Characterization of aromatic hydrocarbons by means of average local ionization energies on their molecular surfaces

The average local ionization energies (i) over bar(s)(r) and the electrostatic potentials V-s(r) have been computed on the molecular surfaces of nine aromatic hydrocarbons, at the Hartree-Fock HF/6-3 1G**//HF/STO-3G and Kohn-Sham B3P86/6-31G**//HF/ STO-3G levels. The surface areas, the fractions covered by positive and negative potentials, and their average values are quite similar by both computational procedures. The average values of (i) over bar(s)(r) and its minima, (i) over bar(s,min) (which indicate the most reactive electronic sites), are smaller in magnitude at the Kohn-Sham level, but the differences from the Hartree-Fock are fairly uniform and the minima occur at essentially the same positions. In nearly all instances, therefore, the same qualitative predictions result from either approach. Most of the (i) over bar(s,min) are found in bond regions and indicate varying degrees of olefin-like behavior. Some (i) over bar(s,min) are located near carbons, and it is proposed that these reflect some radical character. Supporting evidence is cited. (C) 2000 Elsevier Science B.V. All rights reserved.