WEAK INTERMOLECULAR INTERACTIONS IN SOLIDS AND LIQUIDS

Jack D. Dunitz

Organic Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland.

Weak intermolecular interactions are responsible for the very existence of liquids and solids; they hold the organic world together. Our main source of information about the geometrical aspects of such interactions is the analysis of molecular packing patterns in crystals, which can be retrieved with the help of the Cambridge Crystallographic Database. These packing patterns tell us about energetically favorable mutual orientations of molecules, which usually depend on a subtle interplay of many weak interactions, of which hydrogen bonding is only the strongest and best known example. Atom-atom pair potentials are useful for estimating lattice energies of molecular crystals. Short intermolecular contacts are always repulsive, even though they may correspond to stabilizing energy terms. Close packing of molecules, the avoidance of empty space, is seen as the most important feature of crystal packing. However, a slightly less dense structure will have a higher vibrational entropy and may therefore become thermodynamically more stable at elevated temperatures. For liquids, reliable structural information is hard to come by, but extensive thermodynamic data are available for certain classes of compounds. On melting a molecular solid, the packing becomes less dense, but most of the cohesive energy of the solid is retained in the liquid state.