*2.4. Electron Density Analyses*

The electron densities of the equilibrium and transition structures were analyzed using the quantum theory of atoms in molecules (QTAIM) methodology. All complexes have two bond critical points. In the equilibrium structures, the first bond critical point is associated with the covalent O–H bond with ρBCP values of 0.30 au, while the second refers to the hydrogen bond with ρBCP values around 0.05 au. The transition structures have intermediate values of ρBCP between 0.15 to 0.20 au. The <sup>2</sup>ρBCP values are negative for the covalent O–H bonds in the equilibrium and transition structures but positive for the hydrogen bonds. The total energy densities, HBCP, are negative in all structures. Thus, all O–H contacts have some covalent character. The covalency is of medium strength in the equilibrium complexes that have a positive <sup>2</sup>ρBCP and a negative value of HBCP, while the transition structures have much stronger covalent interactions with a negative value of both <sup>2</sup>ρBCP and HBCP [47,48]. Excellent exponential correlations are obtained between ρBCP and HBCP versus the interatomic distance, as illustrated in Figure S1, in agreemen<sup>t</sup> with other reports of these parameters as descriptors of intermolecular interactions [49–51].
