The Electronic Spectrum of Aromatic Molecules VI: The Mesomeric Effect

Abstract

The mesomeric effect is discussed in terms of a mixing of the `locally excited' states of the hydrocarbon, and the `electron transfer' states. If the substituent behaves as an electron donor, the energies of the lowest electron transfer states can be estimated from the ionization potential of the substituent. A substituent which has a small ionization potential will be expected to exert a strong mesomeric effect, since the electron transfer states have low energies. These states may give rise to new bands in the spectrum. The spectrum of aniline is discussed in these terms, and the band at 2350 Å is attributed to an excitation to an electron transfer state. If the ionization potential is large, then, in general, there will be only a weak mesomeric effect. In this case it is only possible to estimate the change in energy of the lowest electronic states. A method is described whereby the inductive effect can be allowed for, and this is applied to find the mesomeric effect of a methyl substituent from the various methylated benzenes.