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.