A Theoretical and Structural Investigation of Thiocarbon Anions

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 C_nS _n²- and C_nS_n (n = 3-6) electron delocalization. Localized molecular orbital contributions to NICS, computed by the individual gauge for localized orbitals method, dissect π effects from the σ single bonds and lone pair influences. C_nS _n^2- (n = 3-5) structures in D_nh symmetry are minima. Their aromaticity decreases with increasing ring size. C ₃S₃^2- is both σ and π aromatic, while C₄S₄^2- and C₅S₅ ^2- are much less aromatic. NICS(0)π, the C-C(π) contribution to NICS(0) (i.e., at the ring center), decreases gradually with ring size. In contrast, cyclic C₆S₆^2- prefers D_2h 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 D_6h minima was confirmed by X-ray structure analysis. Comparisons between thiocarbons and oxocarbons based on dissected NICS analysis show that C_nS_n ^2- (n = 3-5) and C₆S₆^6- are less aromatic in D_nh symmetry than their oxocarbon analogues.