Intermolecular Bonding Features in Solid Iodine

A detailed description of the ability of halogen bonding to control recognition, self-organization, and self-assembly in 12 crystal, combining low-temperature X-ray diffraction experiments and theoretical DFT-D and MP2 studies of charge density, is reported. The bond critical point features were analyzed using the bonding ellipsoids, in order to make them more evident and easier to compare. We showed that one-electron potential, in contrast to Laplacian of electron density, allows the electron concentration and depletion regions in the valence shell of the iodine atoms to be revealed. Thus, it was demonstrated as an effective tool for understanding the molecular recognition processes in iodine crystal, describing the mutually complementary areas of concentration and depletion of electron density in adjacent molecules. This finding was also confirmed in terms of electrostatic potential, especially using the concept of a-hole. The tiny features of the electrostatic component of halogen-halogen interactions were also visualized through the superposition of the gradient fields of electron density and electrostatic potential. The general picture provided significant arguments supporting the distinction between Type-I (van der Waals) and Type-II (Lewis molecular recognition mechanism) I center dot center dot center dot I interactions. The energies of these interactions, evaluated on the basis of empirical relationships with bond critical points parameters, have allowed estimating the lattice energy for crystalline I-2, which has been found in reasonable agreement with the experimental sublimation energy.