MCD of Nonaromatic Cyclic π-Electron Systems. 3. The Perimeter Model for Low-Symmetry “Unaromatic” and “Ambiaromatic” Molecules Derived from 4 N -Electron [ n ]Annulenes

The LCAO version of the perimeter model with overlap through second order is used to treat the ππ* electronic absorption and magnetic circular dichroism (MCD) of low-symmetry molecules with a closed-shell ground state and no degenerate states (no threefold or higher order axis) derived from biradical (antiaromatic) parent 4 N -electron [ n ]annulene perimeters by structural perturbations. If a symmetry plane perpendicular to the molecular plane is present, simple explicit algebraic solutions are obtained. Rules are derived for predicting the intensities, polarizations, and MCD signs of low-energy transitions in this class of molecules from the knowledge of relative magnitudes of MO energy differences, which can be frequently deduced by mere inspection of molecular formulas. On the basis of the results, a generalized nomenclature is proposed for low-energy electronic excited states of all even-electron cyclic π systems with a single perimeter.