Cooperative properties of halogen bonds were investigated with computational experiments based on dispersion-corrected relativistic density functional theory. The bonding mechanism in linear chains of cyanogen halide (X-CN), halocyanoacetylene (X-CC-CN), and 4-halobenzonitrile (X-C6 H4 -CN) were examined for X = H, Cl, Br, and I. Our energy decomposition and Kohn-Sham molecular-orbital analyses revealed the bonding mechanism of the studied systems. Cyanogen halide and halocyanoacetylene chains possess an extra stabilizing effect with increasing chain size, whereas the 4-halobenzonitrile chains do not. This cooperativity can be traced back to charge separation within the σ-electronic system by charge-transfer between the lone-pair orbital of the nitrogen (σHOMO ) on one unit and the acceptor orbital of the C-X (σ*LUMO ) on the adjacent unit. As such, the HOMO-LUMO gap in the σ-system decreases, and the cooperativity increases with chain length revealing the similarity in the bonding mechanisms of hydrogen and halogen bonds.
Keywords: MO theory; cooperativity; energy decomposition analysis; halogen bonding; non-covalent interactions.
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