3.2.4 | Topological Properties
The topological and energetic properties at the BCPs of the interactions between the two molecules in the six dimers were analyzed by comparing the following parameters; the electron density ρ (b), the Laplacian of electron density ∇2ρ (b), the kinetic electron energy density Gb, the potential electron energy density Vb, the sum of Gb and Vb (Hb) and the ratio of Gb to Vb(-Gb/Vb). The results are collected in Table 6. A positive value of ∇2ρ (b) implies an interaction between closed shell complexes; ionic interaction, van der Waals force, or hydrogen bonding, while a negative value of ∇2ρ (b) indicates a shared interaction as in a covalent bond48. Rozas’s study49concluded that ∇2ρ (b) and Hbmay be useful in characterization of the strength of interactions. This means that for weak A…B interactions, where ∇2ρ (b) > 0 and Hb> 0, the interactions are mainly electrostatic; for medium strength interactions, where∇2ρ (b) > 0 and Hb < 0, the interactions are partly covalent in nature, while strong interactions show ∇2ρ (b) < 0 and Hb< 0, these are characteristically covalent. Measures of the covalency in noncovalent interactions include the kinetic electron energy density Gc (positive), the potential electron energy density Vc(negative), and the ratio -Gc/Vc. Values of -Gc/Vc > 1 generally indicate a noncovalent interaction, whereas when -Gc/Vc is < 1, the interaction is covalent in nature. For the dimers investigated here (in Table 6), all ∇2ρ (b) values are positive, the Hc values are positive, and the -Gc/Vc values are greater than 1. This means that these interactions belong to weak interactions of an electrostatic nature.