Cooperative Self‐Assembly in Linear Chains Based on Halogen Bonds

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−C6H4−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. Longer is stronger! Halogen bonds in linear chains become increasingly stronger when the chain increases in size. The origin of this cooperativity effect can be traced back to the enhanced donor‐acceptor charge transfer interaction between the units in the chain.