Turowska-Tyrk Ilona, Trzop Elzbieta
Department of Chemistry, Wrocław University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland.
Acta Crystallogr B. 2003 Dec;59(Pt 6):779-86. doi: 10.1107/s0108768103023255. Epub 2003 Nov 25.
The structural changes in a crystal of 9-methylanthracene (1) during the [4 + 4] photodimerization were monitored by means of X-ray diffraction. This is the first example in the literature of such a study of a [4 + 4] photodimerization. The results obtained were compared with data for the [2 + 2] photodimerization. The shape of the product molecules and their preferred packing can explain the crystal disintegration. This was the reason that the reaction was monitored only to 28% completion. As far as could be determined the reaction proceeds with a constant rate. The cell volume increases at the beginning of the transformation and decreases afterwards. The product molecules do not assume a fixed position in the crystal during the photo-reaction, but move in a smooth way that includes a rotational component. The movements of the reactant are much smaller. Movements of molecules characterized by a rotational component were also observed in the case of the [2 + 2] photodimerization of 5-benzylidene-2-benzylcyclopentanone and 5-benzylidene-2-(4-chlorobenzyl)-cyclopentanone. The distance between the reacting atoms of the adjacent monomer molecules of (1) decreases with the degree of reaction completion, but more slowly than in the case of the [2 + 2] photodimerizations cited above. The orientation of the neighbouring monomer molecules changes during the phototransformation so that the monomer pair resembles the dimer product.
通过X射线衍射监测了9-甲基蒽（1）晶体在[4 + 4]光环化二聚反应过程中的结构变化。这是文献中对[4 + 4]光环化二聚反应进行此类研究的首个实例。将所得结果与[2 + 2]光环化二聚反应的数据进行了比较。产物分子的形状及其优选堆积方式可以解释晶体的分解。这就是该反应仅监测至28%完成度的原因。据测定，反应以恒定速率进行。在转变开始时晶胞体积增大，之后减小。在光反应过程中，产物分子在晶体中并未占据固定位置，而是以一种包含旋转分量的平滑方式移动。反应物的移动则小得多。在5-亚苄基-2-苄基环戊酮和5-亚苄基-2-（4-氯苄基）-环戊酮的[2 + 2]光环化二聚反应中也观察到了具有旋转分量特征的分子移动。（1）相邻单体分子反应原子之间的距离随反应完成程度而减小，但比上述[2 + 2]光环化二聚反应的情况要慢。在光转化过程中，相邻单体分子的取向发生变化，使得单体对类似于二聚产物。
