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 CF3 Br 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 CF3 Br ··· 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 CF3 Br ··· benzene complex under the EEF with strength < 0.005 au is formed by the C-X ··· πC3 and C-X ··· πring halogen bonds, while it has only the C-X ··· πC3 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 CF3 Br and π system on the strength of the EEF.