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

The effects on pi pi* electronic absorption and magnetic circular dichroism (MCD) produced by structural perturbations that convert biradical (antiaro matic) parent 4N-electron [n]annulene perimeters into molecules with a clos ed-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 v ersion of the perimeter model with overlap through second order. Simple rul es are derived for predicting the MCD signs of low-energy transitions in th is class of molecules from the knowledge of relative magnitudes of MO energ y differences, which can be frequently deduced by inspection of molecular f ormulas. (C) 1999 Elsevier Science B.V. All rights reserved.