Magnetic circular dichroism of non-aromatic cyclic π-electron systems. 2. [1] The perimeter model for high-symmetry ‘unaromatic’ and ‘ambiaromatic’ molecules derived from 4 N-electron [ n]annulenes

Abstract

The effects on ππ* electronic absorption and magnetic circular dichroism (MCD) produced by structural perturbations that convert biradical (antiaromatic) parent 4 N-electron [ n]annulene perimeters into molecules with a closed-shell ground state and 3-fold or higher axis of rotational symmetry are analyzed in terms of an algebraic solution for singlet states of the LCAO version of the perimeter model with overlap through second order. Simple rules are derived for predicting the 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 inspection of molecular formulas.