Ethane bond length in 1,2-diphenylethanes

Crystal structures of 1,2-diphenylethane (1), 1,2-bis(4-dimethoxyphenyl)ethane (3), 1,2-bis(2,3-dimethylphenyl)ethane (4), 1,2-bis(2,4-dimethylphenyl)ethane (5) and 1,2-bis(2,4,6-trimethylphenyl)ethane (6) were determined by X-ray diffraction analysis at various temperatures. An apparent shortening of the ethane bond and its temperature dependence were observed and it was interpreted as an artifact caused by the torsional vibration of the C-Ph bonds in crystals. The extent of the shortening was dependent on the compounds and was explained in terms of the difference in the amplitude of the torsional vibration controlled by ortho methyl groups. In the crystal structures of 1, a so far unrecognized orientational disorder was observed at room temperature and 315 K. The disorder, which disappeared at lower temperatures, proved to be dynamic and was ascribed to a conformational interconversion through a very large amplitude torsional vibration. The disorder might be another reason for the exceptionally short ethane bond in 1.