Chemistry Reference
In-Depth Information
interactions, as well as to explore the efficiency of utilization of purposefully
tailored supramolecular architectures to control the reactivity. This has been well
documented by a great number of reports of studied dimerizations [
34
] and poly-
merizations [
35
], only a small selection of which from the works of the most active
researchers in this subject will be mentioned here [
36
-
49
] to illustrate the applica-
tion of X-ray photodiffraction. For instance, the method can be used to determine
the reasons behind unusual solid-state kinetics, as has been demonstrated by Techert
and collaborators [
50
] on [4 + 4] photodimerization of
-9-anthracenecarboxylic
acid. In combination with spectroscopy and theoretical calculations, analysis of this
system unraveled an autocatalytic process. Based on the combined results, at least
three possible product configurations were suggested. Recently, we employed the
X-ray photodiffraction technique to study isomerization of the red mineral realgar
to its yellow polymorph pararealgar [
51
,
52
]. The intermediate phase in which half
of the realgar molecule is retained in its envelope-type conformation, while the
other half is transformed by effective switching of the positions of one sulfur and
one arsenic atom, was observed. The stepwise analysis of the two stages of this
reaction (dark and light) unraveled very complex kinetics, with the several reac-
tions acting as a solid-state autocatalytic set with balanced thermodynamics [
52
].
Photolysis of organic molecules has been employed to access important reactive
intermediates (e.g., radicals), either for studying their properties or in order to
obtain information on the respective reaction mechanisms. A prerequisite for the
analysis of photoinduced radicals is prevention of the recombination reactions,
which can be achieved by physical (separation) or chemical (conversion to less
reactive species) isolation of the reactive intermediates. As an example, we reported
recently the creation of an amidyl radical during a solid-state rearrangement where
a chlorine atom of an aromatic
N
-chloroamide is exchanged with an aromatic
hydrogen atom [
53
]. It was found that, after homolysis of the nitrogen-chlorine
bond, the detached chlorine atom and an aromatic hydrogen atom switch their
positions within the slanted head-to-tail hydrogen-bonded columns of the amide.
Another thoroughly studied group of reactions are the photoinduced linkage
isomerizations, which involve change of the coordination mode of small molecular
ligands coordinated to metal atoms. Due to the relatively small change in the overall
structure at sufficiently large changes in the positions of the non-hydrogen atoms
that facilitates analysis of the intermediate states of the reaction, these systems are
very suitable for X-ray photodiffraction analysis, because in many cases the lattice
has the capacity to sustain the stress exerted by the transformation. Photo- and
pressure-induced linkage isomerizations of Co(III) pentaammine nitro/nitrito com-
plexes were studied extensively by Boldyreva and the collaborators [
54
-
59
]. This
meticulous work has revealed important details on solid-state reactivity, especially,
the effects on the structure of the perturbations that occur as a result of coordination
switching, as well as their macroscopic consequences such as modified morphology
and macroscopic appearance of the crystals. In an early powder diffraction study of
photoirradiated samples, the group of Boldyreva has investigated the linkage nitro-
nitrito isomerization of [Co(NH
3
)
5
NO
2
]Br
2
to [Co(NH
3
)
5
ONO]Br
2
[
59
], which
unraveled the structure of the photochemically produced nitrito isomer and showed
b
