The reasons for different kinetics of the Norrish-Yang reaction in crystals. Structural and spectroscopic studies.

The kinetics of the photochemical Norrish-Yang reaction in single crystals was studied using the example of 1,3,5,7-tetraazatricyclo [3.3.1.13,7] decane 4-(2,4,6-triisopropylbenzoyl)-benzoate by X-ray diffraction and Raman spectroscopy. The reaction in the studied compound proceeds in two stages described by the Johnson-Mehl-Avrami-Kolmogorov model. In the first stage the reaction progresses homogeneously at a constant rate, but for the second mode, which started at ca. 50% conversion, the reaction progresses with autoinhibition. The reasons for this autoinhibition are (i) the changes in the geometry of the reaction center during the second stage, which were not observed in the first stage, (ii) the C-H center dot center dot center dot pi interactions, which become stronger along with the reaction progress, and (iii) the change in character of the carboxylate group toward the carboxylic-like group. The Norrish-Yang reaction also proceeds in crystals at 0.2 GPa, as stated on the grounds of the characteristic changes in the unit cell parameters along with UV irradiation.