The dimerization reaction in crystals of 7-methoxycoumarin has been st
udied in the range 0-6 GPa using FT-IR high-pressure spectroscopy. Und
er UV irradiation the yield of the photodimer decreases linearly with
pressure and at almost-equal-to 8 GPa the photoreaction is inhibited.
Results are interpreted mainly in terms of non-topochemical dimers for
ming at defect sides of the crystal and rapidly propagating via the su
bsequent loss of order in the lattice.