Effects of Halogen Bond, Hydrogen Bond, and π-Tetrel Bond on the Internal CC Bond of Halogenated Ethylene

Depending upon the level of halogenation, the electrostatic potential of the π region above the molecular plane of an alkene can vary over a wide range and can be either positive or negative. The ability of H₂CCHX, H₂CCFX, and F₂CCFX, where X=F, Cl, Br, I, to interact with either a nucleophile or electrophile, is studied by DFT calculations. This binding is compared with the σ-hole CX···N halogen bonding with NH₃. The negative potential above H₂CCHX facilitates ClH···π H-bonding with HCl, while NH₃ can bind to the positive π-hole above F₂CCFX; the partially halogenated H₂CCFX is able to act as both nucleophile and electrophile. The introduction of a σ-hole XB acts to generally weaken the CC bond of H₂CCHX, but the effect is less unambiguous for the more highly halogenated alkenes. These effects are reversed upon the formation of a π-tetrel bond with NH₃. The largest perturbations are associated with the π-HB, which causes a sizable CC bond weakening. Combination of two sorts of intermolecular bonds is cooperative when the central alkene acts as both electron donor and acceptor and anticooperative when it acts as double acceptor.