On the use of koopmans' theorem to estimate negative electron affinities

For 26 molecules the experimentally determined negative electron affinities resulting from electron transmission spectroscopy are compared to the theoretically determined eigenvalues of the lowest unoccupied molecular orbitais (ε{lunate}_LUMO) of the neutral compounds, and to the eigenvalues of the highest occupied molecular orbitals (ε{lunate}_HOMO) of the respective anion. With the 3-21G and 6-31G^* basis sets, the ε{lunate}_LUMO values are found to give qualitatively correct correlations in most, but not all, cases for molecules with similar structures. The ε{lunate}_LUMO data of the anions show a much poorer agreement with experimentally established trends. The inclusion of diffuse functions in the 6-31+G^* basis set leads to a complete breakdown in the correlation of experimental and theoretical data.