Inverse Versus Normal Behavior of Interactions, Elucidated Based on the Dynamic Nature with QTAIM Dual-Functional Analysis

Abstract

In QTAIM dual-functional analysis, H_b(r_c) is plotted versus H_b(r_c) - V_b(r_c)/2 for the interactions, where H_b(r_c) and V_b(r_c) are the total electron energy densities and potential energy densities, respectively, at the bond critical points (BCPs) on the interactions in question. The plots are analyzed by the polar (R, θ) coordinate representation for the data from the fully optimized structures, while those from the perturbed structures around the fully optimized structures are analyzed by (θ_p, κ_p). θ_p corresponds to the tangent line of the plot, and κ_p is the curvature; θ and θ_p are measured from the y-axis and y-direction, respectively. The normal and inverse behavior of interactions is proposed for the cases of θ_p > θ and θ_p < θ, respectively. The origin and the mechanism for the behavior are elucidated. Interactions with θ_p < θ are typically found, although seldom for [F-I-∗-F]^-, [MeS-∗-TeMe]²⁺, [HS-∗-TeH]²⁺ and CF₃SO₂N-∗-IMe, where the asterisks emphasize the existence of BCPs in the interactions and where [Cl-Cl-∗-Cl]^- and CF₃SO₂N-∗-BrMe were employed as the reference of θ_p > θ. The inverse behavior of the interactions is demonstrated to arise when H_b(r_c) - V_b(r_c)/2 and when the corresponding G_b(r_c), the kinetic energy densities at BCPs, does not show normal behavior.

Keywords: ab initio calculations; inverse behavior of interactions; nonbonded interactions; normal behavior of interactions; quantum theory of atoms-in-molecules (QTAIM).