On the role of charge transfer in halogen bonding

The role of charge transfer in halogen bonding is the subject of an ongoing debate and controversy. It is clear from experimental data that charge transfer occurs in halogen bonds, but its contribution to the energetics of the interaction can be evaluated only computationally. Since the charge transfer is not a physically well-defined property, there are multiple computational approaches, which could yield very different results. In this work, we investigate this topic using our recently developed method based on constrained DFT, which allows the quantification of net charge transfer and the associated interaction energy component [Řezáč et al., J. Chem. Theory Comput., 2015]. It is based on the spatial definition of molecular fragments using the superimposed electron density of non-interacting fragments as a reference state free of charge transfer. This definition is close to the intuitive view of charge transfer, yet it removes any arbitrariness in the partitioning of the molecular complex. It has been shown to be very reliable as it avoids the issues encountered in other definitions of charge transfer. For example, the results are independent of the basis set. These calculations are complemented with DFT-SAPT decomposition, which yields the other components of the interaction energy. We have found the energetic contribution of charge transfer to halogen bonding to be rather small, on average about 10% of the interaction energy, which is less than that in hydrogen bonds. Even in very strong halogen bonds, where the absolute value of the charge-transfer energy becomes larger, it is still only a small fraction of the other attractive terms obtained from DFT-SAPT. These results suggest that although it is present, charge transfer is not the determining factor in halogen bonding.