Proximity, photochemical reactivity, and intermolecular H-1-C-13 cross polarization in benzophenone-d(10) and cyclohexane in the zeolite NaX

The photochemistry, photophysics, and solid-state C-13 CPMAS NMR of benzophenone and cyclohexane in the zeolite NaX have been analyzed to investigate their proximity, relative mobility, and intermolecular reactivity. Photochemical irradiation of benzophenone in pure cyclohexane yields benzopinacol and benzhydrol as the predominant products. In contrast, irradiation in the solid state leads to l-cyclohexyl-l,1-diphenylmethanol as the only product by collapse of the radical pair formed after hydrogen abstraction from cyclohexane by excited benzophenone. Phosphorescence analysis at 77 K suggests a highly polar environment, but lack of emission at 300 K in the presence of cyclohexane is assigned to triplet decay via an efficient hydrogen abstraction reaction. Spectral analysis by C-13 CPMAS NMR reveals that benzophenone and cyclohexane are adsorbed in an approximate 1:2 ratio. Changes observed in spinning and static samples before and after photolysis are interpreted in terms of molecular motions that are capable of affecting the spectral line width. That benzophenone and cyclohexane share the zeolite supercages in a close packed arrangement was shown by a relatively efficient H-1-C-13 intermolecular cross polarization from cyclohexane to benzophenone-d(10). Comparison of the CPMAS intensities measured with deuterated and nondeuterated benzophenone samples under identical conditions suggest that intermolecular C-D ... H-R distances between carbon atoms of deuterated benzophenone and hydrogens of cyclohexane have an average value of ca. 2.2 Angstrom.