Substituent Effects on the Electronic Ground State (Singlet versus Triplet) of Indenyl Cations: DFT and CASPT2 Studies

The electronic ground states of cations (singlet versus triplet) attract significant attention in the discovery of novel carbocation chemistry. Although the cyclopentadienyl cation exhibits a triplet ground state because of the antiaromaticity of the closed-shell singlet state, the ground state of the indenyl cation, which is a benzene-fused derivative of the cyclopentadienyl cation, is reportedly a closed-shell singlet state rather than a triplet state. In this study, in order to establish a strategy for controlling the ground state of indenyl cations, the substituent effects at the C2 and C5 positions of their structures were computed using density functional theory and complete active space multiconfiguration second order perturbation levels of theory.