Intermolecular Bonding Features in Solid Iodine

A detailed description of the ability
of halogen bonding to control
recognition, self-organization, and self-assembly in I2 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 σ-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··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 I2, which has been found in reasonable agreement
with the experimental sublimation energy.