Predicting the halogen-n (n = 3-6) synthons to form the "windmill" pattern bonding based on the halogen-bonded interactions.

: The "windmill" pattern cyclic halogen polymers (XBr)3 (X = Cl, Br, I) and (BrY)n (n = 3-6, Y = Cl, Br, I) have been investigated using the density functional theory. Due to the anisotropic distribution of its electron density, the halogen atom can form halogen-bonded interactions by functioning as both electron donor sites and electron acceptor sites. For (XBr)3 (X = Cl, Br, I) trimers, the Cl···Cl interaction is the weakest, and the I···I interaction is the strongest. For (BrY)n (n = 3-6, Y = Cl, Br, I), the Br···Br halogen bonds are the strongest in (BrY)4 tetramers. We predict that the iodine-4 synthon may allow creation of a self-assembled island during crystal growth. The angle formed by the electron-depleted sigma-hole, the halogen atom and the electron-rich equatorial belt perpendicular to the bond direction, together with the halogen-bond angle, can be used to explain the geometries and strength of the halogen-bond interactions. © 2018 Wiley Periodicals, Inc.