Conformational Interconverion of Stilbene-type of Compounds in Crystals.

虽然已经知道分子在晶体中运动，但大多数是围绕平衡结构的微幅热振动或整个分子的转动振动。大幅度的分子内运动或构象间的转换是罕见的。这项研究揭示了某些化合物的晶体中发生了构象转换，分子形状由此发生了很大的变化。对于二苯乙烯类化合物的晶体来说，一种取向无序，其中两个分子围绕其最长轴旋转180ﾟ，彼此近似相关。尽管对这种障碍进行了反复的研究，但尚不清楚这种障碍是动态的还是静态的。本研究在变温X射线晶体分析的基础上，揭示了该无序是动态的。因此，这种无序随着温度的降低而消失。这种无序是由发生踏板运动的构象相互转换引起的。在N-Ph键周围发生扭转振动，使得两个苯环保持彼此平行，并且苯环向晶格的取向不改变。当扭转振动的幅度变大时，具有两种取向之一的分子通过偶氮单元(C-N=N-C)垂直于苯环的构象转变为另一种构象。如果两种构象在晶体中具有不同的稳定性，则两种构象的布居表现出温度依赖性。这一现象与有机晶体中的固态性质和反应性有关。

Although it has been known that molecules are in motion in crystals, most of them are samll amplitude thermal vibrations around equilibrium structures or rotational vibrations of whole molecules. Large amplitude intramolecular motions or conformational interconversions are rare. This study reveals that conformational interconversions by which molecular shapes change greatly take place in the crystals of certain compounds. For the crystals of stilbene-type compounds, an orientational disorder in which two molecules are approximately related to one another by 180ﾟrotation around their longest axes. Although the disorder has been studied repeatedly, it has remained unclear whether the disorder is dynamic or static. This study revealed on the basis of variable temperature X-ray crystallographic analyses that the disorder is dynamic. Thus, the disorder disappears with lowering the temperature. The disorder is accounted for by conformational interconversion during which a pedal motion occurs. The torsional vibration around the N-Ph bonds occurs so that two benzene rings remain parallel to each other and the orientation of the benzene rings to the crystalline lattice do not change. When the amplitude of the torsional vibration becomes much larger, a molecule with one of two orientations changes into the the conformer with the other conformation through the conformation in which the azo unit (C-N=N-C) is perpendicular to the benzene rings. The populations of the two conformers show a temperature dependence if two conformers have different stabilities in crystals. This phenomenon is of interest in relation to solid state properties and reactivities in organic crystals.