THE CENTRAL BOND-LENGTH IN 1,2-DIPHENYLETHANES

Crystal structures of 1,2-diphenylethane (1) and 1,2-bis(2-methylpheny l)ethane (3) were determined at different temperatures by X-ray diffra ction. The observed length of the central C-C bond in 1 at 240 K (1.50 6(5) Angstrom) appears to be considerably shorter than the standard va lue of a C(sp(3))-C(sp(3)) bond (1.541(3) Angstrom). The observed leng th, however, substantially increases with lowering of the temperature. On the contrary, the ethane bond length in 3 (1.534(5) Angstrom and 2 40 K) is normal and remains constant at different temperatures. It is concluded that the anomaly in the ethane bond length of 1,2-diphenylet hanes, which has been attempted to be explained in terms of various el ectronic effects, is an artifact caused by the torsional vibration of the C-Ph bonds in crystals.