Chemistry Reference
In-Depth Information
attached on an optical microscope, allowing temperature ranges from 100 K up to
very high temperatures. The hot/cold stage could be coupled with a calorimeter,
allowing simultaneous heat capacity measurements. One very useful observation is
the birefringence of the crystal that depends on the lattice symmetry and therefore
can be sensitive to changes like structural phase transitions. The birefringence is
easily observed using (partial) polarized light, cross polarizers, or otherwise a
rotating polarizer (which gives the greatest amount of information) [
50
-
53
].
Computational chemistry is of course another technique to obtain theoretical
information on
perfect crystals
at variable temperature. The background for this
approach has been introduced in [
113
] and will not be further discussed here. It is
important to stress that cryo-crystallography is not necessarily an experimental
science, because predictions or explanations obtained from theoretical modeling are
equally important in modern studies.
4 Cryo-Crystallographic Studies
Sometimes crystallographers consider that measuring a crystal at very low temper-
ature is a kind of
panacea
, able to solve all defects of the sample, all kinds of
experimental errors, and enhance the response indefinitely. Young students might
be disappointed to learn that these miracles do not take place. A bad crystal sample
remains as such even at 10 K, and sometimes it becomes even worse because the
cooling process and the residual stress induced by a temperature gradient may
produce further damage to the sample. Many other kinds of experimental problems
and sources of error (for example absorption, extinctions, disorder, etc.) are not
attenuated by the low temperature.
So, what can a scientist expect from a crystallographic study at low temperature?
We give in the following a bunch of examples that cannot be fully comprehensive
but should illustrate the potential of cryo-crystallography.
4.1 Crystal Structure Solution and Refinement
As anticipated, lower temperature increases the number of observations from an
X-ray diffraction data collection (at constant radiation dose). This is however just
one of the advantages that could improve a structure solution or a refinement. In
fact, a reduced thermal motion usually implies a more reliable “standard” model,
given that for smaller atomic displacements the harmonic approximation is more
appropriate and less correlation is found between variables within a least squares
refinement. This returns higher precision of the parameters calculated from those
variables (for example bond distances, bond angles, etc.).
In X-ray diffraction experiments on small organic/organometallic molecules, it
is less likely that low temperature will be really necessary to
solve
a crystal
