Effect of external electric field on C-X ··· π halogen bonds

In this study, ab initio calculations (RI-MP2(full)/aug-cc-pVDZ) are performed to investigate the effect of an external electric field (EEF) on the nature, properties, and structures of C-X ··· π halogen bonds in CF₃Br complexes with π systems (benzene, ethene, and ethyne), for the first time. This EEF effect is analyzed by a myriad of methods, including molecular electrostatic potential (MEP), symmetry adapted perturbation theory (SAPT), natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM), and noncovalent interaction (NCI) methods. A linear relationship is found between RI-MP2 interaction energy and the strength of the EEF, indicating that the stability of C-X ··· π halogen bonds is sensitive to both the strength and direction of the EEF. According to the SAPT analyses, when the EEF is applied along the +z direction (perpendicular to the π plane), the nature of C-X ··· π halogen bonds transforms gradually from dispersion to electrostatic for the CF₃Br ··· benzene complex and from electrostatic to more electrostatic for the other complexes. However, when the EEF is applied along the -z direction, the C-X ··· π halogen bonds in all the complexes tend to be more dispersive in nature. The QTAIM analysis exhibits that the CF₃Br ··· benzene complex under the EEF with strength < 0.005 au is formed by the C-X ··· π_C3 and C-X ··· π_ring halogen bonds, while it has only the C-X ··· π_C3 halogen bond when the strength of the EEF is > 0.005 au. The structural results of the studied complexes show an inverse dependence of intermolecular distance between the CF₃Br and π system on the strength of the EEF.