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

Abstract

The strengths and nature of the σ–n and σ–π bondings were examined in a series of complexes between diiodine and aromatic para- or meta-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 ν(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 ν(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.