Kinetic and Mechanistic Aspects of a Crystal-to-Crystal Photoreaction: Effective use of Single Crystal X-ray Diffraction

X-ray diffraction has been used to reveal precise kinetic data, in situ, for the photodimerisation reaction of a 2-benzyl-5-benzylidenecyclopentanone (BBCP) single crystal. The reaction obeys first order kinetics over a wide temperature range, is a one photon process and likely to be phonon assisted. Below a threshold temperature of 173 K the rate constant decreases and an Arrhenius type plot reveals that the activation energy for the reaction is 13.2 kJ mol-1. The rate constant is slightly reduced at higher temperatures, possibly due to increased thermal motion of the olefin bonds.