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 n S n 2- and C n S 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 n S n 2- (n = 3−5) structures in D nh symmetry are minima. Their aromaticity decreases with increasing ring size. C3S3 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 2 h 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 6 h minima was confirmed by X-ray structure analysis. Comparisons between thiocarbons and oxocarbons based on dissected NICS analysis show that C n S n 2- (n = 3−5) and C6S6 6- are less aromatic in D nh symmetry than their oxocarbon analogues.