Michl's Perimeter Model in MCD Spectroscopy

Abstract

A molecular orbital (MO) theory approach for conceptualising the structure and bonding of aromatic and antiaromatic π-systems is described, since this usually forms the basis of the analysis of the magnetic circular dichroism (MCD) spectroscopy of these compounds. The background to the use of MO theory based on Platt's pioneering research on the forbidden and allowed L and B bands of benzene and Moffitt's work on high symmetry cyclic perimeters is described. The nodal properties of the frontier π-MOs of structural homologues derived from the same high symmetry charged or uncharged CnHn perimeter are consistently retained and the method used to predict the optical properties can be simplified on this basis. Michl's perimeter models for heteroaromatic 4N+2 and antiaromatic 4N π-systems are introduced. Changes in the relative energies of the frontier π-MOs modify the intensities of the major electronic bands in a manner that can be readily predicted by these models. Changes related to the magnetic dipole properties of the optically accessible ππ* states can be studied by MCD spectroscopy, and sign sequences observed in the Faraday 1 or 0 bands can often be used to validate the results of theoretical calculations. The MCD spectroscopy and theoretical calculations of cyclobutadiene dianion (C4H42µ), zinc tetraphenyltetraacenapthoporphyrin, chlorins and benzofuran derivatives provide examples of the application of Michl's 4N+2 perimeter model, while a study of the electronic structures of neutral and reduced hexaphyrin(1.1.1.1.1.1)s provides an example of the application of both the 4N+2 and the 4N models.