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

Interacting Quantum Atoms (IQA) and Interacting Quantum Fragments (IQF) analyses are used to study F 3 C - X ⋯ N H 3 (X=Cl and Br) model complexes in order to determine the origin of halogen bond directionality. IQA allows for the calculation of intra- and interatomic classical and exchange-correlation energies, which can be used to determine the energetic nature of the changes that occur when deviating from the preferred halogen bond approach. The Relative Energy Gradient (REG) method is also applied to rank the IQA energies and reveal which energy contributions best describe the total behavior of the system. Indeed, all the pairwise interactions and atomic self-energies are angularly dependent; some terms favor the linear structure and some tend toward nonlinear arrangements. For instance, when the C-X-N angle is altered, the halogen-nitrogen interaction energy behaves like the total energy of the system while the carbon-nitrogen interaction works against the total energy profile. Furthermore, the REG values reveal that the contribution of the halogen-nitrogen interaction to the total behavior of the system is small. Instead, the secondary interactions (e. g., fluorine-nitrogen and carbon-hydrogen interactions) and atomic self-energies are mainly responsible for the angular preference of these halogen bonds. Finally, IQF calculations followed by REG analysis reveal the importance of the self-energy of the fragments.