Chemistry Reference
In-Depth Information
Coppens [
10
-
13
], Cole [
14
-
16
], Ohashi [
17
,
18
], and Raithby [
19
], and some
aspects were also covered by a publication preceding this review [
20
]. The inten-
tions with this short chapter were to summarize the basic principles of the tech-
nique, to provide a brief overview of the basic ideas behind this method, and - with
systems ranging in size from simple metals and homonuclear molecules to highly
complex systems such as protein crystals - to illustrate the diversity of applications
which could be realized on similar systems in future. Along with the general theme
of this review series, we provide in Sect.
2
of this chapter more details on systems
which are of “chemical” interest. The results included there are by no means
intended to be a collection of the most representative nor of the most important
examples; they were simply selected so as to provide a diverse ensemble of
chemical and physical cases of study aimed to illustrate the potentials of this
technique in a manner comprehensible to a reader with a general chemical back-
ground. Some of the examples are from our laboratory, while other examples were
selected from the works of some of the leading research groups in the field.
The X-ray photodiffraction method has already been successfully applied to
study both chemical and physical processes. The chemical processes (chemical
reactions) normally involve alteration of the bonding topology by breaking or
creating chemical bonds and subsequent conformational changes. Reactions or
processes such as electrocyclizations, cycloadditions, bond isomerizations, dimer-
izations, transfer of atoms or atomic groups, displacements or movements of
molecules or their parts, polymerizations, and bond dissociations have been docu-
mented. The physical processes that were investigated include excitations and
structure of excited states, evolution and structures of exciplexes, progression of
phonons and shock waves, monitoring of lattice dynamics, laser-induced heating
effects, photomagnetization, light-induced spin-crossover, charge transfer, photo-
induced phase transitions, and similar processes. The size of the studied systems
ranges between single atoms or diatomic molecules, to single molecules or ensem-
bles of protein molecules.
1.4 Possible Pitfalls
Despite the unquestionable advantages that it bears for structure elucidation of
photoinduced species, at the present state of development, the applicability of the
X-ray photodiffraction technique is still limited to systems which must conform to a
large number of requirements. Some of the highest hurdles on that road are: (1)
often strong absorption of the excitation light by the photoinduced form (the
product) in the solid state, (2) occasional decrease of the three-dimensional order
of the crystal lattice during photoexcitation as a result of a large structural change
(homogeneous process) and/or inability of the lattice to withstand the internal
pressure created by the evolution of the photoinduced phase (heterogeneous pro-
cess), (3) high thermal instability and difficulties with accumulation of sufficient
(detectable) amount of the product, and (4) other practical problems which are
