6. Seeing Bonds by Electron Difference Density
Richard M. Colgate Professor of Chemistry
Professor McBride elucidates the principles of X-ray crystallography, correlating diffraction patterns to electron distribution in molecules. The lecture details the use of electron difference density maps to visualize chemical bonds and unshared electron pairs, critically examining their observed density compared to theoretical models.
Freshman Organic Chemistry (CHEM 125)
Professor McBride uses a hexagonal "benzene" pattern and Franklin's X-ray pattern of DNA, to continue his discussion of X-ray crystallography by explaining how a diffraction pattern in "reciprocal space" relates to the distribution of electrons in molecules and to the repetition of molecules in a crystal lattice. He then uses electron difference density mapping to reveal bonds, and unshared electron pairs, and their shape, and to show that they are only one-twentieth as dense as would be expected for Lewis shared pairs. Anomalous difference density in the carbon-fluorine bond raises the course's second great question, "Compared to what?"
Seminar notes & discussion
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