objectives

Understandings

• The structural technique of single crystal X-ray crystallography can be used to identify the bond lengths and bond angles of crystalline compounds.

Applications and Skills

• Deduction of the structure of a compound given information from a range of analytical characterization techniques (X-ray crystallography, IR, 1H NMR and MS).

The Theory

• X-rays have just the right wavelength to undergo reflection when falling on regions of electron density. Some of the reflected beams constructively interfere and the intensity increases, while others destructively interfere and the beam strength weakens.
• The result is a series of spots that can be analysed to build up a picture of the regions of electron density within the crystal and hence the positions of the atoms.
• The diffraction of light and other electromagnetic radiation was first studied by Bragg in the University of Leeds.

Information obtained from X-ray Diffraction

• Single-crystal X-ray Diffraction is a non-destructive analytical technique which provides detailed information about the internal lattice of crystalline substances, including unit cell dimensions, bond-lengths, bond-angles, and details of site-ordering.
• Directly related is single-crystal refinement, where the data generated from the X-ray analysis is interpreted and refined to obtain the crystal structure.
• Crystallography is the most unambiguous method for determining structures of small molecules and macromolecules.

Limitations

• As the crystal's repeating unit, its unit cell, becomes larger and more complex, the atomic-level picture provided by X-ray crystallography becomes less well-resolved (more "fuzzy") for a given number of observed reflections.
• Two limiting cases of X-ray crystallography- "small-molecule" and "macromolecular" crystallography - are often discerned.
• Small-molecule crystallography typically involves crystals with fewer than 100 atoms in their asymmetric unit; such crystal structures are usually so well resolved that the atoms can be discerned as isolated "blobs" of electron density.
• By contrast, macromolecular crystallography often involves tens of thousands of atoms in the unit cell.
• Such crystal structures are generally less well-resolved (more "smeared out"); the atoms and chemical bonds appear as tubes of electron density, rather than as isolated atoms.
• In general, small molecules are also easier to crystallize than macromolecules; however, X-ray crystallography has proven possible even for viruses with hundreds of thousands of atoms.

summary

Strengths

1. No separate standards required
2. Non-destructive
3. Detailed crystal structure, including unit cell dimensions, bond-lengths, bond-angles and site-ordering information
4. Determination of crystal-chemical controls on mineral chemistry
5. With specialized chambers, structures of high pressure and/or temperature phases can be determined
6. Powder patterns can also be derived from single-crystals by use of specialized cameras (Gandolfi)

Limitations

1. Must have a single, robust (stable) sample, generally between 50—250 microns in size
2. Optically clear sample
3. Twinned samples can be handled with difficulty
4. Data collection generally requires between 24 and 72 hours