Separation of halogenated benzenes enabled by investigation of halogen-π interactions with carbon materials

The halogen-π (X-π) interaction is an intermolecular interaction between the electron-poor region of bonded halogen atoms and aromatic rings. We report an experimental evaluation of the halogen-π (X-π) interaction using liquid chromatography with carbon-material coated columns providing strong π interactions in the normal phase mode. A C₇₀-fullerene (C70)-coated column showed higher retentions for halogenated benzenes as the number of halogen substitutions increased as a result of X-π interactions. In addition, the strength of the X-π interaction increased in the order of F < Cl < Br < I. Changes to the UV absorption of C70 and the brominated benzenes suggested that the intermolecular interaction changed from the π-π interaction to X-π interaction as the number of bromo substitutions increased. Computer simulations also showed that the difference in dipole moments among structural isomers affected the strength of the π-π interaction. Furthermore, we concluded from small peak shifts in ¹H NMR and from computer simulations that the orbital interaction contributes to the X-π interactions. Finally, we succeeded in the one-pot separation of all isomers of brominated benzenes using the C70-coated column by optimizing the mobile phase conditions.