Exploring the Competition between Halogen Bonding and CH Hydrogen Bonding in Bromoarenes Using an Aryldiyne Template.

Hydrogens connected to benzene rings functionalized with electron-withdrawing groups are found to provide attractions with electron donors that are competitive with the halogen bond attraction to bromines on the same rings. An aryldiyne bridge that adequately templates this CH hydrogen bonding, along with the competing halogen bonding, provides an experimental pathway for looking at these interactions in a systematic way. Calculations using the M06-2x/6-311+G-(2d,p) density functional and basis set, along with mapping of the molecular electrostatic potentials using B3LYP/6-311++G**, support the experimental conclusion that CH hydrogen bonding to bromoarenes can be preferred to halogen bonding due to the greater positive potential on the surface of the hydrogen compared to that of the bromine. Computational evidence suggests a preference for the CH hydrogen bond conformer of the templated system regardless of the arrangement of electron-withdrawing (-F or -CF3) substituents on the haloarenes. At ∼5 kJ/mol or less, the energy differences between CH hydrogen bonding conformers and bromine-based halogen bonding conformers are often small, however, suggesting that selective crystal design using either interaction would be challenging.