π-Electron systems: Recent developments in electronic spectroscopy and the PPP model

Abstract

The PPP model gives good account of the recently measured MCD signs and polarizations of ππ* transitions in hydrocarbons. The previously predicted mirror-image relation between the MCD spectra of two species paired in the sense of alternant symmetry has now been verified experimentally. Consideration of pairing properties suggests a classification of π-electron chromophores into “hard,” whose MCD sign pattern is intrinsic and little affected by substituents, and “soft” (those paired with themselves), whose MCD sign pattern is dominated by the substituents and heteroatoms present. In many instances, prediction of the substituent-induced signs requires no more than an inspection of the Hückel orbitals of the parent hydrocarbon. Agreement with experiment is very good for low-energy transitions.