Explaining the sequence of protonation affinities of cytosine with QTAIM

The results obtained in the QTAIM electron density analysis carried out on B3LYP/6-31G(d, p) full optimized electron densities of the main protonated forms of cytosine indicate that the most stable ones display the largest electron transferences from hydrogens to the ring atoms. QTAIM results contradict the description of the resonance model, but are in line with a proposed qualitative scheme that relates the variations of atomic electron populations to their relative position with regard to the proton.