Substituent effects on the electronic absorption spectra of phenolphthalein and phenolsulphonphthalein monopositive ions

Abstract

The energy of the second band in the electronic absorption spectrum of phenolphthalein and phenolsulphonphthalein monopositive ions, substituted ortho to the two hydroxy-groups, varies linearly with the Hammett σmeta substituent constant and a value of 0·56 can consequently be estimated for the σmeta value of the sulphonic acid (–SO3H) group. The energy of the first band gives no such correlation. An explanation of these observations in terms of Extended Huckel Molecular Orbital calculations is attempted.