1 Introduction
Over the last century, single-crystal X-ray crystallography has developed into the
optimum method for determining the molecular structure of materials in the crystalline state and, as this volume of Structure and Bonding shows, now underpins many
aspects of the physical and life sciences. What the standard crystallographic method
provides is a full three-dimensional picture of the structure of the starting material
and of the product of the reaction if both can be obtained in a crystalline form. What
it does not do is provide a pathway by which the starting material is converted into
the product, and, since the reaction may not occur in the solid state, the immediate
relevance to a solution or gas phase chemical reaction may not be apparent [1].
The reason for this inability to follow a solid-state chemical process is because the
single-crystal X-ray crystallographic experiment is both space and time averaged. In
terms of space averaging, every unit cell in the crystal contributes to the diffraction
pattern obtained, so if some molecules within the crystal are changing and others are
not, an average of the structures will be obtained. This is commonly observed in
crystal structures when, for example, various parts of the molecule adopt different
orientations in different unit cells or lattice solvent molecules adopt different orientations; this is termed disorder [2]. The crystallographic experiments are also time
averaged. Although a single X-ray photon may interact with the electron cloud that
surrounds an atomic nucleus in crystal in 10
À18 s, it has to be remembered that in a
single-crystal X-ray crystallographic experiment, the whole crystal has to be sampled. Typically, the crystal may contain 10
15 molecules or more, and even with
modern laboratory-based diffractometers, the sampling process takes between
minutes and hours, so that the crystal structure obtained is an average over the
whole duration of the data collection [3].
A further problem associated with following chemical or biological processes
within a single crystal is the retention of crystallinity throughout the process since a
loss of crystallinity caused by degradation of the crystal means that a single-crystal
diffraction experiment is no longer viable. This limits the type of reaction that can be
followed. Adding a reagent to a crystal to facilitate a chemical reaction will usually
destroy the crystal, although there are an increasing number of examples, often
associated with crystals of framework structure materials, where small molecules
(liquids or gases) can be introduced into the crystal and can undergo a reversible or
irreversible physical or chemical process [4–6]. A much easier way of facilitating a
single-crystal-to-single-crystal reaction is to use an external medium such as irradiation with light [7–9], application of pressure [10–12] or change in temperature [13–
15] which is less likely to disrupt the crystalline lattice, and such experiments have
been becoming more feasible over the last few decades because of advances in
technology.
Over the last three decades, developments in synchrotron facilities, X-ray detectors, cryogenic apparatus, laser technology, computing power and data storage
capacity have all enabled single-crystal X-ray studies to provide information about
solid-state reactions and dynamic processes that occur in crystals. Now chemical and
Time-Resolved Single-Crystal X-Ray Crystallography
241
Over the last century, single-crystal X-ray crystallography has developed into the
optimum method for determining the molecular structure of materials in the crystalline state and, as this volume of Structure and Bonding shows, now underpins many
aspects of the physical and life sciences. What the standard crystallographic method
provides is a full three-dimensional picture of the structure of the starting material
and of the product of the reaction if both can be obtained in a crystalline form. What
it does not do is provide a pathway by which the starting material is converted into
the product, and, since the reaction may not occur in the solid state, the immediate
relevance to a solution or gas phase chemical reaction may not be apparent [1].
The reason for this inability to follow a solid-state chemical process is because the
single-crystal X-ray crystallographic experiment is both space and time averaged. In
terms of space averaging, every unit cell in the crystal contributes to the diffraction
pattern obtained, so if some molecules within the crystal are changing and others are
not, an average of the structures will be obtained. This is commonly observed in
crystal structures when, for example, various parts of the molecule adopt different
orientations in different unit cells or lattice solvent molecules adopt different orientations; this is termed disorder [2]. The crystallographic experiments are also time
averaged. Although a single X-ray photon may interact with the electron cloud that
surrounds an atomic nucleus in crystal in 10
À18 s, it has to be remembered that in a
single-crystal X-ray crystallographic experiment, the whole crystal has to be sampled. Typically, the crystal may contain 10
15 molecules or more, and even with
modern laboratory-based diffractometers, the sampling process takes between
minutes and hours, so that the crystal structure obtained is an average over the
whole duration of the data collection [3].
A further problem associated with following chemical or biological processes
within a single crystal is the retention of crystallinity throughout the process since a
loss of crystallinity caused by degradation of the crystal means that a single-crystal
diffraction experiment is no longer viable. This limits the type of reaction that can be
followed. Adding a reagent to a crystal to facilitate a chemical reaction will usually
destroy the crystal, although there are an increasing number of examples, often
associated with crystals of framework structure materials, where small molecules
(liquids or gases) can be introduced into the crystal and can undergo a reversible or
irreversible physical or chemical process [4–6]. A much easier way of facilitating a
single-crystal-to-single-crystal reaction is to use an external medium such as irradiation with light [7–9], application of pressure [10–12] or change in temperature [13–
15] which is less likely to disrupt the crystalline lattice, and such experiments have
been becoming more feasible over the last few decades because of advances in
technology.
Over the last three decades, developments in synchrotron facilities, X-ray detectors, cryogenic apparatus, laser technology, computing power and data storage
capacity have all enabled single-crystal X-ray studies to provide information about
solid-state reactions and dynamic processes that occur in crystals. Now chemical and
Time-Resolved Single-Crystal X-Ray Crystallography
241