Existence of untypical halogen-involving interactions in crystal packings: a statistical and first-principles study

There is a common perception by the scientific community that a halogen-involving interaction forms when the distance between the donor atom and the acceptor atom is less than the sum of their van der Waals (vdW) radii. Our previous work, some theoretical predictions and a search in the Cambridge Structural Database (CSD) for crystalline solids with halogen-involving interactions cast doubt on this common perception and lead us to express a challenging question: do halogen-involving interactions exist above the sum of the vdW radii of the atoms in contact? To address this question, we chose 21 couples with similar structures where the main difference between them is the distance between the atoms in contact involved in halogen interactions. To characterize halogen-involving interactions, we resort to (i) periodic and non-periodic density functional theory (DFT) calculations, (ii) quantum theory of atoms in molecules (QTAIM), and (iii) energy decomposition analysis (EDA). In contrast to the common perception, our results indicate that halogen-involving interactions do exist at distances up to 20.74% above the sum of the vdW radii of the atoms in contact which shows the same trend as theoretical predictions. Our results modify the current picture of crystal packing which contains halogen-involving interactions, leading to new approaches in designing and engineering crystals for applications.