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?" 00:00 - Chapter 1. Understanding Diffraction Patterns: Continuing the Case of the Hexagonal "Benzene" 15:10 - Chapter 2. Double Helices and DNA: Even and Offset Planes 29:04 - Chapter 3. Revealing Bonds and Unshared Electron Pairs via Electron Difference Density Maps 43:23 - Chapter 4. The Second Great Question: "Compared with What?" Complete course materials are available at the Open Yale Courses website: http://open.yale.edu/courses This course was recorded in Fall 2008.
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