Complexation of Copper by Cyclohexadienyl Radicals in Cu/ZSM-5 Zeolite

Reorientational Dynamics of Cyclohexadienyl Radicals in High-Silica ZSM-5

“…It was furthermore concluded that superimposed on this reorientation there are jumps between a limited number of sorption sites with distinct orientations of the C 6 axis and that its correlation time amounted to 10-100 µs at 200 K. This is much longer and not compatible with the present finding for C 6 H 6 Mu, a fact which must be related to the much lower Si/Al ratio of 50 of the NaZSM-5 used in the NMR study. It is clear also from previous ALC-µSR results 16,30 that the cyclohexadienyl radical coordinates strongly with the cation.…”

“…It has been used to study the structures of substituted cyclohexadienyl radicals, 13 their reaction kinetics, 14 their translational diffusion on the surface of spherical silica grains, 15 and their interaction with copper ions in ZSM-5 zeolite. 16 Here, it will allow the investigation of the radical reorientation dynamics in high-silica ZSM-5.…”

Spectroscopic characterisation of molecules adsorbed at zeolite surfaces

“…84,85 This has been used to measure hyperfine couplings to nuclei from cations, such as Cu + , Li + , Na + and H + , exchanged into ZSM5 zeolites. 86 The observation of these additional couplings confirms that the cyclohexadienyl radicals (and so the benzene molecules from which they are formed), really do ''sit'' on the cations, as has been supposed; 87 however, the radical does not so much ''sit'' on the H + ion in H-ZSM5 but (at least below ca. 300 K) reacts with it, to yield a cyclohexadiene radical cation.…”

Unpaired Electrons as Probes of Catalytic Systems

“…In addition to the dynamical information that may be revealed, there is another important Cavities Channels aspect of ALC, namely that it can detect hyperfine couplings to other magnetic nuclei which are present in a given paramagnetic system 95,96 . This has been used to measure hyperfine couplings to nuclei from cations, such as Cu + , Li + , Na + and H + , exchanged into ZSM5 zeolites 97 . The observation of these additional couplings confirms that the cyclohexadienyl radicals (and so the benzene molecules from which they are formed), really do "sit" on the cations, as has been supposed 98 ; however, the radical does not so much "sit" on the H + ion in H-ZSM5 but (at least below ca 300 K) reacts with it to yield a cyclohexadiene radical cation 99 .…”

“…With the intention to derive a valence bond description of C -H activation and transition state stabilisation along the reaction coordinate with aldehyde substrates, a detailed EPR and computational study has been made of a critical paramagnetic form of xanthine oxidase (XO). Thus, the g-, 95,97 Mo hyperfine, and the 13 C hyperfine tensors have been determined to provide insight into the electronic origin of key delocalisations within the Mo -O eq -C fragment, and their contribution to C -H bond activation/cleavage and transition state stabilisation. From a natural bond orbital analysis of the enzyme reaction coordinate with aldehyde substrates it is concluded that both Mo = S π → C -H σ* (ΔE = 24.3 kcal mol -1 ) and C -H σ → Mo = S π* (ΔE = 20.0 kcal mol -1 ) back donation are important factors in activating the substrate for C -H bond cleavage.…”