Chen Zhongfang, Sutton Liam R, Moran Damian, Hirsch Andreas, Thiel Walter, Schleyer Paul von Ragué
Computational Chemistry Annex, The University of Georgia, Athens, GA 30602-2525, USA.
J Org Chem. 2003 Nov 14;68(23):8808-14. doi: 10.1021/jo035144l.
Density functional theory energies, geometries, and population analyses as well as nucleus-independent chemical shifts (NICS) have been used to investigate the structural and magnetic evidence for cyclic CnSn(2-) and CnSn (n = 3-6) electron delocalization. Localized molecular orbital contributions to NICS, computed by the individual gauge for localized orbitals method, dissect pi effects from the sigma single bonds and lone pair influences. CnSn(2-) (n = 3-5) structures in Dnh symmetry are minima. Their aromaticity decreases with increasing ring size. C3S3(2-) is both sigma and pi aromatic, while C4S4(2-) and C5S5(2-) are much less aromatic. NICS(0)pi, the C-C(pi) contribution to NICS(0) (i.e., at the ring center), decreases gradually with ring size. In contrast, cyclic C6S6(2-) prefers D2h symmetry due to the balance between aromaticity, strain energy, and the S-S bond energies and is as aromatic as benzene. The theoretical prediction that C6S6(6-) has D6h minima was confirmed by X-ray structure analysis. Comparisons between thiocarbons and oxocarbons based on dissected NICS analysis show that CnSn(2-) (n = 3-5) and C6S6(6-) are less aromatic in Dnh symmetry than their oxocarbon analogues.
密度泛函理论能量、几何结构、布居分析以及核独立化学位移（NICS）已被用于研究环状CnSn(2-)和CnSn（n = 3 - 6）电子离域的结构和磁性证据。通过定域轨道个体规范方法计算得到的定域分子轨道对NICS的贡献，将π效应与σ单键和孤对电子的影响区分开来。Dnh对称性的CnSn(2-)（n = 3 - 5）结构为极小值。它们的芳香性随环尺寸增大而降低。C3S3(2-)既是σ芳香性的又是π芳香性的，而C4S4(2-)和C5S5(2-)的芳香性则弱得多。NICS(0)π，即C - C(π)对NICS(0)（即在环中心处）的贡献，随环尺寸逐渐减小。相比之下，环状C6S6(2-)由于芳香性、应变能和S - S键能之间的平衡而更倾向于D2h对称性，并且其芳香性与苯相当。X射线结构分析证实了C6S6(6-)具有D6h极小值的理论预测。基于剖析的NICS分析对硫代碳和氧代碳的比较表明，Dnh对称性下的CnSn(2-)（n = 3 - 5）和C6S6(6-)比它们的氧代碳类似物芳香性更弱。
