Chemistry Reference
In-Depth Information
reaction and the partial disintegration are operative simultaneously, the assignment
of these effects in the diffraction pattern to one particular factor may be difficult
without more quantitative assessment and consideration of the overall scattering
power and the spatial distribution, shape, and/or intensity of the reflections. In some
cases, although present, the disruption of the crystalline order may not be readily
visually distinguishable from the effects in the diffraction pattern which occur as a
result of the chemical transformation (e.g., increased disorder). The compatibility
of a certain solid-state structure to the geometric perturbations required by some
chemical reaction can also be assessed by trial steady-state experiments aimed at
determining the maximum possible conversion of the reactant to the product
without significant decrease in the crystal quality. In practice, the use of polarized
light may be helpful to detect such intrinsic non-homogeneities. Yet other, more
practical, approaches to overcome the decreased crystallinity problem are prepara-
tion of host-guest crystals or cocrystals with suitable secondary molecules, inclu-
sion of the reactants in molecular containers, or using powder samples instead of
single crystals.
2 Selected Examples of Application of the X-Ray
Photodiffraction Method to Chemical Reactions
and Physical Processes
2.1 Steady-State X-Ray Photodiffraction
If species which appear as products in the reaction have very different absorption
spectra and color from the reactants in the visible region, progress of the reaction
can be analyzed by electron (optical) spectroscopy. If color change can be reverted
by exposure of the product to light or heat, the respective reactions are commonly
termed
photochromic
. Such changes are usually related to significant changes in the
molecular structure (for example, in the degree of electron delocalization) and they
can be easily monitored by spectroscopic methods, which turns such systems into
important and convenient targets for application of the X-ray photodiffraction
method. Many (although not all) of the known organic photochromic systems are
based on chemical reactions, such as electrocyclizations or transfer of chemical
groups, and they provide the possibility to study these processes in detail. As a
typical example, in the case of the widely studied photochromic anil (salicylide-
neaniline) compounds, the photoinduced enol-keto tautomeric equilibrium due to
an intramolecular proton transfer is combined with subsequent
cis
-
trans
isomeri-
zation. X-ray photodiffraction was successfully applied to a number of anils to
solve the dilemma of the identity of the product, which was identified as the
trans
-
keto form [
21
]. Other proton transfer reactions, such as the nitro-assisted proton
transfer in the nitrobenzylpyridines [
22
], have also been studied. The electrocycli-
zation/ring-opening reactions comprise another important family of photochromic
