6 Seeing Bonds by Electron Difference Density

Description: 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?" Complete course materials are available at the Open Yale Courses website: open.yale.edu This course was recorded in Fall 2008.