The nature of halogen bonding: insights from interacting quantum atoms and source function studies

A detailed study of the X...N (X = I, Br) halogen bonds in complexes formed by an extended set of substituted pyridines with D-X molecules (D = X, CN) is reported here. The nature of these interactions has been investigated at different (MP2 and DFT) levels of theory through Bader's quantum theory of atoms in molecules (QTAIM) and Pendás' interacting quantum atoms (IQA) scheme, focusing on the role of the local environment (i.e. the substituent on the pyridine ring and the halogenated residue) on the halogen bond features. We found that the exchange-correlation energy represents a substantial contribution to the IQA total energy, in some cases comparable to (I₂ complexes) or even dominating (ICN complexes) the electrostatic term. Meaningful information is provided by the source function, indicating that the major contribution to the electron density at the bond critical point of the X...N interaction is derived from the halogen atom, while a much lower contribution comes from the nitrogen atom, which acts as either source or sink for electron density. A relevant contribution from distal atoms, including the various electron-donor and electron-withdrawing substituents in different positions of the pyridine ring, is also determined, highlighting the non-local character of the electron density. The existence of possible relationships between binding energies, interaction energies according to IQA, and QTAIM descriptors such as delocalization indices and source function, has been inspected. In general, good correlations are only found when the local environment, external to the directly involved halogen and nitrogen atoms, plays a minor role in the interaction.