A novel electrostatic approach to substituent constants: Doubly substituted benzenes

The most negative-valued molecular electrostatic potential (MESP) minimum (V-min) observed over the benzene ring is proposed as a sensitive quantity for the analysis of the electronic perturbations due to the substituents attached to it. MESP topography of 45 doubly substituted benzenes is mapped at HF/6-31G** level for an appraisal of this proposition. The V-min values are seen to clearly reflect the changes due to the different orientations (para, meta, and ortho) and conformations of the substituents and different mechanisms of electron donation or withdrawal. Good linear correlations are obtained with V-min and the experimentally determined sigma values of the ortho- and meta-disubstituted benzenes. New quantities, D-p, D-m, and D-o, introduced in this work as the substituent pair-constants respectively for the para, meta, and ortho arrangements, provide a quantitative measure of the simultaneous effect of two substituents over the aromatic nucleus. The predictive power of these quantities is checked in the case of some triply substituted benzenes using an equation V-min = V-ben + Sigma Delta V-min(mono) - Sigma D-p - Sigma D-m - Sigma D-o where V-ben is the MESP minimum of benzene and Delta V-min(mono) is the difference between the monosubstituted benzene V-min and V-ben. These predicted values are in fairly good agreement with the MESP values obtained at HF/6-31G** level.