Hyperfine Interactions and Molecular Motion of the Mu−Ethyl Radical in Faujasites:  NaY, HY, and USY

The adsorption and dynamical behavior of the Mu−ethyl radical (MuC 2 H 4 ) in NaY, HY, and USY faujasites was investigated by the muon spin resonance (μSR) technique, at loadings of one to five ethene molecules per supercage and over a temperature range of ca. 5−500 K (for NaY). The temperature dependences of both the muon and proton hyperfine coupling constants (Hfc's) are reported and compared with similar studies of MuC 2 H 4 in different environments. Both transverse field (TF) μSR and avoided level crossing resonance (ALC) μSR spectra were recorded, with information on molecular motion mainly provided by the ALC line shapes. The muon Hfc's show only a small sensitivity to different frameworks and loadings but exhibit significant (∼10%) shifts at low temperatures, in comparison with bulk values, due to binding of the ethyl radical to cations at S II sites in NaY and to framework hydroxyls in the case of HY(USY). The Δ 1 resonances are symmetric and quite broad at the lower temperatures studied, but dramatically further broaden near room temperature, seen also in the TF relaxation rates, suggesting that the Mu−ethyl radical either desorbs from or hops between its binding sites at the higher temperatures. An Arrhenius estimate of the activation energy for desorption gives ∼ 20 kJ/mol, consistent with the dipolar interaction energy between the Mu−ethyl radical and an NaY cluster. The observation of such highly broadened Δ 1 ALC lines at the higher temperatures contrasts with the largely static line widths reported previously for the Mu−cyclohexadienyl radical (MuC 6 H 6 ) in NaY. Sharper Δ 0 ALC lines for both the α and β protons of MuC 2 H 4 appear near the same temperatures at which the Δ 1 lines overly broaden, and which persist to the highest temperatures (350 K). For NaY, the α proton resonances also broaden further at these temperatures. For both NaY and particularly HY, the temperature dependence of the α proton Hfc's indicates considerable distortion of the Mu−ethyl radical geometry, due to its binding to zeolite sites. Recently published calculations of binding energies and Hfc's for ethyl radicals in NaY and HY suggest a much stronger binding of the MuC 2 H 4 radical than seems warranted by the data and pose as well a conundrum in comparison with earlier results for MuC 6 H 6 in NaY. On the other hand, the temperature dependence of the isotropic muon Hfc's found from the T-atom model for NaY employed in these calculations is in excellent agreement with experiment.

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