Principles of Diffraction

Question 1

What is diffraction?

Answer 1

Diffraction is the process by which a system of waves is spread out by interaction with matter. If we can create a lens, the diffracted waves can be used to recreate an image. Otherwise, we can only measure the waves and reconstruct using a computer.

Question 2

Why do we need to use diffraction?

Answer 2

We need to see atoms, but they are too small to see with light. We need to use something with a small enough wavelength (e.g. x-rays, electrons or neutrons). We can’t build an x-ray or neutron microscope because we can’t make lenses for x-rays or neutrons. Therefore, we have to measure diffraction from the molecule and work backwards.

Question 3

What are some common diffraction experiments?

Answer 3

• x-ray crystallography
• electron crystallography
• neutron crystallography
• x-ray powder diffraction
• small angle x-ray scattering (SAXS)

Question 4

Why do we use cystals for diffraction?

Answer 4

Any interaction with an atom that can tell us about its position will also seriously affect that atom due to radiation damage. A solution to this is to observe lots of copies of the molecule at once by using a crystal.

Question 5

What can we learn from x-ray crystallography?

Answer 5

• Bond lengths (atomic radii, bond orders)
• Bond angles, bonding densities (chemical bonding, molecular orbital theory)
• Surface structure (catalytic chemistry)
• Biological structure (biological function, drug design)