DFT study of ionization potentials for aza‐substituted aromatic rings

Abstract

AbstractThe 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 = −εHOMO) and as the difference of single‐point energy calculations for the corresponding cation and for the neutral form (IPV = Ecation − Eneutral). From our theoretical results most of the first ionizations come from the nitrogen lone pair and not from the highest occupied π‐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 π‐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. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003