π-Hole Halogen Bonds Are Sister Interactions to σ-Hole Halogen Bonds

Halogen’s σ-hole interactions are found throughout the chemical, biological, and crystal engineering literature. These interactions are directional and may be of van der Waals type, weak, strong, or even very strong, with binding energies ranging between −0.01 and −100 kcal mol–1. When occurring between interacting neutral molecular entities, the energy range is often between −0.01 and −8.0 kcal mol–1. This study was undertaken to show there is crystallographic evidence that a covalently bonded halogen in a molecule can present with p/π-holes that are capable of engaging in attractive interactions with nucleophiles on an identical, or dissimilar, neighboring molecule, and that the strength of the interaction between the p/π-hole halogen bonded dimers can vary between −4.0 and −12.0 kcal mol–1 depending on the mode of that interaction and whether other interactions are involved. To this end, density functional theory at the M06 level with and without the effect of dispersion, in conjunction with the aug-cc-pVTZ (aug-cc-pVTZ-PP for iodine) basis set, suggested for the study of halogen bonding, was employed. Various charge density-based models were applied and the results of bonding modes are elucidated. Analysis of molecular electrostatic surface potentials revealed the electrophilicity of the σ- and p/π-holes on the surfaces of covalently bonded halogens in molecules, which were useful indicators for the formation of σ- and p/π-holes halogen bonding interactions in the dimers studied. Individual interactions identified may be weak, but since they are collective, they strengthen the geometry of the dimers formed. Furthermore, periodic calculations of two crystalline systems (CF3ICl2 and CF3IClF) reveal that they are direct bandgap materials, indicating the importance of halogen bonding in the design and subsequent growth of such materials for application in optoelectronics.