Fluorine as a Lewis acid: A symmetry-adapted perturbation theory based on density functional theory and interacting quantum atoms study of noncovalent interactions in the NCF⋯NH3 complex.

Abstract

Different computational methods are used to investigate the nature of interaction in the NCF⋯NH3 model complex, in which the fluorine atom acts as a Lewis acid and forms a noncovalent bond with the ammonia (Lewis base). Symmetry-adapted perturbation theory based on density functional theory (SAPT(DFT)) indicates that the noncovalent interaction in the NCF⋯NH3 complex is mainly electrostatics. However, dispersion and induction terms also play important roles. Although fluorine noncovalent interactions are typically classified as halogen bonds, they are somewhat different from the well-known halogen bonds of iodine, bromine, and chlorine. The halogen bonds of NCCl⋯NH3 and NCBr⋯NH3 are directional and the C  X  N (X = Cl or Br) angle tends to be linear. In contrast, the fluorine interaction in NCF⋯NH3 is not directional; the interaction energy shows no sensitivity to the angular (C  F  N) distortions, and the energy profile is flat over a wide angular range (from 180° to about 140°). However, for the angles less than 130°, the energy curve shows a clear angular dependence and the interaction between NCF and NH3 becomes stronger as the C  F  N angle decreases. It seems that at the tighter angles, a tetrel-bonded NCF⋯NH3 complex is preferred. Moreover, interacting quantum atoms (IQA) analysis shows that the competition between different intra-atomic and interatomic interactions determines the geometry of NCF⋯NH3 complex.