σ-aromaticity in hexa-group 16 atom-substituted benzene dications: A theoretical study

C₆I₆²⁺ has been reported to have a σ-aromatic character since removal of two σ anti-bonding electrons localized on iodines results in fulfilling Hückel (4n+2) rules for I ₆²⁺ as well as C₆ parts. To search for molecules possessing similar character, hexa-group 16 atom-substituted benzene dications C₆(ChH)₆²⁺ (Ch = S, Se, Te) and their derivatives are examined for aromatic character by using nucleus-independent chemical shift (NICS). For these dications, in which iodines in C ₆I₆²⁺ are replaced by group 16 atoms, negative NICS values larger in magnitude than for benzene are found when a σ anti-bonding orbital localized on group 16 atoms is unoccupied. To clarify the origin of large negative NICS values, they are decomposed into individual molecular orbitals. It has been shown that both π bonding orbitals on C ₆ and σ bonding orbitals on Se₆ or Te₆ contribute to the negative NICS values, indicating that the aromaticity of these dications have a substantial σ character as well as π characters. Aromaticity of group 14 and 15 atom-substituted benzene dications is also discussed.