THEORETICAL STUDIES ON THE ELECTRONIC SPECTRA OF SUBSTITUTED AROMATIC MOLECULES: PART I. THE INTRAMOLECULAR ELECTRON TRANSFER MODEL

A simple empirical model, based on molecular orbital theory without configuration interaction, is used to interpret the K band transitions of substituted aromatic molecules as involving intramolecular electron transfer between substituent and hydrocarbon. For electron-donating substituents, such as amino, methoxyl, chloro, bromo, and iodo, the transfer is from substituent to hydrocarbon, while for electron-attracting substituents, such as nitro, formyl, acetyl, and cyano, the charge transfer is from hydrocarbon to substituent. Such a treatment is successful for "long field molecules" (benzene, diphenyl, terphenyl, styrene, and stilbene), but for "round field" molecules agreement between experiment and theory was achieved only for electron-donating substituents. The theory, like less empirical theories, overestimates the relative effect of a substituent in the 2-position compared with substitution in the 1-position.