Nature of Halogen Bonds: An Interacting Quantum Atoms (IQA) and Relative Energy Gradient (REG) Study

Halogen bonds are non-covalent interactions in which a covalently bonded halogen atom (X) in a molecule ( ) acts as an electrophile and interacts with a nucleophilic site (Y) in the same or another molecule [1]. The halogen bonds tendency toward linearity is the most important features of them. The halogen bonds are highly directional and the R-X-Y angle, θ, tends toward 180° [2]. The interacting quantum atoms (IQA) scheme [3] is used to investigate the orientation preference and directional features of the halogen bonds in the (X= Cl and Br) model complexes. The relative energy gradient (REG) method[4] is also applied to rank these components and find the term(s) that best describe the total behaviour of the system. The linear structures are energetically the most stable geometries and the complexes become less stable as the C-X-N angle deviates from linearity. To find the origin of the directionality in the studied halogen bonds, a scan of θ from 180° to 90° in the steps of 10 degrees was performed. Halogen-nitrogen interaction (i.e. the halogen bond) is attractive and more favourable in the linear arrangement, however, its REG shows that this term plays a small role in the total behavior of the system. Instead, the secondary repulsive interactions (e.g. fluorine-nitrogen, carbonhydrogen interactions) and also atomic self-energies are mainly responsible for the directionality of halogen bond.