Conformational Interconverion of Stilbene-type of Compounds in Crystals.

虽然已知分子在晶体中是运动的，但大多数是围绕平衡结构的小振幅热振动或整个分子的旋转振动。大振幅的分子内运动或构象间转换是罕见的。这项研究揭示了在某些化合物的晶体中发生的使分子形状发生很大变化的构象相互转换。对于二苯乙烯类化合物的晶体，一种取向紊乱，其中两个分子围绕它们的最长轴以180个<s:1>旋转的方式彼此近似相关。虽然这种疾病已经被反复研究，但它是动态的还是静态的仍然不清楚。本研究在变温x射线晶体学分析的基础上揭示了这种无序是动态的。因此，随着温度的降低，无序性消失。这种紊乱是由构象转换引起的，在此过程中踏板运动发生。在N-Ph键周围发生扭转振动，使得两个苯环保持彼此平行，并且苯环与晶格的方向不会改变。当扭转振动的振幅变得很大时，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具有另一种构象的构象。当两种构象在晶体中具有不同的稳定性时，其居数表现出温度依赖性。这种现象与有机晶体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.