Nature of E2X2 σ(4c-6e) of the X---E-E---X type at naphthalene 1,8-positions and model, elucidated by X-ray crystallographic analysis and QC calculations with the QTAIM approach.

The nature of E2X2 σ(4c-6e) of the X-*-E-*-E-*-X type is elucidated for 1-(8-XC10H6)E-E(C10H6X-8')-1' [(1) E, X = S, Cl; (2) S, Br; (3) Se, Cl; (4) Se, Br] after structural determination of (1), (3) and (4), together with model A [MeX---E(H)-E(H)---XMe (E = S and Se; X = Cl and Br)]. The quantum theory of atoms-in-molecules dual functional analysis (QTAIM-DFA) is applied. The total electron energy densities Hb(rc) are plotted versus Hb(rc) - Vb(rc)/2 for the interactions at the bond critical points (BCPs; *), where Vb(rc) show the potential energy densities at the BCPs. Data for the perturbed structures around the fully optimized structures are employed for the plots, in addition to those of the fully optimized structures. The plots were analysed using the polar coordinate (R, θ) representation of the data of the fully optimized structures. Data containing the perturbed structures were analysed by (θp, κp), where θp corresponds to the tangent line of the plot and κp is the curvature. Whereas (R, θ) shows the static nature, (θp, κp) represents the dynamic nature of interactions. E-*-E are all classified as shared shell (S) interactions for (1)-(4) and as weak covalent (Cov-w) in nature (S/Cov-w). The nature of pure CS (closed shell)/typical-HB (hydrogen bond) with no covalency is predicted for E-*-X in (1) and (3), regular CS/typical-HB nature with covalency is predicted for (4), and an intermediate nature is predicted for (2). The NBO energies evaluated for E-*-X in (1)-(4) are substantially larger than those in model A due the shortened length at the naphthalene 1,8-positions. The nature of E2X2 of σ(4c-6e) is well elucidated via QTAIM-DFA.