含直鏈型硫化合物”2,5-Dimethyl-3,4-trimethylene-6a-thiathiophthene”之電子密度研究

Abstract

Thiathiophthene(TTP)為一具有兩個五圓雜環及硫-硫-硫直鏈的平面分子，由於此種分子擁有非常不尋常的的鍵結特性及芳香性質，因此引起了相當大的興趣與討論。為了嘗試瞭解此種獨特的鍵結情況，我們利用X-ray單晶繞射技術及DFT理論計算來研究其衍生物2,5-dimethyl-3,4-trimethylene-6a -thiathiophthene (C10H12S3)。我們分別在室溫(295K)及低溫(100K)收取了繞射數據，室溫時空間群C 2/m，而當溫度降低到100K時，空間群轉變成P 21/n，其原因是原本室溫時垂直二轉軸的鏡面消失了，此種對稱降低的情形也同樣地顯現在TTP的另一個衍生物2,5-Dimethyl-6a-thiathiophthene上。室溫時，兩硫硫之間的距離皆為2.33419(8)Å，但當溫度降到100K後，兩硫硫之間的鍵長明顯地變不一樣了，距離分別為2.3274(5)及2.3393(5) Å。為了更深入瞭解此鍵結特性，在實驗方面，利用X光單晶繞射的結果獲得分子的總電荷密度分佈後，引入Coppens等人所提出的多極模型精算理論，將球形原子模型擴展到非球形原子模型，使得電荷更符合分子內原子的狀態。在理論方面，我們利用密度泛函理論的方法求得分子的電子密度分佈，所選用的基底函數為B3LYP 6-31G**，硫的部份則多加一個擴散函數。由實驗與理論兩方面的結果配合拓樸學分析，使我們更瞭解分子的鍵結型態，也證實了中央的硫原子為電子密度較低的，兩側的硫為電子密度較高的。此外，利用費米孔洞分析也證明了兩個五圓環的芳香性及兩硫原子之間為稍弱的共價鍵性質。最後，我們再利用X光吸收光譜來進一步對不同的硫做探討。

Thiathiophthene(TTP), a planar molecule with two fused heterocyclic five-membered rings and essentially a linear S-S-S bond, is a molecule of great interest due to the unusual bonding characters and the possible aromatic properties of the two five-memberd rings. In order to understand the remarkable bonding properties, the electron density distribution of one of the derivatives, i.e. 2,5-dimethyl-3,4-trimethylene-6a -thiathiophthene (C10H12S3), was investigated both by single crystal X-ray diffraction and by DFT calculations. The X-ray crystal structure of C10H12S3 was studied both at 295 K and at 100 K. The Space group is C2/m at 295 K, which is transformed to P21/n at 100 K. The mirror symmetry perpendicular to 2-fold axis disappears at low temperature. Such reduction of symmetry elements was also found in a previous study on the 2,5-dimethyl-6a-thiathiophthene. The bond lengths of two S-S bonds are crystallographically same [2.3341(8)Å] at 295K, but are significantly different [2.3274(5) and 2.3393(5) Å] at low temperature. The experimental electron density is produced according to multipole model. The theoretical electron density is calculated by DFT calculation, where the basis set of 6-31G** is used for all the atoms but an additional diffuse function is added for S atom. Results on the electron density distribution will be presented in terms of deformation density, Laplacian maps and the topological properties. Sulfur K-edge X-ray absorption spectroscopy (XAS) is also undertaken to further our knowledge on the electronic configuration of S atom.