Natural bond orbital, nuclear magnetic resonance analysis and hybrid-density functional theory study of σ-aromaticity in Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆.

Natural bond orbital (NBO), nuclear magnetic resonance (NMR) analysis and hybrid-density functional theory based method (B3LYP/Def2-TZVPP) were used to investigate the correlation between the nucleus-independent chemical shifts [NICS, as an aromaticity criterion], σ Al(1)-X2(b) → σ*Al(3)-X₄(b) electron delocalizations and the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆ to 2AlX₃ (X = F, Cl, Br, I). The results obtained showed that the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆ decrease from Al₂F₆ to Al₂I₆. Like aromatic molecules, these compounds have relatively significant negative NICSiso(0) values. Clearly, based on magnetic criteria, they exhibit aromatic character and make it possible to consider them as σ-delocalized aromatic species, such as Möbius σ-aromatic species. The σ-aromatic character which is demonstrated by their NICSiso(0) values decreases from Al₂F₆ to Al₂I₆. The NICSiso values are dominated by the in-plane σ₂₂ (i.e., σyy, the plane containing halogen atoms bridged) chemical shift components. The increase of the NICSiso values explains significantly the decrease of the corresponding dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆. Importantly, the NBO results suggest that in these compounds the dissociation energies are controlled by the stabilization energies associated with σ Al(1)-X2(b) →σ*Al(3)-X4(b) electron delocalizations. The decrease of the stabilization energies associated with σ Al(1)-X₂(b) →σ*Al(3)-X4(b) electron delocalizations is in accordance with the variation of the calculated NICSiso values. The correlations between the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆, σ Al(1)-X₂(b) →σ*Al(3)-X4(b) electron delocalizations, natural atomic orbitals (NAOs) and NICSiso values have been investigated.