Chemistry Reference
In-Depth Information
2.2.4 Radiation Damage and Crystal Instability
It may be that an important source of systematic errors can be corrected by
monitoring standard intensities at regular intervals during data collection. Chemical
damage of organic materials by X-rays and other forms of ionizing radiation is
classified as
primary
or
secondary
. Primary damage is caused by interaction between
the radiation beam and electrons of the compounds and it mainly depends on the
radiation dose [
31
]. On the other hand, secondary damage, caused by the reactions of
the resulting radiolytic products is typically reduced at low temperature [
32
,
33
],
though not completely removed. Another source of instability is the loss of solvated
molecules that might occur even at temperatures well below the boiling point of this
solvent (thus at temperatures at which the vapor tension of the solvent as pure liquid
would be very small). De-solvation often damages the crystal, creating extended
defect or fractures that severely affect the sample quality. Of course, by lowering
the temperature all these phenomena can be reduced or even completely avoided for
the entire duration of an experiment. Cooling the sample may not be the only way to
solve the problem; in fact coating the sample properly, such as with a poly-fluorinated
oil, or enclosing it in a closed capillary, may also be adopted. These alternatives allow
temperature dependent investigation of the sample (even above room temperature).
Protective oil is a good alternative to glue for crystal mounting because of the
large residual stress that epoxies can cause. Sensitive species must be handled with
special care, because exposure to oxidative atmosphere or anyway to ambient
temperature may produce irreversible damage. For this reason, mounting the
sample in an inert atmosphere within glove boxes is sometimes necessary. If the
sample must be kept at low temperature while under the microscope, special
equipment is necessary as proposed by Stalke [
34
] or by Hope [
35
].
3 Cryo-Crystallographic Techniques
There are several techniques to cool crystal sample in order to carry out crystallo-
graphic studies and they depend on: (1) which investigation is carried out; (2) what
is the target temperature; (3) how rapid shall be the experiment; (4) the available
budget. In the following we will analyze these methods.
3.1 Single Crystal Diffraction
The diffraction from single crystals is certainly one of the most accurate techniques
to obtain detailed information on the disposition of atoms in a solid. The crystals
typically are very small (~ 10
3
mm
3
), and they are selected and mounted in air on
very small supports like glass fibers (or other amorphous and low absorbing
