3. Structure Factors (2)

In this section we will look at the structure factors of a slightly larger structure.

The applet now shows a larger molecule, marked by the black crosses, in a unit cell, and the corresponding structure factors and electron density. The electron density shows peaks for the seven atoms (each of 6 electrons). The origin term is 42 electrons.

Note that there are now no other structure factors as strong as the origin term. The next strongest reflection is (2,4) for which |F|=31. Most of the reflections are much weaker.

As the size of the structure increases, it soon becomes impossible to find a set of planes which pass through the centres of every atom. Most reflections represent Bragg planes for which some atoms fall on the planes, and some fall between them. For larger molecules, all reflections are `weak' reflections.

Try clearing the map, and then set these reflections in order, to build up a low resolution image of the molecule:

The first two reflections define roughly where the molecule is. The next three begin to give the molecule shape, and after 7 reflections the ring is visible, although individual atoms are not distinguished (indeed there appear to be 4 rather than 5).