Halogen−π Interactions between Benzene and X₂/CX₄ (X = Cl, Br): Assessment of Various Density Functionals with Respect to CCSD(T)

Various types of interactions between halogen (X) and π moiety (X−π interaction) including halogen bonding play important roles in forming the structures of biological, supramolecular, and nanomaterial systems containing halogens and aromatic rings. Furthermore, halogen molecules such as X₂ and CX₄ (X = Cl/Br) can be intercalated in graphite and bilayer graphene for doping and graphene functionalization/modification. Due to the X−π interactions, though recently highly studied, their structures are still hardly predictable. Here, using the coupled-cluster with single, double, and noniterative triple excitations (CCSD­(T)), the Møller–Plesset second-order perturbation theory (MP2), and various flavors of density functional theory (DFT) methods, we study complexes of benzene (Bz) with halogen-containing molecules X₂ and CX₄ (X = Cl/Br) and analyze various components of the interaction energy using symmetry adapted perturbation theory (SAPT). As for the lowest energy conformers (S1), X₂–Bz is found to have the T-shaped structure where the electropositive X atom-end of X₂ is pointing to the electronegative midpoint of CC bond of the Bz ring, and CX₄–Bz has the stacked structure. In addition to this CX₄–Bz (S1), other low energy conformers of X₂–Bz (S2/S3) and CX₄–Bz (S2) are stabilized primarily by the dispersion interaction, whereas the electrostatic interaction is substantial. Most of the density functionals show noticeable deviations from the CCSD­(T) complete basis set (CBS) limit binding energies, especially in the case of strongly halogen-bonded conformers of X₂–Bz (S1), whereas the deviations are relatively small for CX₄–Bz where the dispersion is more important. The halogen bond shows highly anisotropic electron density around halogen atoms and the DFT results are very sensitive to basis set. The unsatisfactory performance of many density functionals could be mainly due to less accurate exchange. This is evidenced from the good performance by the dispersion corrected hybrid and double hybrid functionals. B2GP-PLYP-D3 and PBE0-TS­(Tkatchenko-Scheffler)/D3 are well suited to describe the X−π interactions adequately, close to the CCSD­(T)/CBS binding energies (within ∼1 kJ/mol). This understanding would be useful to study diverse X−π interaction driven structures such as halogen containing compounds intercalated between 2-dimensional layers