Koopmans' theorem for acidic protons.

The famous Brønsted acidity, which is relevant in many areas of experimental and synthetic chemistry, but also in biochemistry and other areas, is investigated from a new perspective. Nuclear electronic orbital methods, which explicitly account for the quantum character of selected protons, are applied. The resulting orbital energies of the proton wavefunction are interpreted and related to enthalpies of deprotonation and acid strength in analogy to the Koopmans' theorem for electrons. For a set of organic acids, we observe a correlation which indicates the validity of such a NEO-Koopmans' approach and opens up new opportunities for the computational investigation of more complex acidic systems.