X-ray diffraction has been used to reveal precise kinetic data for the single-crystal-to-single-crystal photodimerization reaction of 2-benzyl-5-benzylidenecyclopentanone. The reaction obeys pseud-first-order kinetics. The reaction rate dependence on temperature is changed markedly at a threshold temperature of 200 K. This phenomenon is related to a change in the intermolecular distance of the two reactive olefin groups in association with temperature. A dimeric pair formation of the monomer molecules on crystallization is suggested to explain the structural similarity of the monomer and dimer crystals. Intermolecular interaction energy of the pair structure has been estimated by using a precise ab initio molecular orbital calculation.
