Cooperativity between fluorine-centered halogen bonds: investigation of substituent effects.

This article analyzes the substitution effects on cooperativity between fluorin-centered halogen bonds in NCF · · · NCF · · · NCX and CNF · · · CNF · · · CNX complexes, where X = H, F, Cl, CN, OH, and NH₂. These effects are investigated theoretically in terms of geometric and energetic features of the complexes, which are computed by ab initio methods. The topological analysis, based on the quantum theory of atoms in molecules (QTAIM), is used to characterize the interactions and analyze their enhancement with varying electron density at bond critical points. It is found that the complexes with electron-donating groups exhibit a strong cooperativity, while a much weaker cooperativity occurs in the NCF · · · NCF · · · NCCN and CNF · · · CNF · · · CNCN trimers. An excellent correlation is found between the cooperative energy in the ternary complexes and the calculated three-body interaction energies. The energy decomposition analysis (EDA) indicates that the electrostatic and dispersion effects play a main role in the cooperativity of fluorine-centered halogen bonding.