional motion of the C-Ph bonds in crystals. The true length of the ethylene bond in (E)-stilbenes is safely estimated to be 1.35-1.36 angstrom.