THE CENTRAL BOND-LENGTH IN 1,2-DIPHENYLETHANES

Crystal structures of 1,2-diphenylethane (1) and 1,2-bis(2-methylphenyl)ethane (3) were determined at different temperatures by X-ray diffraction. The observed length of the central C-C bond in 1 at 240 K (1.506(5) Angstrom) appears to be considerably shorter than the standard value of a C(sp(3))-C(sp(3)) bond (1.541(3) Angstrom). The observed length, however, substantially increases with lowering of the temperature. On the contrary, the ethane bond length in 3 (1.534(5) Angstrom and 240 K) is normal and remains constant at different temperatures. It is concluded that the anomaly in the ethane bond length of 1,2-diphenylethanes, which has been attempted to be explained in terms of various electronic effects, is an artifact caused by the torsional vibration of the C-Ph bonds in crystals.