Structural changes induced in crystals by photochemical reactions were presented.
The changes concern: a) the distances between neighbouring reactant
molecules and their mutual orientation in the case of intermolecular reactions,
b) the distances and angles between fragments of a molecule for intramolecular
reactions, c) the position of molecules in crystals, d) geometry of hydrogen bonds,
e) cell constants, and f) the content of product molecules in crystals. For most intramolecular
reactions, the distances between reactive atoms are constant for a long
time in phototransformation and decrease rapidly at its end (Figs. 3 and 5). In the
case of intermolecular reactions, the distances between reactive atoms of reactant
molecules decrease linearly along with the phototransformation of crystals (Fig. 7).
Additionally, unreacted molecules become, to a certain degree, similar to product
molecules in terms of their shape (Figs, 4 and 8). Reactant and product molecules
do not assume a fixed place in crystals. Product molecules change their orientation
towards that of which is observed in a pure product crystal and reactant molecules
gradually move away from the position they took in pure reactant crystals. All this
has an influence on the geometry of hydrogen bonds existing in crystals (Fig. 9).
The above-mentioned structural transformations find their expression in values of
cell constants (Fig. 10). The factors influencing the photoreactivity of molecules in
crystals were also described. Knowledge of crystal and molecular structures of partly
reacted crystals, determined thanks to X-ray structure analysis which is a branch
of crystallography, reveals the behaviour of molecules in crystals in which photochemical
reactions proceed and helps to understand a pathway of these reactions.
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