What makes molecular iodine solid? Not London dispersion forces, but halogen bonds

Area of Science:

• Solid-state chemistry
• Quantum chemistry
• Spectroscopy

Background:

• Intermolecular interactions in iodine (I₂) crystals were traditionally attributed to London dispersion (LD) forces.
• The precise nature of these interactions remains a subject of scientific inquiry.

Purpose of the Study:

• To systematically investigate I₂⋯I₂ interactions in iodine crystals.
• To clarify the dominant intermolecular forces governing the solid-state structure of iodine.

Main Methods:

• Quantum mechanical calculations
• Spectroscopic analysis (Raman spectroscopy)
• Theoretical analyses (electron localization function, atoms-in-molecules)

Main Results:

• Halogen bonding (XB) was identified as the dominant interaction in the most stable I₂⋯I₂ dimer.
• Frequency splitting in II stretching modes observed in Raman spectra correlates with XB-induced bond weakening.
• Systematic evidence supports halogen bonding over LD forces in I₂ crystals.

Conclusions:

• The intrinsic intermolecular interaction in iodine crystals is halogen bonding, not London dispersion forces.
• This finding redefines the understanding of solid-state iodine structure and its intermolecular interactions.