C-Centered Radicals in γ-Irradiated H-ZSM-5 Exposed to CO

The radical reactive intermediates play an important role in many catalytic and radiation reactions. To study their geometry and electronic structure, time-resolved methods or stabilization at low temperature in inert matrixes is requisite. In this Article, we present the EPR study on carbon-centered radicals formed in γ-irradiated H-ZSM-5 zeolite exposed to 13CO. Two EPR doublets with different 13C hyperfine splittings were assigned to two carbon centered radicals stabilized in different sites. According to DFT calculations, the anisotropic doublet A with Ax = 30.2 mT, Ay = 27.4 mT, and Az = 26.0 mT represents the centers in which •+CO radical cation interacts with lattice oxygen placed between Al and Si atoms or between two Si atoms. The second doublet B with Aiso = 21.4 mT was assigned to •CO radical attached to the oxygen of terminal silanol group. The analysis of radical electronic structure proved the sp2 hybridization of 13C atoms in all observed centers. These centers react with oxygen even at 77 ...