Kinetic and Mechanistic Study on Single-Crystal-to-Single-Crystal Photodimerization of 2-Benzyl-5-Benzylidenecyclopentanone Utilizing X-Ray Diffraction

Single crystals of 2-benzyl-5-benzylidenecyclopentanone (BBCP) undergo a single-crystal-to-single-crystal photodimerization reaction when irradiated with UV light. We have developed an X-ray diffraction technique whereby the precise monomer/dimer composition can be determined at any time during the reaction, in situ, revealing the kinetics of the process. Using this method it was demonstrated that the BBCP dimerization displayed a pseudo first order reaction rate over a wide temperature range and the rate constant was proportional to the incident UV photon flux. In addition, it was found that the rate constant increased over a temperature range of 132-200 K yet at higher temperatures it actually decreased. Analysis of the separation of olefinic groups in a BBCP monomer crystal at a number of different temperatures was used to explain the temperature dependence of the reaction rate.

Fig. 1.