Fluxional Halogen Bonds Versus Interlayer Stacking - Theory Meets Experiment.

Area of Science:

• Solid-state chemistry
• Supramolecular chemistry
• Crystallography

Background:

• Halogen bonds (XBs) are directional interactions crucial in crystal engineering.
• Bifurcated XBs are rare in solids and energetically unfavorable at the molecular level.

Purpose of the Study:

• To investigate the dynamic behavior of a symmetric bifurcated XB in a cocrystal.
• To reconcile experimental observations with theoretical calculations regarding XB behavior and crystal transitions.

Main Methods:

• High-resolution X-ray diffraction experiments at varying temperatures.
• Theoretical calculations using revised van der Waals density functional theory.
• Analysis of electron densities and interaction types.

Main Results:

• A reversible crystal-to-crystal transition observed at 130 K for the 1,4-diiodobenzene and 1,4-dinitrobenzene cocrystal.
• The transition shifts the symmetric bifurcated XB to an asymmetric, more linear arrangement.
• XB is fluxional and entropy-favored in the high-temperature phase, becoming asymmetric and Gibbs free energy-favored at low temperatures.
• Calculations and experiments show weak interaction strengths, with molecular stacking being the dominant contact.

Conclusions:

• The dynamic behavior of bifurcated XBs can reconcile their presence in crystals with their molecular energetics.
• Entropy plays a significant role in stabilizing the high-temperature symmetric phase.
• The observed transition is driven by subtle energy differences, highlighting the fluxional nature of these interactions.