Knowledge Bank

A set of reasonable assumptions permits the formulation of an algebraic expression for the MCD signs of those $\Pi \Pi \ast$ transitions in cyclic conjugated molecules which can be, at least remotely, related to the $Lb,La,$ and $Ba$ states of their perimeter. The signs for a wide variety of monocyclic and polycyclic hydrocarbons, heterocycles, and their substituted derivatives, neutral and charged, can be predicted upon inspection of these expressions without any computations, using simple qualitative notions of MO theory. Classification of cyclic $\Pi$-electron chromophores, based on expected response to substitution, is proposed. The simple model has been tested on low-energy transitions in well over a hundred compounds. Examples of comparison with experiment will be given. For instance, a physical basis is offered for the well-known empirical correlation of MCD signs and Intensities with Hammett $\sigma p$ constants in the case of benzene derivatives. The experimental spectra also provide a safer identification of the controversial $Lb$ state of anthracene and reveal new excited states in several molecules, e.g., azulene. A rationale is offered for the apparently insignificant role played by magnetic $\Pi \Pi \ast -n\Pi \ast$ mixing, and for some of the good fortune noted in previous numerical PPP work (small number of significant contributions to low-energy B terms, negligible origin-dependence).