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Temperature dependence of isotropic hyperfine coupling constants of the mu-ethyl radical in faujasites Bridges, Michael David

Abstract

In this complete study of the isotropic hyperfine coupling constants (hfccs) of the Mu-ethyl radical in the NaY, HY and USY faujasites, across a temperature range of 5 to 500 K, the effects of changing the loading of guest molecules, and the type and number of cations in the zeolite was studied (while comparing the results to those of bulk ethene and silica-bound ethene), giving important information on the interactions of the radical with the faujasitic environments. This is the first such report of a neutral alkyl free-radical in an unperturbed faujasite environment, and the first report of ethyl radical hfccs in any zeolite. This is also the first time that proton hfccs for an alkyl radical have been measured in zeolites. Apart from changes in the magnitude of the proton hfccs, their temperature trends are similar to those of the bulk. At low temperatures, the muon hfccs values obtained were 10-15% higher than those observed in the bulk, an unprecedented hfcc shift for alkyl radicals and is the second largest environment-induced shift ever observed. Despite these shifts, the Mu-hfcc temperature-dependent trends follow the general shape of the bulk, indicating that intramolecular rotation about the Mu-ethyl radical's C-C bond is not strongly influenced by binding within the faujasite. Changing the loading of guest molecules in faujasites has a small, but present, effect on the Mu-ethyl radical's muon hfccs. Strangely, increases in loading have the opposite effect in HY than in NaY, potential evidence that differing cation site locations in NaY and HY, relative to the surrounding framework, cause microscopic differences in radical binding. By fitting an empirical equation offered by Roduner et al. to our data, we determined classical barriers to intramolecular rotation, which ranged from 1.16 to 1.36 (± 0.03 to 0.09) kJ mol⁻¹. Temperature-independent µALCR Δ₁ linewidths help support our supposition that the Mu-radical binds within the zeolite framework. Also, loss of Δ₁ peak intensity as temperature is increased indicates some sort of radical motion or isotropic averaging. This is the first case in which the Δ₁ resonance has disappeared with temperature for a zeolite-bound Mu-radical.

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