Understanding Intermolecular Interactions using Metastable Clusters and Assemblies

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Solids are formed when molecules stick together. The strength of the attraction is not the same for all orientations of two molecules, however, as some parts of a molecule are stickier than others. Thus, even simple molecules have complex interactions that make it difficult to predict the structure of a crystal based on the chemical structure of the individual molecules. With support from the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Professor S. Alex Kandel of the University of Notre Dame is exploring how molecules crystallize from solution. The insights gained from the project are advancing our understanding of materials science, as well as have far-reaching commercial implications in the manufacture of pharmaceuticals. The project includes training of graduate students, and involvement of undergraduate and high-school students in scientific research.Molecular clusters and crystals are formed by injecting microliter-sized droplets of solutions containing small organic molecules into vacuum. As the solvent evaporates, solid structures are formed, deposited onto clean substrates, and characterized using scanning tunneling microscopy (STM). The deposition method captures a range of geometries and molecular configurations, not just the structures near the thermodynamic minimum-free-energy state. The ability to characterize multiple metastable structures provides detailed insight into the complex potential surface created by several interacting functional groups on two or more molecules. The experimental approach compares a range of chemically similar systems to determine how, for a given molecule, each specific functional group contributes to the intermolecular interactions that ultimately control the formation of clusters and larger structures. Molecular-scale STM images are also compared with measurements made on gas-phase ionic clusters using electrospray-ionization mass spectrometry, and the results of both are examined in the light of theoretical calculations of cluster geometries.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.