Strengths, nature and spectral implications of the σ–n <i>vs</i>. σ–π halogen bonding between diiodine and aromatic amines: a computational study

The strengths and nature of the σ-n and σ-π bondings were examined in a series of complexes between diiodine and aromatic <i>para</i>- or <i>meta</i>-substituted N,N-dimethylamines, XDMA (X = Me₂N, MeO, H, F, Br, and CN). Computational analysis showed that, while the most negative electrostatic potentials on the surfaces of the aromatic amines were located over the substituents X and/or the centers of the aromatic rings, diiodine formed the strongest σ-n bonds with the amino groups and somewhat weaker σ-π bonds with the aromatic carbons. Energy decomposition analysis indicated that the electrostatic attraction represented the largest contributor to bonding, and the molecular orbital interaction accounted for about one-third of the attractive energy in all complexes. The stronger orbital interactions involved in the bonding of I₂ with the amino group led to higher UV-vis absorption band energies (by approximately 1.2 eV) of these σ-n associations, compared to the σ-π complexes formed by the same pair of molecules. Although both σ-n and σ-π bondings were accompanied by charge transfer from the aromatic amine to diiodine, these interactions resulted in markedly different structural and spectroscopic (IR and ¹H NMR) changes in the electron donors. In particular, σ-n bonding of I₂ with the amino group led to a lengthening of the C-N bonds between the aromatic ring and nitrogen, a decrease in the stretching frequency <i>ν</i>(CN), and downfield shifts of the aromatic proton signals. In contrast, σ-π bonding of I₂ with the aromatic ring resulted in C-N bond shortening, a decrease in <i>ν</i>(CN), and upfield shifts of the NMR signals of the protons attached to the carbons involved in halogen bonding. The distinct characteristics of the σ-n and σ-π complexes presented in the current work will facilitate the analysis of the experimental spectral data in different types of supramolecular bondings in chemical and biochemical systems.