The nature of BE–*–AE–*–AE–*–BE σ(4c–6e) is primarily elucidated at naphthalene 1,8-positions: while the weak covalent nature is predicted for all AE–*–AE, the HB nature with covalency or the CT-MC (MC formation through CT) nature is for AE–*–BE.

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AIM-DFA (AIM dual functional analysis) is applied to the E–E′ bonds (E, E′ = S and Se) in R-cystine (1), its derivatives and MeEE′Me. The nature of E–E′ is elucidated by (θp, κp: dynamic behavior) and (R, θ: static behavior), through AIM-DFA.

The nature of EX σ(4c-6e) of the X-*-E-*-E-*-X type is elucidated for 1-(8-XCH)E-E(CHX-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 H(r) are plotted versus H(r) - V(r)/2 for the interactions at the bond critical points (BCPs; *), where V(r) 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 (θ, κ), where θ corresponds to the tangent line of the plot and κ is the curvature. Whereas (R, θ) shows the static nature, (θ, κ) 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 EX of σ(4c-6e) is well elucidated via QTAIM-DFA.

The nature of the E–E’ bonds (E, E’ = S and Se) in glutathione disulfide (1) and derivatives 2–3, respectively, was elucidated by applying quantum theory of atoms-in-molecules (QTAIM) dual functional analysis (QTAIM-DFA), to clarify the basic contribution of E–E’ in the biological redox process, such as the glutathione peroxidase process. Five most stable conformers a–e were obtained, after applying the Monte-Carlo method then structural optimizations. In QTAIM-DFA, total electron energy densities Hb(rc) are plotted versus Hb(rc) − Vb(rc)/2 at bond critical points (BCPs), where Vb(rc) are potential energy densities at BCPs. Data from the fully optimized structures correspond to the static nature. Those containing perturbed structures around the fully optimized one in the plot represent the dynamic nature of interactions. The behavior of E–E’ was examined carefully. Whereas E–E’ in 1a–3e were all predicted to have the weak covalent nature of the shared shell interactions, two different types of S–S were detected in 1, depending on the conformational properties. Contributions from the intramolecular non-covalent interactions to stabilize the conformers were evaluated. An inverse relationship was observed between the stability of a conformer and the strength of E–E’ in the conformer, of which reason was discussed.