Comparison of the AIM and Hirshfeld totals, σ, and π charge distributions: A study of protonation and hydride addition processes

A comparison of atomic charges has been carried out using two different atomic partitionings: one provided by the theory of Atoms in Molecules (AIM) and one based on the Hirshfeld scheme. The systems studied were the neutral and protonated forms of O- or N-containing compounds, some of them bearing two functional groups. The results obtained with both partitionings display important discrepancies, in particular, for the charges of the C-X groups (X = O or N), for their variation upon protonation, and for the final charge of the proton. However, separating the a from the π changes upon protonation, and a clear correlation was found for the π contributions that were calculated using the AIM and Stockholder schemes. This allowed the conclusion that the classical resonance forms do not agree with the π charge variations (obtained with both the AIM and Hirshfeld methods) that are produced by protonation. In fact, though the π charges calculated with both methods are different, they provide the same conclusions about the π charge redistribution upon protonation. The main difference between the results obtained with both partitionings pertains to how the σ electron population is distributed among the atoms of the molecule.