Intermolecular Embraces and Intermolecular Energies

ABSTRACT

Control of the assembly of molecules in the crystalline state requires management of all possible intermolecular interactions, and their energies. Any one molecule is surrounded by and interacts with parts of the surface of several others, and it is the balance of all the intermolecular interaction energies that determines the lattice enthalpy, and therefore the product structure. In this context of competition between different intermolecular contacts, simplification occurs when intermolecular motifs operate in concert, as in multiple aryl embraces. Structural hierarchies of extended embrace motifs occur in crystals, analogous to the secondary ⇒ tertiary ⇒ quaternary construction of biological macromolecules. Some principles for the analysis and interpretation of intermolecular space in crystals are discussed.

Richly abundant data on the packing of molecules in crystals provides the basis for the description of intermolecular motifs, but this data is only geometrical. A different property, intermolecular energy, is determinative in the assembly of molecules. In strong contrast to the abundance of geometrical data, there is a serious scarcity of experimental thermodynamic data about many types of intermolecular motifs, including embraces. However, intermolecular potentials for chemically diverse systems can be calculated efficiently by appropriate density functional methods. Key concepts relating intermolecular potentials, the van der Waals distance criterion, and distributions of intermolecular distances, are reiterated. Charged polyatomic molecules, prevalent in coordination chemistry, raise significant questions about the relative magnitudes and influences of the additional electrostatic components of intermolecular energy. Some of the issues and uncertainties in this, for instance in the multiple phenyl embraces that occur between homocharged molecules, are discussed.

Keywords:

• aryl embrace motif
• coordination complex
• coulombic energy
• crystal packing
• intermolecular potential
• polyatomic ion
• van der Walls

Acknowledgments

This research is supported by the Australian Research Council, the University of New South Wales, and Australian Partnership for Advanced Computing. I thank my coworkers, Dr. Marcia Scudder and the students named in the references.