Experimentally Derived Charge Density of Halogen Bonds to Heavy Pnictogens: Revealing Electronic Structure and Validating Density Functional Theory Approaches

We present the combined experimental and computational charge density study of halogen-bonded cocrystals containing highly unusual interactions involving heavy pnictogen acceptors. Multipole refinement against high-resolution X-ray diffraction data shows the presence of bond critical points along the I···P, I···As, and I···Sb interaction paths, as well as clear nucleophilic and electrophilic features on the electrostatic potential surfaces, marking direct evidence for the bonding character of these interactions. The experimental results are then used as a benchmark for theoretical predictions obtained from a wide variety of density functional theory (DFT) methods, allowing a direct comparison of DFT-derived charge density distributions with experiment. It is shown that calculations with different DFT functionals result in widely different interaction geometries, energies, and charge density characteristics. The presented work demonstrates the value of experimental charge density analysis in benchmarking the computational methods for reliable prediction of the structural properties of halogen-bonded materials.