Structural transformations in a crystal during the photochemical reaction of 2-benzyl-5-benzylidenecyclopentanone

The studies presented are the first example in which the substrate and prod uct molecules were monitored separately over many stages during the chemica l reaction progress in a crystal to understand the reaction path of the mol ecules in detail during the whole transformation. The BBCP., 2-benzyl-5-ben zylidenecyclopentanone, single crystal was irradiated without any destructi on over several steps. After each of step the X-ray structure with a differ ent substrate/product ratio and with a disorder was determined. The study r evealed that the photochemical [2+2]-cycloaddition reaction of BBCP in a cr ystal does not exhibit cooperative character and can be regarded as of the first-order. During the transformation the product molecules do not adopt t he position of the molecules in the final pure dimer crystal. but move grad ually and smoothly in this direction. The movement of the molecules was als o observed for the substrate. This remarkable behaviour of the product and reactant molecules bears a rotational component. The distance between react ion centres of adjacent BBCP substrate molecules decreases fluently with th e progress of the reaction, but it does not get smaller than a sum of van d er Waals radii.