Natural bond orbital, nuclear magnetic resonance analysis and hybrid-density functional theory study of σ-aromaticity in Al2F6, Al2Cl6, Al2Br6 and Al2I6
Date
2013-02-11
Journal Title
Journal ISSN
Volume Title
Publisher
Journal of Molecular Modeling
Abstract
Natural bond orbital (NBO), nuclear magnetic res onance (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], σAl(1)-X2(b) →
σ*Al(3)-X4(b) electron delocalizations and the dissociation ener gies 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 mag netic 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 corre sponding 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 σAl(1)-X2(b) →σ*Al(3)-
X4(b) electron delocalizations. The decrease of the stabilization
energies associated with σAl(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 dis sociation energies of Al2F6, Al2Cl6, Al2Br6 and Al2I6, σAl(1)-
X2(b) →σ*Al(3)-X4(b) electron delocalizations, natural atomic
orbitals (NAOs) and NICSiso values have been investigated.
Description
Keywords
chemical engineering