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

Halogen bonds (indicated by R - X ... Y) are non-covalent interactions in which a covalentlybonded halogen atom (X) in a molecule (R - X ) acts as an electrophile and interacts with anucleophilic site (Y) in the same or another molecule [1]. The halogen bonds tendency towardlinearity is the most important features of them. The halogen bonds are highly directional and theR-X-Y angle, θ, tends toward 180° [2]. The interacting quantum atoms (IQA) scheme [3] is used toinvestigate the orientation preference and directional features of the halogen bonds in theF(3) C- X... NH(3) (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 totalbehaviour of the system.The linear structures are energetically the most stable geometries and the complexesbecome less stable as the C-X-N angle deviates from linearity. To find the origin of thedirectionality in the studied halogen bonds, a scan of θ from 180° to 90° in the steps of 10degrees was performed.Halogen-nitrogen interaction (i.e. the halogen bond) is attractive and more favourable in thelinear arrangement, however, its REG shows that this term plays a small role in the total behaviorof the system. Instead, the secondary repulsive interactions (e.g. fluorine-nitrogen, carbonhydrogeninteractions) and also atomic self-energies are mainly responsible for the directionalityof halogen bond.

کلیدواژه ها:

Halogen Bond ، Directionality ، IQA ، REG