DFT study of ionization potentials for aza-substituted aromatic rings

The ionization potentials of 20 azines and azanaphthalenes have been studied using Becke's three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Paar (B3LYP) method with 6-31G** basis set at optimized geometries. Vertical ionization potentials (IPv) have been calculated using Koopman's theorem (IPv = -epsilon(HOMO)) and as the difference of single-point energy calculations for the corresponding cation and for the neutral form (IPv = E-cation - E-neutral). From our theoretical results most of the first ionizations come from the nitrogen lone pair and not from the highest occupied v-orbital. Good agreement is found with experimental results when IPv is calculated from unrestricted B3LYP calculations of the neutral and the cationic species. The first pi-ionization follows Koopman's theorem for all monocyclic and bi-cyclic systems under study. The lone pair repulsion effect is acceptably described by the energy of the highest n-orbital energy either for azines and azanaphthalenes, The annulation effect in the charged species is not reflected in the n-lone pair orbital energy of the neutral form and, consequently, it is not possible to find a fit that follows Koopman's theorem for lone pair orbital energies, (C) 2002 Wiley Periodicals, Inc.