Unusual Bond Length of Organic Molecules in Crystals

最近，我们发现(E)-二苯乙烯类化合物的X-射线结构中乙烯键的异常缩短以及分子结构与温度的显着依赖关系。这一现象归因于C-Ph键的大幅度扭转运动产生的伪影。这个项目的目的是为我们之前报道的解释和这一现象的一般性获得进一步的证据。X-射线晶体分析具有(E)-二苯乙烯骨架的化合物，其中C-Ph键的内部旋转受到亚烷链的严重限制。实验测得的乙烯键长为1.35-1.36@90，与分子力学计算的结果一致。结果与普通(E)-二苯乙烯的结果形成鲜明对比，并有力地支持了(E)-二苯乙烯X射线结构中异常的乙烯短键是由晶体中C-Ph键的大幅度扭转运动引起的伪影的解释。(E)-二苯乙烯键的真实长度为1.35-1.36@90。用X射线衍射法测定了不同温度下1，2-二苯乙烷的晶体结构。在240K(1,506(5)@90)观察到的1，2-二苯乙烷的中心C-C键的长度似乎比C(sp^3)-C(sp^3)键的标准值1,541(3)@90短得多。然而，观察到的长度随着温度的降低而显著增加。结果表明，1，2-二苯基乙烷的乙烷键长反常是由晶体中C-Ph键的扭转振动引起的，并试图用各种电子效应来解释。

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.