Charge Density Analysis of Heterohalogen (Cl···F) and Homohalogen (F···F) Intermolecular Interactions in Molecular Crystals: Importance of the Extent of Polarizability

Experimental charge density distribution in 2-chloro-4-fluorobenzoic acid and 4-fluorobenzamide has been carried out using high resolution X-ray diffraction data collected at 100 K using Hansen-Coppens multipolar formalism of electron density. These compounds display short Cl···F and F···F interactions, respectively. The experimental results are compared with the theoretical charge densities using theoretical structure factors obtained from periodic quantum calculation at the B3LYP/6-31G** level. The topological features were derived from Bader’s “atoms in molecules” (AIM) approach. Intermolecular Cl···F interaction in 2-chloro-4-fluorobenzoic acid is attractive in nature (type II interaction) while the nature of F···F interactions in 4-fluorobenzamide shows indication of a minor decrease in repulsion (type I interaction), though the extent of polarization on the fluorine atom is arguably small.