Experimentally derived charge density of halogen bonds to heavy pnictogens: revealing electronic structure and validating DFT 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 of high resolution X-ray diffraction data shows the presence of bond critical points along the I···P, I···As and I···Sb interaction paths, 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 produce result in widely different interaction geometries, energies and charge density characteristics. The presented work demonstrates the importance of selecting the appropriate DFT methods for reliable prediction of structural properties of halogen-bonded materials, as well as shows the unique value of experimental charge density analysis in benchmarking the computational methods.