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

The average ionization energy, Formula: see text, is introduced and is demonstrated to be useful as a guide to chemical reactivity in aromatic systems. Formula: see text 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 Formula: see text at the 6-31G* level, using STO-3G optimized geometries. Formula: see text has been computed on molecular surfaces defined by the contour of constant electronic density equal to 0.002 electrons/bohr 3 , for a series of aromatic systems. This surface Formula: see text provides site specific predictions for preferred positions of electrophilic aromatic substitution. Relative reactivity toward electrophiles increases as the magnitudes of the smallest Formula: see text values Formula: see text for these systems decrease. An excellent relationship, with a correlation coefficient of 0.99, has been found between the Hammett constants and Formula: see text; this allowed us to predict the values of these constants for the substituents NHF and NF 2 , for which they were previously not known. Keywords: average local ionizations energy, chemical reactivity, electrophilic aromatic substitution, molecular surfaces, Hammett constants.