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.