Quantitative characterization of weak interactions

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Through this award, funded by the Chemical Structure, Dynamics, and Mechanisms Program of the Division of Chemistry, Prof. A. Alan Pinkerton and Dr. Elizabeth Zhurova from the University of Toledo, will quantitatively characterize important weak interactions between atoms or molecules in the solid state using advanced, high resolution, X-ray crystallographic techniques. The electron density distribution will be determined from the intensities of the X-rays scattered from single crystals, and quantified by fitting the parameters of an atom-centered multipole expansion to the observed data. A range of intra- and inter-molecular interactions that have been previously identified from structural data, many with "non-bonded" distances significantly below the sum of the Van der Waals radii, will be studied. Currently potential interactions between 20 different atom pairs have been identified for 20 different molecular solids. Information from this study will provide new insight into a wide spectrum of scientific areas such as materials design/crystal engineering and biological ligand-receptor binding/cell signaling. Properties of all materials are intimately related to the distribution of electrons between the atoms and molecules. For example, diamond and graphite are composed entirely of carbon atoms, but have vastly different properties, diamond being very hard, and graphite being a soft lubricant. This is easily understood as the distribution of electrons in diamond leads to strong covalent bonds in three dimensions, whereas graphite is strongly bonded in two dimensions, and weakly bonded in the third. The importance of strong chemical bonds has been recognized by generations of chemists, and their characterization by theory and experiment is the material for text books. The importance of weak interactions, with the exception of hydrogen bonds, has not been systematically studied using X-ray diffraction techniques until now, and represents a new investigative direction in chemistry. This pursuit will allow graduate students and postdoctoral fellows to acquire specialized training in cryogenics, and in high resolution X-ray crystallography. The new methods for cryogenic data collection and analysis, followed by workshops to train others, will push the limits of what is experimentally obtainable. This project will also be integrated into an outreach program to introduce high school students to the science of crystallography.