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

The nature of intermolecular interactions in iodine (I₂) crystals has long been a subject of interest, traditionally attributed to London dispersion (LD) forces. Herein, we systematically investigate I₂⋯I₂ interactions through quantum mechanical calculations combined with spectroscopic and theoretical analyses. Focusing on three I₂⋯I₂ dimers extracted from the crystal structure, the nature of these interactions was further clarified using electron localization function (ELF) analysis and topological atoms-in-molecules (AIM). The results confirm XB is dominated in dimer 1, the most stable dimer. Comparisons of experimental Raman spectra with calculated spectra for dimer 1 and crystal fragments revealed frequency splitting of II stretching modes, directly linked to XB-induced weakening of specific II bonds. Collectively, these results provide the first systematic evidence that the intrinsic intermolecular interaction in I₂ crystals is halogen bonding, rather than LD forces. This work redefines our understanding of iodine's solid-state structure and the nature of the intermolecular interactions.