Proton affinity of substituted naphthalenes

Abstract

The absolute proton affinity (PA) of aromatic carbons of monosubstituted naphthalenes with CH3, OH, CHO, NO2 and Cl substituents was calculated at the MP2(fc)/6–31G**//HF/6–31G* + ZPVE(HF/6–31G*) level of theory. Increments corresponding to unsubstituted positions within the naphthalene skeleton were estimated. They can be used in estimating PAs of polysubstituted naphthalenes by using a simple additivity rule based on the independent substituent approximation (ISA). It is shown that increments are good indicators of the electrophilic substitution reactivity. The proton affinities of a large number of polysubstituted methylnaphthalenes was examined employing the additivity equation. It was found that the protonated forms, which exhibit the largest PAs, correspond to arenium ions observed by NMR spectroscopy in superacid media. Copyright © 1999 John Wiley & Sons, Ltd.