MCD of nonaromatic cyclic pi-electron systems. 3. The perimeter model for low-symmetry "unaromatic" and "ambiaromatic" molecules derived from 4N-electron [n]annulenes

The LCAO version of the perimeter model with overlap through second order i s used to treat the pi pi* electronic absorption and magnetic circular dich roism (MCD) of low-symmetry molecules with a closed-shell ground state and no degenerate states (no threefold or higher order axis) derived from birad icai (antiaromatic) parent 4N-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 deriv ed for predicting the intensities, polarizations, and MCD signs of low-ener gy transitions in this class of molecules from the knowledge of relative ma gnitudes of MO energy differences, which can be frequently deduced by mere inspection of molecular formulas. On the basis of the results, a generalize d nomenclature is proposed for low-energy electronic excited states of all even-electron cyclic pi systems with a single perimeter.