Fluxional halogen bonds in linear complexes of tetrafluorodiiodobenzene with dinitrobenzene.

The fluxional nature of halogen bonds (XBs) in small molecular clusters, supramolecules, and molecular crystals has received considerable attention in recent years. In this work, based on extensive density-functional theory calculations and detailed electrostatic potential (ESP), natural bonding orbital (NBO), non-covalent interactions-reduced density gradient (NCI-RDG), and quantum theory of atoms in molecules (QTAIM) analyses, we unveil the existence of fluxional halogen bonds (FXBs) in a series of linear (IC₆F₄I)_m(OONC₆H₄NOO)_n (m + n = 2-5) complexes of tetrafluorodiiodobenzene with dinitrobenzene which appear to be similar to the previously reported fluxional hydrogen bonds (FHBs) in small water clusters (H₂O)_n (n = 2-6). The obtained $\mathrm{GS}\rightleftharpoons \mathrm{TS}\rightleftharpoons {\mathrm{GS}}^{\text{'}}$ fluxional mechanisms involve one FXB in the systems which fluctuates reversibly between two linear CI···O XBs in the ground states (GS and GS') via a bifurcated CI  O₂N van der Waals interaction in the transition state (TS). The cohesive energies (E_coh) of these complexes with up to four XBs exhibit an almost perfect linear relationship with the numbers of XBs in the systems, with the average calculated halogen bond energy of E_coh/XB = 3.48 kcal·mol^-1 in the ground states which appears to be about 55% of the average calculated hydrogen bond energy (E_coh/HB = 6.28 kcal·mol^-1) in small water clusters.