Use of semi-empirical models for calculation of B terms in MCD spectra. PPP model for [4 N + 2] annulenes: a rule for substituent and heteroatom effects

Abstract

Using the PPP model at monoexcited CI level and several additional simplifying assumptions, the sign and magnitude of the lowest MCD (magnetic circular dichroic) band of a [4 N + 2] annulene of arbitrary size perturbed by one or more substituents and/or heteroatoms are evaluated in closed form and related to the difference between the energy separation of the two highest occupied molecular orbitals and the energy separation of the two lowest free molecular orbitals. This difference can be usually deduced by inspection from the nature and relative location of substituents, permitting facile sign predictions without computations. The results establish a relation to the perimeter model for polycyclic hydrocarbons and a relation between MCD and other forms of spectroscopy, in particular photoelectron transmission spectroscopy.