Structural transformations in crystals induced by radiation and pressure. Part 4. The complex influence of high pressure on the path and kinetics of the [2 + 2] photodimerization

Abstract

The results from monitoring structural changes brought about by the [2 + 2] photodimerization of 2-benzyl-5-benzylidenecyclopentanone (BBCP) in crystals at high pressure were presented. This is one of the two first examples of such studies for an intermolecular reaction. The influence of pressure of 0.5 GPa on the extent of changes caused by the photochemical reaction in the cell volume, in the molecular orientation and in the distance between reactive atoms is the same or similar to for 0.1 MPa of pressure, however, the character of the changes in the above-mentioned distance is different. The reason for the observed close similarity between the reaction path at 0.1 MPa and 0.5 GPa is a very good fit for dimer molecules to the crystal lattice of the monomer. The influence of high pressure itself on (a) the volume of the unit cell, (b) the volume of the free space in the unit cell, (c) the geometry of intermolecular interactions and (d) the distance between directly reacting atoms of two adjacent molecules was also described. The reaction rate is greater at 0.7 GPa than at 0.5 GPa; however, the difference is small. This was explained by the consistency between structural changes that came about by the photochemical reaction and by high pressure.