Average local ionization energies on the molecular surfaces of aromatic systems as guides to chemical reactivity

Abstract

The average ionization energy, , is introduced and is demonstrated to be useful as a guide to chemical reactivity in aromatic systems. is rigorously defined within the framework of self-consistent-field molecular orbital (SCF-MO) theory and can be interpreted as the average energy needed to ionize an electron at any point in the space of a molecule. An abinitio SCF-MO approach has been used to calculate at the 6-31G* level, using STO-3G optimized geometries. has been computed on molecular surfaces defined by the contour of constant electronic density equal to 0.002 electrons/bohr³, for a series of aromatic systems. This surface provides site specific predictions for preferred positions of electrophilic aromatic substitution. Relative reactivity toward electrophiles increases as the magnitudes of the smallest values for these systems decrease. An excellent relationship, with a correlation coefficient of 0.99, has been found between the Hammett constants and ; this allowed us to predict the values of these constants for the substituents NHF and NF₂, for which they were previously not known. Keywords: average local ionizations energy, chemical reactivity, electrophilic aromatic substitution, molecular surfaces, Hammett constants.