Unusual Bond Length of Organic Molecules in Crystals

点击翻译按钮获取中文摘要

Recently we found an unusual shortening of the ethlene bond and a significant temperature dependence of the molecular structure for the X-ray structures of (E)-stilbenes. This phenomenon was ascribed to an artifact that originates from the large amplitude torsional motion of the C-Ph bonds. The puroposes of this project are to get further evidence for our interpretation reported previously and to the generality of the phenomenon.X-ray crystallographic analyzes of the compound having the (E)-stilbene skeleton in which the internal rotation of the C-Ph bonds is severely restricted by the alkylene chains. The observed ethylene bond length is 1.35-1.36 @90, independent of the temperature, which agrees well with the length estimated from the molecular mechanics calculatiojns. The results make a sharp contrast with those for ordinary (E)-stilbenes and strongly support the interpretation that unusual short ethlene 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 ethlene bond in (E)-stilbenes is safely estimated to be 1.35-1.36 @90.Crystal structures of 1,2-diphenylethanes were determined at different temperatures by X-ray diffraction. The observed length of the central C-C bond of 1,2-diphenylethane at 240 K (1,506 (5) @90) appears to be considerably shorter than the standard value of a C (sp^3)-C (sp^3) bond 1,541 (3) @90. The observed length, however, substantially increases with lowering the temperature. 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.