A NOVEL ELECTROSTATIC APPROACH TO SUBSTITUENT CONSTANTS - DOUBLY SUBSTITUTED BENZENES

The most negative-valued molecular electrostatic potential (MESP) mini mum (V-min) observed over the benzene ring is proposed as a sensitive quantity for the analysis of the electronic perturbations due to the s ubstituents attached to it. MESP topography of 45 doubly substituted b enzenes is mapped at HF/6-31G* level for an appraisal of this proposi tion. The V-min values are seen to clearly reflect the changes due to the different orientations (para, meta, and ortho) and conformations o f the substituents and different mechanisms of electron donation or wi thdrawal. Good linear correlations are obtained with V-min and the exp erimentally determined sigma values of the ortho- and meta-disubstitut ed benzenes. New quantities, D-p, D-m, and D-o, introduced in this wor k as the substituent pair-constants respectively for the para, meta, a nd ortho arrangements, provide a quantitative measure of the simultane ous effect of two substituents over the aromatic nucleus. The predicti ve power of these quantities is checked in the case of some triply sub stituted 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 mini mum of benzene and Delta V-min(mono) is the difference between the mon osubstituted benzene V-min and V-ben. These predicted values are in fa irly good agreement with the MESP values obtained at HF/6-31G* level.