NO2···NO2 Contacts under Compression: Testing the Forces in Soft Donor-Acceptor Interactions

Intermolecular forces and energies are not directly observable and cannot be retrieved from a crystal structure determination, which simply pictures the resulting equilibrium between forces. In this work, using compression to sample the repulsive part of potentials, we show that high pressure studies may give insight in the nature of intermolecular forces. We focus our attention on controversial π∗ ← n interactions between NO2 groups, which exhibit several conformations. Using X-ray diffraction and ab initio calculations, we describe the high pressure behavior and phase transition of 4-amino-4′-nitrobiphenyl. To deepen our understanding of the nature of NO2···NO2 interactions, we used several theoretical tools, including pairwise atomic potential energies and forces. The results show that this interaction is associated with a mild stabilization. Since the repulsive forces are experimentally found to be weaker than those of other contacts, we conclude that also the attractive forces are weak, although in excess of the repulsive ones.