Semi-empirical calculations of ?-electron affinities for some conjugated organic molecules

Abstract

Energy levels have been calculated for some conjugated systems containing C, N, and O atoms using a semi-empiricalmethod based upon a variableβ-γ modification of the Pariser-Parr-Pople approximation to the Hartree-Pock equation. Koopmans' theorem is used to relate the calculated energy of the lowest vacant molecular orbital, ɛLVMO, to the adiabatic electron affinity of a molecule. The approach is identical to that used previously by Kunii and Kuroda [13]. An excellent correlation is found between electron affinities deduced from recent beam experiments and ɛLVMO. This relationship is used to predict electron affinities for over 100 other organic molecules. In addition, excited state energies for negative ions are calculated, and good agreement is found with the available experimental data. Bound excited states are also predicted for some organics which contain the =C(CN)2 substructure. The additive contribution of group substitutions to the electron affinity is discussed for the case of CN substitutions to ethylene, benzene, and naphthalene.