On the treatment of LUMO energies for their use as descriptors

Calculated energies of lowest unoccupied molecular orbitals (LUMOs) are frequently employed as descriptors in studies of quantitative structure-activity relationships and linear free energy relationships involving electron transfer. However, the quantum chemical programs with which these are carried out, whether Hartree-Fock or density functional theory, do not treat orbitals of different character, for example, C=C π(∗) and C-Cl σ(∗), consistently, nor is there consistency among different families of compounds. These problems can be ameliorated with the use of the experimental equivalent of the LUMO energy, the vertical attachment energy (VAE), or by shifting and scaling LUMO energies to a training set of available VAEs in similar compounds. Examples from the literature are used to illustrate these points.