Intermolecular atom-atom bonds in crystals?

In the latest issue of IUCrJ, three essays have been published that consider the concept of interatomic bonds in crystals from different perspectives.

The debate is framed by Professor Jack D. Dunitz [Dunitz, J. D. (2015). IUCrJ, 2, 157-158; doi: 10.1107/S2052252515002006], who questions whether the transfer of the concept of interatomic bonds to the world of intermolecular arrangements in crystals is tenable. His essay poses some specific questions: (1) Should the observation of short distances between pairs of atoms on the peripheries of different molecules in crystals be regarded as evidence of specific intermolecular bonding between the atoms concerned? (2) If the answer is not "yes" but "no" or "perhaps" or "sometimes", how are we to distinguish the bonding atom–atom interaction from the energetically neutral or anti-bonding type?

Thakur, T.S. et al. (2015), IUCrJ, 2, 159-160; doi: 10.1107/S205225251500189X consider Dunitz’s questions from a chemical and crystal engineering perspective. They believe that in most cases the answer to the first question is “yes”, in a few cases "perhaps" and in the rarest of cases "no". This is evident from experimental observations that individual atom-atom contacts are, more often than not, adequate to exert a controllable structure-directing influence over molecular crystal structures. Merely because some short contacts in crystal structures might be repulsive, or even destabilizing, this should not dismiss all short contacts as not being of any significance in determining crystal packing.

Lecomte, C. et al. (2015), IUCrJ, 2,161-163; doi: 10.1107/S2052252515002067 approach the discussion from a more fundamental position, articulating the validity and usefulness of the bond path concept. Within the framework of Bader’s Quantum Theory of Atoms In Molecules (QTAIM) theory, observing bond paths and their associated bond critical points provide sufficient conditions to establish bonding interactions, whether intra- or inter-molecular. Thus, they suggest that the answer to Dunitz’s second question can be found in the analysis of high-quality diffraction data. In a crystal engineering context, QTAIM theory provides the theoretical basis for the supramolecular synthon approach.

The debate provides an illustration of how chemical and crystallographic concepts come together to frame the vibrant research field of crystal engineering. To read these and other articles in the current issue of IUCrJ, go to http://journals.iucr.org/m/issues/2015/02/00/.