Overview:

Electron Densities and Structure Factors:

The big question is "What do molecules look like?" We'd like to scatter radiation (X- rays) off the atoms (electron density) and use a microscope to reformulate the image. However, atoms are too small. Optical methods don't work because at the wavelengths necessary to see small things like atoms ( =1-2 Å ) there aren't optical elements like lenses that can be used to reconstruct the image of the scattering objects.

Instead, we can use computational methods to take the effects of X-ray diffraction from crystals and generate the electron densities giving rise to the scattering of X-rays. For examples, see the diffraction photos and the electron density map sectioned on plastic sheets.

Here's the basic equation we'll try to understand as the term progresses.

This is a Fourier series relating the electron density, , and information from the diffraction pattern, .

Electron density

• atoms are in the high parts of the electron density
• three dimensional function
• "real space"
• x,y,z are the coordinates in this system
• axes are the edges of the unit cell

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copyright © Ron Stenkamp stenkamp@u.washington.edu Most recent update 1/7/97